4 Обмен энергией (1160073), страница 8
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Flavoproteins are often very complex; somehave, in addition to a flavin nucleotide, tightly bound inorganic ions(iron or molybdenum, for example) capable of participating in electrontransfers.SummaryLiving cells constantly perform work and thus require energy for the maintenance of highly organized structures, for the synthesis of cellular components, for movement, for the generation ofelectrical currents, for the production of light, andfor many other processes.
Bioenergetics is thequantitative study of energy relationships andenergy conversions in biological systems. Biological energy transformations obey the laws of thermodynamics. All chemical reactions are influencedby two forces: the tendency to achieve the most stable bonding state (for which enthalpy, H, is a useful expression) and the tendency to achieve thehighest degree of randomness, expressed as entropy, S. The net driving force in a reaction is AG,the free-energy change, which represents the neteffect of these two factors: AG = Atf - T AS.
Cellsrequire sources of free energy to perform work.The standard free-energy change, AG0', is aphysical constant characteristic for a given reaction, and can be calculated from the equilibriumconstant for the reaction: AG°' = -RT In K'eq. Theactual free-energy change, AG, is a variable, whichdepends on AG°' and on the concentrations ofreactants and products: AG = AG°' + RT In([products]/[reactants]).
When AG is large and negative, the reaction tends to go in the forward direction; when it is large and positive, the reactiontends to go in the reverse direction; and whenAG = 0, the system is at equilibrium. The freeenergy change for a reaction is independent of thepathway by which the reaction occurs. Free-energychanges are also additive; the net chemical reaction that results from the successive occurrence ofreactions sharing a common intermediate has anoverall free-energy change that is the sum of theAG values for the individual reactions.ATP is the chemical link between catabolismand anabolism.
Its exergonic conversion to ADPand Pi, or to AMP and PPi? is coupled to a largenumber of endergonic reactions and processes. Ingeneral, it is not ATP hydrolysis, but the transferof phosphate or adenylate from ATP to a substrateor enzyme molecule that couples the energy of ATPbreakdown to endergonic transformations of substrates. By these group transfer reactions ATP pro-vides the energy for anabolic reactions, includingthe synthesis of informational molecules, and forthe transport of molecules and ions across membranes against concentration and electrical potential gradients.
Muscle contraction is one of severalexceptions to this generalization; ATP hydrolysisdrives the conformational changes in myosin thatproduce contraction in muscle.Cells contain other metabolites with large, negative, free energies of hydrolysis, including phosphoenolpyruvate, 1,3-bisphosphoglycerate, andphosphocreatine.
These high-energy compounds,like ATP, have a high phosphate group transferpotential; they are good donors of the phosphategroup. Thioesters also have high free energies ofhydrolysis.Biological oxidation-reduction reactions can bedescribed in terms of two half-reactions, each witha characteristic standard reduction potential, EQ'.When two electrochemical half-cells, each containing the components of a half-reaction, are connected, electrons tend to flow to the half-cell withthe higher reduction potential. The strength of thistendency is proportional to the difference betweenthe two reduction potentials (AE), and is a functionof the concentrations of oxidized and reduced species.
The standard free-energy change for an oxidation-reduction reaction is directly proportional tothe difference in standard reduction potentials ofthe two half-cells: AG°' = -nJ^E'o.Many biological oxidation reactions are dehydrogenations in which one or two hydrogen atoms(electron and proton) are transferred from asubstrate to a hydrogen acceptor. Oxidationreduction reactions in cells involve specializedelectron carrier cofactors. NAD and NADP are thefreely diffusible cofactors of many dehydrogenasesof cells. Both cofactors accept two electrons and oneproton. FAD and FMN, the flavin nucleotides,serve as tightly bound prosthetic groups of flavoproteins. They can accept either one or two electrons.
In many organisms, a central energyconserving process is the stepwise oxidation of glucose to CO2, in which the energy of oxidation isconserved in ATP as electrons are passed to O2.Chapter 13 Principles of Bioenergetics395Further ReadingBioenergetics and ThermodynamicsAtkins, P.W. (1984) The Second Law, ScientificAmerican Books, Inc., New York.A well-illustrated and elementary discussion of thesecond law and its implications.Blum, H.F. (1968) Time's Arrow and Evolution, 3rdedn, Princeton University Press, Princeton, NJ.Cantor, C.R. & Schimmel, P.R.
(1980) BiophysicalChemistry, W.H. Freeman and Company, SanFrancisco.This and the next two books are outstanding advanced treatments of thermodynamics.Dickerson, R.E. (1969) Molecular Thermodynamics, W.A. Benjamin, Inc., Menlo Park, CA.Edsall, J.T. & Gutfreund, H.
(1983) Biothermodynamics: The Study of Biochemical Processes atEquilibrium, John Wiley & Sons, Inc., New York.Ingraham, L.L. & Pardee, A.B. (1967) Free energyand entropy in metabolism. In Metabolic Pathways, 3rd edn, Vol. I (Greenberg, D.M., ed), pp. 146, Academic Press, Inc., New York.Klotz, I.M. (1967) Energy Changes in BiochemicalReactions, Academic Press, Inc., New York.Brief and nonmathematical introduction to thermodynamics for biochemists, with many illustrative examples.Morowitz, H.J. (1970) Entropy for Biologists: AnIntroduction to Thermodynamics, Academic Press,Inc., New York.A good introduction to thermodynamics in biology,not limited to a discussion of entropy.Rothman, T.
(1989) Science a la Mode, PrincetonUniversity Press, Princeton, NJ.Chapter 4, "The Evolution of Entropy," is an excellent discussion of entropy in biology.van Holde, K.E. (1985) Physical Biochemistry, 2ndedn, Prentice-Hall, Inc., Englewood Cliffs, NJ.Chapters 1 through 3 cover the thermodynamic concepts discussed in this chapter.Phosphate Group Transfers and ATPAlberty, R.A.
(1969) Standard Gibbs free energy,enthalpy and entropy changes as a function of pHand pMg for several reactions involving adenosinephosphates. J. Biol. Chem. 244, 3290-3302.This research paper documents the strong dependence of the free energy of ATP hydrolysis on theBock, R.M. (1960) Adenine nucleotides and properties of pyrophosphate compounds. In The Enzymes,2nd edn, Vol. 2 (Boyer, P.D., Lardy, H., & Myrback,K., eds), pp. 3-38, Academic Press, Inc., New York.Bridger, W.A. & Henderson, J.F.
(1983) Cell ATP,John Wiley & Sons, Inc., New York.The chemistry of ATP, the role of ATP in metabolicregulation, and the catabolic and anabolic roles ofATP.Hanson, R.W. (1989) The role of ATP in metabolism. Biochem. Educ. 17, 86-92.Excellent summary of the chemistry and biology ofATP.Harold, F.M. (1986) The Vital Force: A Study ofBioenergetics, W.H. Freeman and Company, NewYork.A beautifully clear discussion of thermodynamicsin biological processes.Jencks, W.P. (1990) How does ATP make work?Chemtracts—Biochem. Mol.
Biol. 1, 1-13.A clear and sophisticated description of ATP energytransductions in ion transport, muscle contraction,oxidative phosphorylation, and photophosphorylation.Kalckar, H.M. (1969) Biological Phosphorylations:Development of Concepts, Prentice-Hall, Inc., Englewood Cliffs, NJ.An historical account by one of the central participants in the study of biological phosphorylations.Lipmann, F. (1941) Metabolic generation and utilization of phosphate bond energy. Adv. Enzymol. 11,99-162.The classic description of the role of high-energyphosphate compounds in biology.Pullman, B.
& Pullman, A. (1960) Electronicstructure of energy-rich phosphates. Radiat. Res.Suppl. 2, pp. 160-181.An advanced discussion of the chemistry of ATPand other "energy-rich" compounds.Westheimer, F.H. (1987) Why nature chose phosphates. Science 235, 1173-1178.A chemist's description of the unique suitability ofphosphate esters and anhydrides for metabolictransformations.396Part III Bioenergetics and MetabolismBiological Oxidation-Reduction ReactionsDolphin, D., Avramovic, O., & Poulson, R.
(eds)(1987) Pyridine Nucleotide Coenzymes: Chemical,Biochemical, and Medical Aspects, John Wiley &Sons, Inc., New York.An excellent two-volume collection of authoritativereviews. Among the most useful of these are thechapters by Kaplan, Westheimer, Veech, and Ohnoand Ushio.Latimer, W.M.
(1952) Oxidation Potentials, 2ndedn, Prentice-Hall, Inc., New York.Montgomery, R. & Swenson, C.A. (1976) Quantitative Problems in the Biochemical Sciences, 2ndedn, W.H. Freeman and Company, San Francisco.Segel, I.H. (1976) Biochemical Calculations, 2ndedn, John Wiley & Sons, Inc., New York.Problems1.
Entropy Changes during Egg DevelopmentConsider an ecosystem consisting of an egg in anincubator. The white and yolk of the egg containproteins, carbohydrates, and lipids. If fertilized,the egg is transformed from a single cell to a complex organism. Discuss this irreversible process interms of the entropy changes in the system, surroundings, and universe.
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