Bioenergetics

Bioenergetics provides a quantitative description of the transformation of materials and energy in living systems. Most biochemical reactions occur in pathways, in which other reactions continuously add substrates and remove products. The rate of reactions depends on the properties of the enzymes (large proteins produced in cells) that catalyze the reaction. Substrates bind at the active sites of enzymes, where they are converted to products and later released. Enzymes are highly specific for given substrates and products. Inhibitors of enzymes decrease the rate of reaction. 

Mitochondria contain deoxyribonucleic acid (DNA) and ribosomes, protein-producing organelles in the cytoplasm. The DNA directs the ribosomes to produce proteins as enzymes (biological catalysts) in ATP production. 

The need for energy by the cell regulates the TCA cycle, which acts in concert with the electron transfer chain and the ATPase to produce ATP in the inner mitochondrial membrane. The cell has limited amounts of ATP, ADP, 

Electron transport has three major stages (1) transfer of electrons from NADH to coenzyme Q, (2) electron transport from coenzyme Q to cytochrome c, and (3) electron transport from cytochrome c to oxygen. These stages are briefly described below. 

Coenzyme Q passes electrons through iron-sulfur complexes to cytochromes b and ch which transfer the electrons to cytochrome c. In the ferric Fe3+ state, the heme iron can accept one electron and be reduced to the ferrous state Fe2+. Since the cytochromes carry one electron at a time, two molecules on each cytochrome complex are reduced for every molecule of NADH that is oxidized. The electron transfer from coenzyme Q to cytochrome c produces energy, which pumps protons across the inner mitochondrial membrane. The proton gradient produces one ATP for every coenzyme Q-hydrogen that transfers two electrons to cytochrome c. Electrons from FADH2, produced by reactions such as the oxidation of succinate to fumarate, enter the electron transfer chain at the coenzyme Q level. 

The energy requirements of the cells of living organisms are met by energy set free by chemical reactions involving the oxidation of organic nntrients by air oxygen. These reactions can conditionally be written as 

The pathway of the metabolic process converting the original nutrients, which are of rather complex composition, to the simple end products of COj and HjO is long and complicated and consists of a large number of intermediate steps. Many of them are associated with electron and proton (or hydrogen-atom) transfer from the reduced species of one redox system to the oxidized species of another redox system. These steps as a rule occur, not homogeneously (in the cytoplasm or intercellular solution) but at the surfaces of special protein molecules, the enzymes, which are built into the intracellular membranes. Enzymes function as specific catalysts for given steps. 

Considering the many steps involved in the overall reaction, we may ask abont the chemistry (stoichiometry) and energetics of each intermediate step. Throngh the work of numerous biochemists, the chemistry of most steps has now been determined and the enzymes revealed which catalyze each of these steps. 

The high catalytic activity of enzymes has a number of sources. Every enzyme has a particular active site configured so as to secure intimate contact with the substrate molecule (a strictly defined mutual orientation in space, a coordination of the electronic states, etc.). This results in the formation of highly reactive substrate-enzyme complexes. The influence of tfie individual enzymes also rests on the fact that they act as electron shuttles between adjacent redox systems. In biological systems one often sees multienzyme systems for chains of consecutive steps. These systems are usually built into the membranes, which secures geometric proximity of any two neighboring active sites and transfer of the product of one step to the enzyme catalyzing the next step. 

A remarkable feature of the bioenergetic oxidation reactions of nutrients in cells is the fact that they are always coupled to another reaction, that of synthesis of the energy-rich chemical substance adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and phosphate (oxidative phosphorylation Engelgardt and Ljubimova, 1939)  

The energy released by electron transfer can be used in the transport of protons through the membrane. One of the proton conduction mechanisms in proteins is through a chain of hydrogen bonds in the protein, i.e. a Grotthus mechanism (Section 2.9), similar to the mechanism of proton movement in ice. Protons are injected and removed by the various oxidation/reduction reactions which occur in the cell there is no excess of protons or electrons in the final balance, and the reaction cycle is self-sustaining. 

The mediator is generated electrochemically and reacts with a biological molecule by homogeneous electron transfer  

The mediator is linked to the electrode surface, forming a surface-modified electrode, and the biological molecule links itself to the mediator layer by heterogeneous electron transfer. 

In biologic systems, the change in free energy (the energy available to do useful work at constant pressure and temperature) is defined by the equation 

For a biochemical reaction, the change in free energy can be used to predict the direction in which the reaction will proceed. 

If AG is negative, the reaction will proceed spontaneously with the release of energy. If AG is positive, the reaction will not proceed spontaneously. If 

AG is 0, the reaction is at equilibrium, and, although substrates react to form products and products react to form substrates, there is no net change in the concentrations. 

The equilibrium constant and the change in free energy 1. At equilibrium, AG = 0 and 

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The current frontiers for the subject of non-equilibrium thennodynamics are rich and active. Two areas dommate interest non-linear effects and molecular bioenergetics. The linearization step used in the near equilibrium regime is inappropriate far from equilibrium. Progress with a microscopic kinetic theory [38] for non-linear fluctuation phenomena has been made. Carefiil experiments [39] confinn this theory. Non-equilibrium long range correlations play an important role in some of the light scattering effects in fluids in far from equilibrium states [38, 39]. 

Harold F M 1986 The Vital Force A Study of Bioenergetics (New York Freeman)... 

Deak J, Richard L, Pereira M, Chui H-L and Miller R J D 1994 Picosecond phase grating spectroscopy applications to bioenergetics and protein dynamics Meth. Enzymol. 232 322-60... 

As important as nucleotides of adenosine are to bioenergetics that is not the only indispensable part they play m biology The remainder of this chapter describes how these and related nucleotides are the key compounds m storing and expressing genetic information... 

Mitochondria Bioenergetics, Biogenesis and Membrane Structure, Packer, L., and Gomez-Pnyon, A., eds. New York Academic Press. 

Cramer, W. A., and Kn2iff, D. B., 1990. Energy Transduction in Biological Membranes—A Textbook of Bioenergetics. New York Springer-Verlag, 545 pp. A textbook on bioenergetics by two prominent workers in photosyn diesis. 

From the perspective of general energetics, the long span of human prehistoric development can be seen as the quest for a more efficient use of somatic energy, the muscular exertions used primarily to secure a basic food supply and then to gradually improve shelters, acquire more material possessions, and evolve a variety of cultural expressions. This quest was always limited by fundamental bioenergetic considerations Fifty to ninety watts is the limit of nsefnl work that healthy adults can snstain for prolonged periods of time (of course, short bursts of effort could reach hundreds of watts). 

Kostyuk, P. G. Electrical events during active transport of ions through biological membranes, in Topic in Bioelectrochemistry and Bioenergetics, Vol. 2, (ed.) Milazzo, G., New York, Wiley 1978... 

Skou, J. C. Mitochondria biogenesis and bioenergetics, Biomembranes molecular arrangements and transport mechanisms, Vol. 28, S. 39, North-Holland, American Elsevier 1972... 

Many processes in living organisms are closely linked to energy transfer and to charge transfer complexes. Therefore, studies of the properties of PCSs are important in solving certain problems of bioenergetics, enzymatic catalysis, photoinduced carcinogenesis, etc. 

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