Chapter 1
The Molecular Logic of Life
Living organisms are composed of lifeless molecules. When these molecules are isolated and examined individually, they conform to all the physical and chemical laws that describe the behavior of inanimate matter. Yet living organisms possess extraordinary attributes not shown by any random collection of molecules. In this chapter, we first consider the properties of living organisms that distinguish them from other collections of matter. After arriving at a broad definition of life, we can describe a set of principles that characterize all living organisms. These principles underlie the organization of organisms and the cells that make them up, and they provide the framework for this book. They will help you to keep the larger picture in mind while exploring the illustrative examples presented in the text.
What distinguishes all living organisms from all inanimate objects? First, they are structurally complicated and highly organized. They possess intricate internal structures (Fig. 1–1a) and contain many kinds of complex molecules. By contrast, the inanimate matter in our environment – clay, sand, rocks, seawater – usually consists of mixtures of relatively simple chemical compounds.
Second, living organisms extract, transform, and use energy from their environment (Fig. 1–1b), usually in the form of either chemical nutrients or the radiant energy of sunlight. This energy enables living organisms to build and maintain their own intricate structures and to do mechanical, chemical, osmotic, and other types of work. By contrast, inanimate matter does not use energy in a systematic way to maintain structure or to do work. Inanimate matter tends to decay toward a more disordered state, to come to equilibrium with its surroundings.
The third and most characteristic attribute of living organisms is the capacity for precise self-replication and self-assembly (Fig. 1–1c), a property that can be regarded as the quintessence of the living state. A single bacterial cell placed in a sterile nutrient medium can give rise to a billion identical “daughter” cells in 24 hours. Each of the cells contains thousands of different molecules, some extremely complex; yet each bacterium is a faithful copy of the original, constructed entirely from information contained within the genetic material of the original cell. By contrast, mixtures of inanimate matter show no capacity to grow and reproduce in forms identical in mass, shape, and internal structure, generation after generation.
Figure 1–1  Some characteristics of living matter. (a) Microscopic complexity and organization are apparent in this thin section of vertebrate muscle tissue, viewed with the electron microscope. (b) The lion uses organic compounds obtained by eating other animals to fuel intense bursts of muscular activity. The zebra derives energy from compounds in the plants it consumes; the plants derive their energy from sunlight. (c) Biological reproduction occurs with near-perfect fidelity.
Erwin Schrödinger
1887–1961
The ability to self-replicate has no true analog in the nonliving world, but there is an instructive analogy in the growth of crystals in saturated solutions. Crystallization produces more material identical in lattice structure with the original “seed” crystal. Crystals are much less complex than the simplest living organisms, and their structure is static, not dynamic as are living cells. Nonetheless, the ability of crystals to “reproduce” themselves led the physicist Erwin Schrödinger to propose in his famous essay “What Is Life?” that the genetic material of cells must have some of the properties of a crystal. Schrödinger’s 1944 notion (years before the modern understanding of gene structure was achieved) describes rather accurately some of the properties of deoxyribonucleic acid, the material of genes.
Each component of a living organism has a specific function. This is true not only of macroscopic structures such as leaves and stems or hearts and lungs, but also of microscopic intracellular structures such as the nucleus or chloroplast. Even individual chemical compounds in cells have specific functions. The interplay among the chemical components of a living organism is dynamic; changes in one component cause coordinating or compensating changes in another, with the result that the whole ensemble displays a character beyond that of the individual constituents. The collection of molecules carries out a program, the end result of which is the reproduction of the program and the self-perpetuation of that collection of molecules.
The molecules of which living organisms are composed conform to all the familiar laws of chemistry, but they also interact with each other in accordance with another set of principles, which we shall refer to collectively as the molecular logic of life. These principles do not involve new or as yet undiscovered physical laws or forces. Instead, they are a set of relationships characterizing the nature, function, and interactions of biomolecules.
If living organisms are composed of molecules that are intrinsically inanimate, how do these molecules confer the remarkable combination of characteristics we call life? How is it that a living organism appears to be more than the sum of its inanimate parts? Philosophers once answered that living organisms are endowed with a mysterious and divine life force, but this doctrine (vitalism) has been firmly rejected by modern science. The basic goal of the science of biochemistry is to determine how the collections of inanimate molecules that constitute living organisms interact with each other to maintain and perpetuate life. Although biochemistry yields important insights and practical applications in medicine, agriculture, nutrition, and industry, it is ultimately concerned with the wonder of life itself.
A massive oak tree, an eagle that soars above it, and a soil bacterium that grows among its roots appear superficially to have very little in common. However, a hundred years of biochemical research has revealed that living organisms are remarkably alike at the microscopic and chemical levels (Fig. 1–2). Biochemistry seeks to describe in molecular terms those structures, mechanisms, and chemical processes shared by all organisms and to discover the organizing principles that underlie life in all of its diverse forms.
Figure 1–2  Diverse living organisms share common chemical features. The eagle, the oak tree, the soil bacterium, and the human share the same basic structural units (cells), the same kinds of macromolecules (DNA, RNA, proteins) made up of the same kinds of monomeric subunits (nucleotides, amino acids), the same pathways for synthesis of cellular components, and the same genetic code and evolutionary ancestors.
Although there is a fundamental unity to life, it is important to recognize at the outset that very few generalizations about living organisms are absolutely correct for every organism under every condition. The range of habitats in which organisms live, from hot springs to Arctic tundra, from animal intestines to college dormitories, is matched by a correspondingly wide range of specific biochemical adaptations. These adaptations are integrated within the fundamental chemical framework shared by all organisms. Although generalizations are not perfect, they remain useful. In fact, exceptions often illuminate scientific generalizations.
Most of the molecular constituents of living systems are composed of carbon atoms covalently joined with other carbon atoms and with hydrogen, oxygen, or nitrogen. The special bonding properties of carbon permit the formation of a great variety of molecules. Organic compounds of molecular weight (Mr) less than about 500, such as amino acids, nucleotides, and monosaccharides, serve as monomeric subunits of proteins, nucleic acids, and polysaccharides, respectively. A single protein molecule may have 1,000 or more amino acids, and deoxyribonucleic acid has millions of nucleotides.
monomeric subunits, letters of English alphabet (26 different kinds), deoxyribonucleotides (4 different kinds), amino acids (20 different kinds), ordered linear sequences, English words, deoxyribonucleic acid (DNA), protein, for a segment of 8 subunits, the number of different sequences possible = 268 or 2.1 × 1011, 48 or 65,536, 208 or 2.56 × 1010
Each cell of the bacterium Escherichia coli (E. coli) contains more than 6,000 different kinds of organic compounds, including about 3,000 different proteins and a similar number of different nucleic acid molecules. In humans there may be tens of thousands of different kinds of proteins, as well as many types of polysaccharides (chains of simple sugars), a variety of lipids, and many other compounds of lower molecular weight.
To purify and to characterize thoroughly all of these molecules would be an insuperable task were it not for the fact that each class of macromolecules (proteins, nucleic acids, polysaccharides) is composed of a small, common set of monomeric subunits. These monomeric subunits can be covalently linked in a virtually limitless variety of sequences (Fig. 1–3), just as the 26 letters of the English alphabet can be arranged into a limitless number of words, sentences, or books.
Figure 1–3  Monomeric subunits in linear sequences can spell infinitely complex messages. The number of different sequences possible (S) depends on the number of different kinds of subunits (N) and the length of the linear sequence (L): S = NL. For polymers the size of proteins (L ≈ 1,000), S is very large, and for nucleic acids, for which L may be many millions, S is astronomical.
Deoxyribonucleic acids (DNA) are constructed from only four different kinds of simple monomeric subunits, the deoxyribonucleotides, and ribonucleic acids (RNA) are composed of just four types of ribonucleotides. Proteins are composed of 20 different kinds of amino acids. The eight kinds of nucleotides from which all nucleic acids are built and the 20 different kinds of amino acids from which all proteins are built are identical in all living organisms.
Most of the monomeric subunits from which all macromolecules are constructed serve more than one function in living cells. The nucleotides serve not only as subunits of nucleic acids, but also as energy-carrying molecules. The amino acids are subunits of protein molecules, and also precursors of hormones, neurotransmitters, pigments, and many other kinds of biomolecules.
From these considerations we can now set out some of the principles in the molecular logic of life:

All living organisms have the same kinds of monomeric subunits.

There are underlying patterns in the structure of biological macromolecules.

The identity of each organism is preserved by its possession of distinctive sets of nucleic acids and of proteins.
Energy is a central theme in biochemistry: cells and organisms depend upon a constant supply of energy to oppose the inexorable tendency in nature for decay to the lowest energy state. The synthetic reactions that occur within cells, like the synthetic processes in any factory, require the input of energy. Energy is consumed in the motion of a bacterium or an Olympic sprinter, in the flashing of a firefly or the electrical discharge of an eel. The storage and expression of information cost energy, without which structures rich in information inevitably become disordered and meaningless. Cells have evolved highly efficient mechanisms for capturing the energy of sunlight, or extracting the energy of oxidizable fuels, and coupling the energy thus obtained to the many energy-consuming processes they carry out.
In the course of biological evolution, one of the first developments must have been an oily membrane that enclosed the water-soluble molecules of the primitive cell, segregating them and allowing them to accumulate to relatively high concentrations. The molecules and ions contained within a living organism differ in kind and in concentration from those in the organism’s surroundings. The cells of a freshwater fish contain certain inorganic ions at concentrations far different from those in the surrounding water (Fig. 1–4). Proteins, nucleic acids, sugars, and fats are present in the fish but essentially absent from the surrounding water, which instead contains carbon, hydrogen, and oxygen atoms only in simpler molecules such as carbon dioxide and water. When the fish dies, its contents eventually come to equilibrium with those of its surroundings.
[K+]fish > [K+]lake, [Na+]fish > [Na+]lake, [Cl−]fish > [Cl−]lake, [K+]fish = [K+]lake, [Na+]fish = [Na+]lake, [Cl−]fish = [Cl−]lake, K+, Na+, Cl−, DNA, RNA, protein, lipids, etc., monomeric subunits, NH3, CO2, HPO42−
Figure 1–4  Living organisms are not at equilibrium with their surroundings. Death and decay restore the equilibrium. During growth, energy from food is used to build complex molecules and to concentrate ions from the surroundings. When the organism dies, it loses its ability to derive energy from food. Without energy, the dead body cannot maintain concentration gradients; ions leak out. Inexorably, macromolecular components decay to simpler compounds. These simple compounds serve as nutritional sources for phytoplankton, which are then eaten by larger organisms. (By convention, square brackets denote concentration – in this case, of ionic species.)
Figure 1–5  A dynamic steady state results when the rate of appearance of a cellular component is exactly matched by the rate of its disappearance. In (a), a protein (hemoglobin) is synthesized, then degraded. In (b), glucose derived from food (or from carbohydrate stores) enters the bloodstream in some tissues (intestine, liver), then leaves the blood to be consumed by metabolic processes in other tissues (heart, brain, skeletal muscle). In this scheme, r1, r2, etc., represent the rates of the various processes. The dynamic steady-state concentrations of hemoglobin and glucose are maintained by complex mechanisms regulating the relative rates of the processes shown here.
Although the chemical composition of an organism may be almost constant through time, the population of molecules within a cell or organism is far from static. Molecules are synthesized and then broken down by continuous chemical reactions, involving a constant flux of mass and energy through the system. The hemoglobin molecules carrying oxygen from your lungs to your brain at this moment were synthesized within the past month; by next month they will have been degraded and replaced with new molecules. The glucose you ingested with your most recent meal is now circulating in your bloodstream; before the day is over these particular glucose molecules will have been converted into something else, such as carbon dioxide or fat, and will have been replaced with a fresh supply of glucose. The amount of hemoglobin and glucose in the blood remains nearly constant because the rate of synthesis or intake of each just balances the rate of its breakdown, consumption, or conversion into some other product (Fig. 1–5). The constancy of concentration does not, therefore, reflect chemical inertness of the components, but is rather the result of a dynamic steady state.
precursors (20 amino acids), synthesis, r1 → hemoglobin (in erythrocyte) → degradation, r2, breakdown products (20 amino acids), when r1 = r2, the concentration of hemoglobin is constant, food (carbohydrates), ingestion, r1 → glucose (in blood) → utilization, r2, r3, r4, waste CO2, storage fats, other products, when r1 = r2 + r3 + r4, the concentration of glucose in blood is constant
Living cells and organisms must perform work to stay alive and to reproduce themselves. The continual synthesis of cellular components requires chemical work; the accumulation and retention of salts and various organic compounds against a concentration gradient involves osmotic work; and the contraction of a muscle or the motion of a bacterial flagellum represents mechanical work. Biochemistry examines the processes by which energy is extracted, channeled, and consumed, so it is essential to develop an understanding of the fundamental principles of bioenergetics.
Consider the simple mechanical example shown in Figure 1–6. An object at the top of an inclined plane has a certain amount of potential energy as a result of its elevation. It tends spontaneously to slide down the plane, losing its potential energy of position as it approaches the ground. When an appropriate string-and-pulley device is attached to the object, the spontaneous downward motion can accomplish a certain
Figure 1–6  (Top) The downward motion of an object releases potential energy that can do work. The potential energy made available by spontaneous downward motion (an exergonic process, represented by the pink box) can be coupled to the upward movement of another object (an endergonic process, represented by the blue box). (Bottom) A spontaneous (exergonic) chemical reaction (B→C) releases free energy, which can pull or drive an endergonic reaction (A→B) when the two reactions share a common intermediate, B. The exergonic reaction B→C has a large, negative free-energy change (ΔGB→C), and the endergonic reaction A→B has a smaller, positive free-energy change (ΔGA→B). The free-energy change for the overall reaction A→C is the arithmetic sum of these two values (ΔGA→C). Because the value of ΔGA→C is negative, the overall reaction is exergonic and proceeds spontaneously.
mechanical example, work done in raising object, loss of potential energy of position, chemical example, free energy, G, reaction coordinate, A → B → C, ΔGA→B (positive), ΔGA→C (negative), ΔGB→C (negative), endergonic, exergonic
amount of work, an amount never greater than the change in potential energy of position. The amount of energy actually available to do work (called the free energy) will always be somewhat less than the total change in energy, because some energy is dissipated as the heat of friction. The greater the elevation of the object relative to its final position, the greater the change in energy as it slides downward, and the greater the amount of work that can be accomplished.
In the chemical analog of this mechanical example (Fig. 1–6, bottom), a reactant, B, is converted into a product, C. The compounds B and C each contain a certain amount of potential energy, related to the kind and number of bonds in each type of molecule. This energy is analogous to the potential energy in an elevated object. Some of the energy is available to do work when B is converted into C by a chemical reaction that involves no change in temperature or pressure. This portion of the energy, the free energy, is designated G (for J. Willard Gibbs, who developed much of the theory of chemical energetics), and the change in free energy during the conversion of B to C is ΔG.
We can define a system as all of the reactants and products, the solvent, and the immediate atmosphere – in short, everything within a defined region of space. The system and its surroundings together constitute the universe. If the system exchanges neither matter nor energy with its surroundings, it is said to be closed. The magnitude of the free-energy change for a process proceeding toward equilibrium depends upon how far from equilibrium the system was in its initial state. In the mechanical example, no spontaneous sliding will occur once the object has reached the ground; the object is then at equilibrium with its surroundings, and the free-energy change for sliding along the horizontal surface is zero.
In chemical reactions in closed systems, the process also proceeds spontaneously until equilibrium is reached. The free-energy change (ΔG) for a chemical reaction is a quantitative expression of how far the system is from chemical equilibrium. Reactions that proceed with the release of free energy are exergonic, and because the products of such reactions have less free energy than the reactants, ΔG is negative. Chemical reactions in which the products have more free energy than the reactants are endergonic, and for these reactions ΔG is positive. When all of the chemical species in the system are at equilibrium, the free-energy change for the reaction is zero, and no further net conversion of reactants into products will occur without the input of energy or matter from outside the system.
As in the mechanical example, some of the energy released in a spontaneous process can accomplish work – chemical work in this case. In living systems, as in mechanical processes, part of the total energy change in the chemical reaction is unavailable to accomplish work. Some is dissipated as heat, and some is lost as entropy, a measure of energy due to randomness, which we will define more rigorously later.
How is free energy from a chemical reaction channeled into energy-requiring processes in living organisms? In the mechanical example in Figure 1–6, it is clear that if one sliding object is coupled to another object on another inclined plane, the energy released by the spontaneous downward sliding of one may be harnessed to produce upward motion of the other, a motion that cannot occur spontaneously. This is a direct analogy to a biochemical process in which the energy released in an exergonic chemical reaction can be used to drive another reaction that is endergonic and would not proceed spontaneously. The reactions
in this system are coupled because the product of one (compound B) is a reactant in the other. This coupling of an exergonic reaction with an endergonic one is absolutely central to the free-energy exchanges that occur in all living systems. In biological energy coupling, the simultaneous occurrence of two reactions is not enough. The two reactions must be coupled in the sense of Figure 1–6 (bottom); the two reactions share an intermediate, B.
A living organism is an open system; it exchanges both matter and energy with its surroundings. Living organisms use either of two strategies to derive free energy from their surroundings: (1) they take up chemical components from the environment (fuels), extract free energy by means of exergonic reactions involving these fuels, and couple these reactions to endergonic reactions; or (2) they use energy absorbed from sunlight to bring about exergonic photochemical reactions, to which they couple endergonic reactions.

Living organisms create and maintain their complex, orderly structures at the expense of free energy from their environment.

Exergonic chemical or photochemical reactions are coupled to endergonic processes through shared chemical intermediates, channeling the free energy to do work.
Figure 1–7  During metabolic transductions, entropy increases as the potential energy of complex nutrient molecules decreases. Living organisms (a) extract energy from their environment, (b) convert some of it into useful forms of energy to produce work, and (c) return some energy to the environment as heat, together with end-product molecules that are less well organized than the starting fuel, increasing the entropy of the universe.
The first law of thermodynamics, developed from physics and chemistry but fully valid for biological systems as well, describes the energy conservation principle:

In any physical or chemical change, the total amount of energy in the universe remains constant, although the form of the energy may change.

Not until the nineteenth century did physicists discover that energy can be transduced (converted from one form to another), yet living cells have been using that principle for eons. Cells are consummate transducers of energy, capable of interconverting chemical, electromagnetic, mechanical, and osmotic energy with great efficiency (Fig. 1–7). Biological energy transducers differ from many familiar machines that depend on temperature or pressure differences. The steam engine, for example, converts the chemical energy of fuel into heat, raising the temperature of water to its boiling point to produce steam pressure that drives a mechanical device. The internal combustion engine, similarly, depends upon changes in temperature and pressure. By contrast, all parts of a living organism must operate at about the same temperature and pressure, and heat flow is therefore not a useful source of energy. Cells are isothermal, or constant-temperature, systems.

Living cells are chemical engines that function at constant temperature.

potential energy, nutrients in environment (complex molecules such as sugars, fats), sunlight, energy transductions, chemical transformations within cells → cellular work: chemical synthesis, mechanical work, osmotic and electrical gradients, light production, genetic information transfer, entropy increase, metabolic end products (simple molecules such as CO2, H2O), heat
thermonuclear fusion, 4H → 4He + positrons + electromagnetic radiation (light), photons of visible light
Figure 1–8  Sunlight is the ultimate source of all biological energy. Thermonuclear reactions in the sun produce energy that is transmitted to the earth as light and converted into chemical energy by plants and certain microorganisms.
Virtually all of the energy transductions in cells can be traced to a flow of electrons from one molecule to another, in the oxidation of fuel or in the trapping of light energy during photosynthesis. This electron flow is “downhill”, from higher to lower electrochemical potential; as such, it is formally analogous to the flow of electrons in an electric circuit driven by an electrical battery. Nearly all living organisms derive their energy, directly or indirectly, from the radiant energy of sunlight, which arises from the thermonuclear fusion reactions that form helium in the sun (Fig. 1–8). Photosynthetic cells absorb the sun’s radiant energy and use it to drive electrons from water to carbon dioxide, forming energy-rich products such as starch and sucrose. In doing so, most photosynthetic organisms release molecular oxygen into the atmosphere. Ultimately, nonphotosynthetic organisms obtain energy for their needs by oxidizing the energy-rich products of photosynthesis, passing electrons to atmospheric oxygen to form water, carbon dioxide, and other end products, which are recycled in the environment. All of these reactions involving electron flow are oxidation–reduction reactions. Thus, other principles of the living state emerge:

The energy needs of virtually all organisms are provided, directly or indirectly, by solar energy.

The flow of electrons in oxidation–reduction reactions underlies energy transduction and energy conservation in living cells.

All living organisms are dependent on each other through exchanges of energy and matter via the environment.
free energy, G, reactants (A), activation barrier (transition state, ‡), ΔG‡uncat, ΔG‡cat, ΔG, products (B), reaction coordinate (A → B)
Figure 1–9  The energetic course of a chemical reaction. A high activation barrier, representing the transition state, must be overcome in the conversion of reactants (A) into products (B), even though the products are more stable than the reactants – as indicated by a large, negative free-energy change (ΔG). The energy required to overcome the activation barrier is the activation energy (ΔG‡). Enzymes catalyze reactions by lowering the activation barrier. They bind the transition-state intermediates tightly, and the binding energy of this interaction effectively reduces the activation energy from ΔG‡uncat to ΔG‡cat. (Note that the activation energy is unrelated to the free-energy change of the reaction, ΔG.)
The fact that a reaction is exergonic does not mean that it will necessarily proceed rapidly. The reaction coordinate diagram in Figure 1–6 (bottom) is actually an oversimplification. The path from reactant to product almost invariably involves an energy barrier, called the activation barrier (Fig. 1–9), that must be surmounted for any reaction to occur. The breaking and joining of bonds generally requires the prior bending or stretching of existing bonds, creating a transition state of higher free energy than either reactant or product. The highest point in the reaction coordinate diagram represents the transition state.
Activation barriers are crucial to the stability of biomolecules in living systems. Although, when isolated from other cellular components, most biomolecules are stable for days or even years, inside cells they often undergo chemical transformations within milliseconds. Without activation barriers, biomolecules within cells would rapidly break down to simple, low-energy forms. The lifetime of complex molecules would be very short, and the extraordinary continuity and organization of life would be impossible.
Virtually every cellular chemical reaction occurs because of enzymes – catalysts that are capable of greatly enhancing the rate of specific chemical reactions without being consumed in the process (Fig. 1–10). Enzymes, as catalysts, act by lowering this energy barrier between reactant and product. The activation energy (ΔG‡; Fig. 1–9) required to overcome this energy barrier could in principle be supplied by heating the reaction mixture, but this option is not available in living cells. Instead, during a reaction, enzymes bind reactant molecules in the transition state, thereby lowering the activation energy and enormously accelerating the rate of the reaction. The relationship between the activation energy and reaction rate is exponential; a small decrease in ΔG‡ results in a very large increase in reaction rate. Enzyme-catalyzed reactions commonly proceed at rates up to 1010- to 1014-fold greater than the uncatalyzed rates.
amount of product formed (B), with enzyme, reaction: A → B, without enzyme (uncatalyzed), time
Figure 1–10  An enzyme increases the rate of a specific chemical reaction. In the presence of an enzyme specific for the conversion of reactant A into product B, the rate of the reaction may increase a millionfold or more over that of the uncatalyzed reaction. The enzyme is not consumed in the process; one enzyme molecule can act repeatedly to convert many molecules of A to B.
Enzymes are, with a few exceptions we will consider later, proteins. Each enzyme protein is specific for the catalysis of a specific reaction, and each reaction in a cell is catalyzed by a different enzyme. Thousands of different types of enzymes are therefore required by each cell. The multiplicity of enzymes, their high specificity for reactants, and their susceptibility to regulation give cells the capacity to lower activation barriers selectively. This selectivity is crucial in the effective regulation of cellular processes.
The thousands of enzyme-catalyzed chemical reactions in cells are functionally organized into many different sequences of consecutive reactions called pathways, in which the product of one reaction becomes the reactant in the next (Fig. 1–11). Some of these sequences of enzyme-catalyzed reactions degrade organic nutrients into simple end products, in order to extract chemical energy and convert it into a form useful to the cell. Together these degradative, free-energy-yielding reactions are designated catabolism. Other enzyme-catalyzed pathways start from small precursor molecules and convert them to progressively larger and more complex molecules, including proteins and nucleic acids; such synthetic pathways invariably require the input of energy, and taken together represent anabolism. The network of enzyme-catalyzed pathways constitutes cellular metabolism.
Figure 1–11  An example of a typical synthetic (anabolic) pathway. In the bacterium E. coli, threonine is converted to isoleucine in five steps, each catalyzed by a separate enzyme. (Only the main reactants and products are shown here.) Threonine, in turn, was synthesized from a simpler precursor. Both threonine and isoleucine are precursors of much larger and more complex molecules: the proteins. (The letters A to F correspond to those in Fig. 1–14.)
threonine (A), enzyme 1 → α-ketobutyrate (B), enzyme 2 → α-aceto-α-hydroxybutyrate (C), enzyme 3 → α,β-dihydroxy-β-methylvalerate (D), enzyme 4 → α-keto-β-methylvalerate (E), enzyme 5 → isoleucine (F)
Figure 1–12  (a) Structural formula and (b) ball-and-stick model for adenosine triphosphate (ATP). The removal of the terminal phosphate of ATP is highly exergonic, and this reaction is coupled to many endergonic reactions in the cell.
Figure 1–13  ATP is the chemical intermediate linking energy-releasing to energy-requiring cell processes. Its role in the cell is analogous to that of money in an economy: it is “earned/produced” in exergonic reactions and “spent/consumed” in endergonic ones.
Cells capture, store, and transport free energy in a chemical form. Adenosine triphosphate (ATP) (Fig. 1–12) functions as the major carrier of chemical energy in all cells. ATP carries energy between metabolic pathways by serving as the shared intermediate that couples endergonic reactions to exergonic ones. The terminal phosphate group of ATP is transferred to a variety of acceptor molecules, which are thereby activated for further chemical transformation. The adenosine diphosphate (ADP) that remains after the phosphate transfer is recycled to become ATP, at the expense of either chemical energy (during oxidative phosphorylation) or solar energy in photosynthetic cells (by the process of photophosphorylation). ATP is the major connecting link (the shared intermediate) between the catabolic and anabolic networks of enzyme-catalyzed reactions in the cell (Fig. 1–13).
These linked networks of enzyme-catalyzed reactions are virtually identical in all living organisms.
stored nutrients, ingested foods, solar photons, catabolic reaction pathways (exergonic), ADP + HPO42−, ATP, CO2, NH3, H2O, simple products, precursors, anabolic reaction pathways (endergonic), osmotic work, mechanical work, complex biomolecules, other cellular work
Not only can living cells simultaneously synthesize thousands of different kinds of carbohydrate, fat, protein, and nucleic acid molecules and their simpler subunits, they can also do so in the precise proportions required by the cell. For example, when rapid cell growth occurs, the precursors of proteins and nucleic acids must be made in large quantities, whereas in nongrowing cells the requirement for these precursors is much reduced. Key enzymes in each metabolic pathway are regulated so that each type of precursor molecule is produced in a quantity appropriate to the current requirements of the cell. Consider the pathway shown in Figure 1–14 (see also Fig. 1–11), which leads to the synthesis of isoleucine (one of the amino acids, the monomeric subunits of proteins). If a cell begins to produce more isoleucine than is needed for protein synthesis, the unused isoleucine accumulates. High concentrations of isoleucine inhibit the catalytic activity of the first enzyme in the pathway, immediately slowing the production of the amino acid. Such negative feedback keeps the production and utilization of each metabolic intermediate in balance.
threonine, A, enzyme 1 → B → C → D → E → F, isoleucine
Figure 1–14  Regulation of a biosynthetic pathway by feedback inhibition. In the pathway by which isoleucine is formed in five steps from threonine (Fig. 1–11), the accumulation of the product isoleucine (F) causes inhibition of the first reaction in the pathway by binding to the enzyme catalyzing this reaction and reducing its activity. (The letters A to F represent the corresponding compounds shown in Fig. 1–11.)
Living cells also regulate the synthesis of their own catalysts, the enzymes. Thus a cell can switch off the synthesis of an enzyme required to make a given product whenever that product is available ready-made in the environment. These self-adjusting and self-regulating properties allow cells to maintain themselves in a dynamic steady state, despite fluctuations in the external environment.

Living cells are self-regulating chemical engines, adjusted for maximum economy.
The continued existence of a biological species requires that its genetic information be maintained in a stable form and, at the same time, expressed with very few errors. Effective storage and accurate expression of the genetic message defines individual species, distinguishes them from one another, and assures their continuity over successive generations.
Among the seminal discoveries of twentieth-century biology are the chemical nature and the three-dimensional structure of the genetic material, DNA. The sequence of deoxyribonucleotides in this linear polymer encodes the instructions for forming all other cellular components and provides a template for the production of identical DNA molecules to be distributed to progeny when a cell divides.
Figure 1–15  Two ancient scripts. (a) The Prism of Sennacherib, inscribed in about 700 B.C., describes in characters of the Assyrian language some historical events during the reign of King Sennacherib. The Prism contains about 20,000 characters, weighs about 50 kg, and has survived almost intact for about 2,700 years. (b) The single DNA molecule of the bacterium E. coli, seen leaking out of a disrupted cell, is hundreds of times longer than the cell itself and contains all of the encoded information necessary to specify the cell’s structure and functions. The bacterial DNA contains about 10 million characters (nucleotides), weighs less than 10−10 g, and has undergone only relatively minor changes during the past several million years. The black spots and white specks are artifacts of the preparation.
Perhaps the most remarkable of all the properties of living cells and organisms is their ability to reproduce themselves with nearly perfect fidelity for countless generations. This continuity of inherited traits implies constancy, over thousands or millions of years, in the structure of the molecules that contain the genetic information. Very few historical records of civilization, even those etched in copper or carved in stone, have survived for a thousand years (Fig. 1–15). But there is good evidence that the genetic instructions in living organisms have remained nearly unchanged over very much longer periods; many bacteria have nearly the same size, shape, and internal structure and contain the same kinds of precursor molecules and enzymes as those that lived a billion years ago.
Hereditary information is preserved in DNA, a long, thin organic polymer so fragile that it will fragment from the shear forces arising in a solution that is stirred or pipetted. A human sperm or egg, carrying the accumulated hereditary information of millions of years of evolution, transmits these instructions in the form of DNA molecules, in which the linear sequence of covalently linked nucleotide subunits encodes the genetic message.
The capacity of living cells to preserve their genetic material and to duplicate it for the next generation results from the structural complementarity between the two halves of the DNA molecule (Fig. 1–16). The basic unit of DNA is a linear polymer of four different monomeric subunits, deoxyribonucleotides (see Fig. 1–3), arranged in a precise linear sequence. It is this linear sequence that encodes the genetic information. Two of these polymeric strands are twisted about each other to form the DNA double helix, in which each monomeric subunit in one strand pairs specifically with the complementary subunit in the opposite strand. In the enzymatic replication or repair of DNA, one of the two strands serves as a template for the assembly of another, structurally complementary DNA strand. Before a cell divides, the two DNA strands separate and each serves as a template for the synthesis of a complementary strand, generating two identical double-helical molecules, one for each daughter cell. If one strand is damaged, continuity of information is assured by the information present on the other strand.
Figure 1–16  The complementary structure of double-stranded DNA accounts for its accurate replication. DNA is a linear polymer of four subunits, the deoxyribonucleotides deoxyadenylate (A), deoxyguanylate (G), deoxycytidylate (C), and deoxythymidylate (T), joined covalently. Each nucleotide has the intrinsic ability, due to its precise three-dimensional structure, to associate very specifically but noncovalently with one other nucleotide: A always associates with its complement T, and G with its complement C. In the double-stranded DNA molecule, the sequence of nucleotides in one strand is complementary to the sequence in the other; wherever G occurs in strand 1, C occurs in strand 2; wherever A occurs in strand 1, T occurs in strand 2. The two strands of the DNA, held together by a large number of hydrogen bonds (represented here by vertical blue lines) between the pairs of complementary nucleotides, twist about each other to form the DNA double helix. In DNA replication, prior to cell division, the two strands of the original DNA separate and two new strands are synthesized, each with a sequence complementary to one of the original strands. The result is two double-helical DNA molecules, each identical to the original DNA.

Genetic information is encoded in the linear sequence of four kinds of subunits of DNA.

The double-helical DNA molecule contains an internal template for its own replication and repair.

strand 1, strand 2, old strand 1, new strand 2, new strand 1, old strand 2
Figure 1–17  The gradual accumulation of mutations over long periods of time results in new biological species, each with a unique DNA sequence. At top is shown a short segment of a gene in a hypothetical progenitor organism. With the passage of time, changes in nucleotide sequence (mutations, indicated here by colored boxes) occur, one at a time, resulting in progeny with different DNA sequences. These mutant progeny themselves undergo occasional mutations, yielding their own progeny differing by two or more nucleotides from the original sequence.
Despite the near-perfect fidelity of genetic replication, infrequent, unrepaired mistakes in the replication process produce changes in the nucleotide sequence of DNA, representing a genetic mutation (Fig. 1–17). Incorrectly repaired damage to one of the DNA strands has the same effect. Mutations can change the instructions for producing cellular components. Many mutations are deleterious or even lethal to the organism; they may, for example, cause the synthesis of a defective enzyme that is not able to catalyze an essential metabolic reaction.
Occasionally the mutation better equips an organism or cell to survive in its environment. The mutant enzyme might, for example, have acquired a slightly different specificity, so that it is now able to use as a reactant some compound that the cell was previously unable to metabolize. If a population of cells were to find itself in an environment where that compound was the only available source of fuel, the mutant cell would have an advantage over the other, unmutated (wild-type) cells in the population. The mutant cell and its progeny would survive in the new environment, whereas wild-type cells would starve and be eliminated.
Chance genetic variations in individuals in a population, combined with natural selection (survival of the fittest individuals in a challenging or changing environment), have resulted in the evolution of an enormous variety of organisms, each adapted to life in a particular ecological niche.
time, mutation 1, mutation 2, mutation 3, mutation 4, mutation 5, mutation 6
Carolus Linnaeus
1707–1778
Charles Darwin
1809–1882
Biochemistry has confirmed and greatly extended evolutionary theory. Carolus Linnaeus recognized the anatomic similarities and differences among living organisms and provided a framework for assessing the relatedness of different species. Charles Darwin gave us a unifying hypothesis to explain the phylogeny of modern organisms – the origin of different species from a common ancestor. Biochemistry has begun to reveal the molecular anatomy of cells of different species – the sequences of subunits in nucleic acids and proteins and the three-dimensional structures of individual molecules of nucleic acid and protein. There is a reasonable prospect that when the twenty-first century dawns, we will know the entire nucleotide sequence of all of the genes that make up the biological heritage of a human.
At the molecular level, evolution is the emergence over time of different sequences of nucleotides within genes. With new genetic sequences being experimentally determined almost daily, biochemists have an enormously rich treasury of evidence with which to analyze evolutionary relationships and to refine evolutionary theory. The molecular phylogeny derived from gene sequences is consistent with, but in many cases more precise than, the classical phylogeny based on macroscopic structures.
Molecular structures and mechanisms have been conserved in evolution even though organisms have continuously diverged at the level of gross anatomy. At the molecular level, the basic unity of life is readily apparent; crucial molecular structures and mechanisms are remarkably similar from the simplest to the most complex organisms. Biochemistry makes it possible to discover the unifying features common to all life. This book examines many of these features: the mechanisms for energy conservation, biosynthesis, gene replication, and gene expression.
The information in DNA is encoded as a linear (one-dimensional) sequence of the nucleotide units of DNA, but the expression of this information results in a three-dimensional cell. This change from one to three dimensions occurs in two phases. A linear sequence of deoxyribonucleotides in DNA codes (through the intermediary, RNA) for the production of a protein with a corresponding linear sequence of amino acids (Fig. 1–18). The protein folds itself into a particular three-dimensional shape, dictated by its amino acid sequence. The precise three-dimensional structure (native conformation) is crucial to the protein’s function as either catalyst or structural element. This principle emerges:
Figure 1–18  Linear sequences of deoxyribonucleotides in DNA, arranged into units known as genes, are transcribed into ribonucleic acid (RNA) molecules with complementary ribonucleotide sequences. The RNA sequences are then translated into linear protein chains, which fold spontaneously into their native three-dimensional shapes. Individual proteins sometimes associate with other proteins to form supramolecular complexes, stabilized by numerous weak interactions.

The linear sequence of amino acids in a protein leads to the acquisition of a unique three-dimensional structure by a self-assembly process.

Once a protein has folded into its native conformation, it may associate noncovalently with other proteins, or with nucleic acids or lipids,
to form supramolecular complexes such as chromosomes, ribosomes, and membranes (Fig. 1–18). These complexes are in many cases self-assembling. The individual molecules of these complexes have specific, high-affinity binding sites for each other, and within the cell they spontaneously form functional complexes.

Individual macromolecules with specific affinity for other macromolecules self-assemble into supramolecular complexes.

gene 1, gene 2, gene 3, transcription of DNA sequence into RNA sequence → RNA 1, RNA 2, RNA 3, translation on the ribosome of RNA sequence into protein sequence and folding of protein into native conformation → protein 1, protein 2, protein 3 → formation of supramolecular complex
The forces that provide stability and specificity to the three-dimensional structures of macromolecules and supramolecular complexes are mostly noncovalent interactions. These interactions, individually weak but collectively strong, include hydrogen bonds, ionic interactions among charged groups, van der Waals interactions, and hydrophobic interactions among nonpolar groups. These weak interactions are transient; individually they form and break in small fractions of a second. The transient nature of noncovalent interactions confers a flexibility on macromolecules that is critical to their function. Furthermore, the large number of noncovalent interactions in a single macromolecule makes it unlikely that at any given moment all the interactions will be broken; thus macromolecular structures are stable over time.

Three-dimensional biological structures combine the properties of flexibility and stability.

The flexibility and stability of the double-helical structure of DNA are due to the complementarity of its two strands and the many weak interactions between them. The flexibility of these interactions allows strand separation during DNA replication (see Fig. 1–16); the complementarity of the double helix is essential to genetic continuity.
Noncovalent interactions are also central to the specificity and catalytic efficiency of enzymes. Enzymes bind transition-state intermediates through numerous weak but precisely oriented interactions. Because the weak interactions are flexible, the complex survives the structural distortions as the reactant is converted into product.
The formation of noncovalent interactions provides the energy for self-assembly of macromolecules by stabilizing native conformations relative to unfolded, random forms. The native conformation of a protein is that in which the energetic advantages of forming weak interactions counterbalance the tendency of the protein chain to assume random forms. Given a specific linear sequence of amino acids and a specific set of conditions (temperature, ionic conditions, pH), a protein will assume its native conformation spontaneously, without a template or scaffold to direct the folding.
We can now summarize the various principles of the molecular logic of life:

A living cell is a self-contained, self-assembling, self-adjusting, self-perpetuating isothermal system of molecules that extracts free energy and raw materials from its environment.

The cell carries out many consecutive reactions promoted by specific catalysts, called enzymes, which it produces itself.

The cell maintains itself in a dynamic steady state, far from equilibrium with its surroundings. There is great economy of parts and processes, achieved by regulation of the catalytic activity of key enzymes.

Self-replication through many generations is ensured by the self-repairing, linear information-coding system. Genetic information encoded as sequences of nucleotide subunits in DNA and RNA specifies the sequence of amino acids in each distinct protein, which ultimately determines the three-dimensional structure and function of each protein.

Many weak (noncovalent) interactions, acting cooperatively, stabilize the three-dimensional structures of biomolecules and supramolecular complexes.

At no point in our examination of the molecular logic of living cells have we encountered any violation of known physical laws; nor have we needed to define new physical laws. The organic machinery of living cells functions within the same set of laws that governs the operation of inanimate machines, but the chemical reactions and regulatory processes of cells have been highly refined during evolution.
This set of principles has been most thoroughly validated in studies of unicellular organisms (such as the bacterium E. coli), which are exceptionally amenable to biochemical and genetic study. Although multicellular organisms must solve certain problems not encountered by unicellular organisms, such as the differentiation of the fertilized egg into specialized cell types, the same principles have been found to apply. Can such simple and mechanical statements apply to humans as well, with their extraordinary capacity for thought, language, and creativity? The pace of recent biochemical progress toward understanding such processes as gene regulation, cellular differentiation, communication among cells, and neural function has been extraordinarily fast, and is accelerating. The success of biochemical methods in solving and redefining these problems justifies the hope that the most complex functions of the most highly developed organism will eventually be explicable in molecular terms.
The relevant facts of biochemistry are many; the student approaching this subject for the first time may occasionally feel overwhelmed. Perhaps the most encouraging development in twentieth-century
biology is the realization that, for all of the enormous diversity in the biological world, there is a fundamental unity and simplicity to life. The organizing principles, the biochemical unity, and the evolutionary perspective of diversity, provided at the molecular level, will serve as helpful frames of reference for the study of biochemistry.
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