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Such regulation could not occur if anabolic and catabolicpathways were catalyzed by the same set of enzymes, operating in onedirection for anabolism, the opposite for catabolism. Inhibition of anenzyme involved in catabolism would also inhibit the reaction sequence in the anabolic direction. Catabolic and anabolic pathways thatconnect the same two end points (a fatty acid and acetate, for example)may employ many of the same enzymes, but invariably at least one ofthe steps is catalyzed by different enzymes in the catabolic and theanabolic directions, and these enzymes are the sites of separate regulation.
It is also common for such paired catabolic and anabolic pathwaysto occur in different cellular compartments. Fatty acid catabolism, forexample, occurs in mitochondria, whereas the synthesis of fatty acidstakes place in the cytosol. The concentrations of intermediates, enzymes, and regulators can be maintained at different levels in differentcompartments, further contributing to the separate regulation of catabolic and anabolic reaction sequences. These devices for separation ofanabolic and catabolic processes will be of particular interest in ourdiscussions of metabolism.Metabolic pathways are regulated at three levels.
The first andmost immediately responsive form of regulation is through the actionPart III Bioenergetics and Metabolismof allosteric enzymes, which are capable of changing their catalyticactivity in response to stimulatory or inhibitory modulators (p. 230).We shall meet examples of allosteric regulation throughout the following chapters. Metabolic control is exerted at a second level in higherorganisms by hormonal regulation. Hormones are chemical messengers released by one tissue that stimulate or inhibit some process inanother tissue. Hormones serve to coordinate the metabolic activitiesof different tissues, and their actions and effects are generally on asomewhat longer time scale than those of allosteric effectors. The thirdlevel of metabolic regulation is control of the rate of a metabolic step byregulating the concentration of its enzyme in the cell.
The concentration of an enzyme at any given time is the result of a balance betweenits rate of synthesis and its rate of degradation, both of which aresubject to regulation on a time scale of minutes to hours.The number of metabolic transformations that occur in a typicalcell can seem overwhelming to a beginning student. Fortunately, thereare recurring patterns in the metabolic pathways that make learningeasier.
Certain types of reactions occur in many different metabolicpathways but always employ the same coenzyme(s) and the same general mechanism. Many of the coenzymes are derived from vitamins(see Table 8-2), compounds essential in the diets of animals. The coenzymes are critical to the reaction mechanisms in which they participate. Once you have learned the general mechanism of a reaction, including the role of the cofactor, the recurring pattern in a variety ofmetabolic pathways will be easily recognizable. In the chapters thatfollow, we will usually discuss the general mechanism for each of thesereactions when we first encounter the cofactor in its typical role.In the first half of Part III we consider the major catabolic pathways by which cells obtain energy from the oxidation of various fuels:first, the central pathways of hexose conversion to triose (Chapter 14)and triose oxidation to carbon dioxide (Chapter 15); then the pathwaysof fatty acid oxidation (Chapter 16) and amino acid oxidation(Chapter 17).
Chapter 18 is the pivotal point of our discussion of metabolism; it concerns chemiosmotic energy coupling, the universal mechanism in which a transmembrane electrochemical potential, producedeither by substrate oxidation or by light absorption, drives the synthesis of ATP.The second half of this part describes the major anabolic pathwaysby which cells use ATP to produce carbohydrates (Chapter 19), lipids(Chapter 20), and amino acids and nucleotides (Chapter 21) from simpler precursors.
Finally, in Chapter 22 we step back from the detailsof the metabolic pathways and consider how those pathways areregulated and integrated in mammals by hormonal mechanisms.We begin our study of intermediary metabolism with an introduction to bioenergetics (Chapter 13). But before we begin, a final word.Try not to forget that the myriad reactions described on these pagestake place in, and play crucial roles in, living organisms.
Ask of eachreaction and of each pathway, "What is accomplished for the cell or theorganism by this reaction or pathway? How does this pathway interconnect with the other pathways occurring simultaneously in the samecell to produce the energy and products required for cell maintenanceand growth? How do the multilayered regulatory mechanisms cooperate to balance metabolic and energetic inputs and outputs, achievingthe dynamic steady state of life?" Learned with this perspective, metabolism provides fascinating and revealing insights into life.363C H A P T E RPrinciples of BioenergeticsLiving cells and organisms must perform work to stay alive, to grow,and to reproduce themselves. The ability to harness energy from various sources and to channel it into biological work is a fundamentalproperty of all living organisms; it must have been acquired very earlyin the process of cellular evolution.
Modern organisms carry out a remarkable variety of energy transductions, conversions of one form ofenergy to another. They use chemical energy in fuels to bring about thesynthesis of complex molecules from simple precursors, producingmacromolecules with highly ordered structure. They also convert thechemical energy of various fuels into concentration gradients and electrical gradients, motion, heat, and even, in a few organisms such asfireflies, light.
Photosynthetic organisms transduce light energy intoall of these other forms of energy.The chemical mechanisms that underlie biological energy transductions have fascinated and challenged biologists for centuries. Antoine Lavoisier, before he lost his head in the French Revolution, recognized that animals somehow transform chemical fuels (foods) into heatand that this process of respiration is essential to life. He observed thatAntoine Lavoisier1743-1794. . . in general, respiration is nothing but a slow combustion of carbon andhydrogen, which is entirely similar to that which occurs in a lighted lampor candle, and that, from this point of view, animals that respire are truecombustible bodies that burn and consume themselves.
. . . One may saythat this analogy between combustion and respiration has not escaped thenotice of the poets, or rather the philosophers of antiquity, and which theyhad expounded and interpreted. This fire stolen from heaven, this torch ofPrometheus, does not only represent an ingenious and poetic idea, it is afaithful picture of the operations of nature, at least for animals thatbreathe; one may therefore say, with the ancients, that the torch of lifelights itself at the moment the infant breathes for the first time, and itdoes not extinguish itself except at death.*In this century, biochemical studies have revealed much of the chemistry of energy transductions in living organisms.
Biological energytransductions obey the same physical laws that govern all other natural processes. It is therefore essential for a student of biochemistry tounderstand these laws and the ways in which they apply to the flow ofenergy in the biosphere. In this chapter we first review the laws of* From a memoir by Armand Seguin and Antoine Lavoisier, dated 1789, quoted in Lavoisier, A.
(1862) Oeuures de Lavoisier, Imprimerie Imperiale, Paris.364Chapter 13 Principles of Bioenergeticsthermodynamics and the quantitative relationships among free energy, enthalpy, and entropy. We then describe the special role of ATPin biological energy exchanges. Finally, we consider the importance ofoxidation-reduction reactions in living cells, the energetics of suchelectron transfer reactions, and the electron carriers commonly employed as cofactors of the enzymes that catalyze these reactions.Bioenergetics and ThermodynamicsBioenergetics is the quantitative study of the energy transductionsthat occur in living cells and of the nature and function of the chemicalprocesses underlying these transductions.
Although many of the principles of thermodynamics have been introduced in earlier chapters andmay be familiar to you, it is worth reviewing the quantitative aspectsof these principles.Biological Energy Transformations Follow theLaws of ThermodynamicsMany quantitative observations made by physicists and chemists onthe interconversion of different forms of energy led to the formulation,in the nineteenth century, of two fundamental laws of thermodynamics. The first law is the principle of the conservation of energy: in anyphysical or chemical change, the total amount of energy in the universeremains constant, although the form of the energy may change.
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