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ESS102H1 (104)
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6 Biological Processes.doc

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Department
Earth Sciences
Course
ESS102H1
Professor
John Ferris
Semester
Winter

Description
6 Biological Mediation of Oxidation-Reduction Reactions The capacity of living organisms to mediate oxidation-reduction reactions is surpassed only by the tremendous diversity of life on Earth, particularly among microorganisms that inhabit virtually every conceivable environment where liquid water is found. Redox processes are essential in bioenergetics and intermediary metabolism, and intimately couple all biological entities physicochemically to their surroundings. This coupling further sustains a wide spectrum of vital and dynamic relationships that underpin intense biogeochemical cycling of redox sensitive elements such as carbon, oxygen, nitrogen, iron, and sulfur. 6.1 Redox Reactions and Energy Flow In oxidation-reduction reactions, electrons are donated from the oxidation of reduced ions or molecules characterized by a low half-cell Eh, to oxidants with a higher half-cell Eh. Such reactions are characterized by equilibrium constants and reaction quotients K eqQ (i.e., bias towards a greater proportion of products compared to reactants at equilibrium) with a corresponding value for ΔG < 0 as inferred from the general relationship ΔG = −nFΔEh With ΔEh = Eh reduction(electronaccepoxidation(electrondonor) Spontaneous chemical reactions (i.e., ΔG < 0; K > eq are, in thermodynamic terms, exergonic. This is especially true of oxidation reactions, and means that work is done as energy is released by the reaction and moves into the surrounding region. As a consequence, the reaction is accompanied by an increase in entropy that is manifest in physical terms by the breaking of molecular bonds or relaxation of an unstable ionic configuration. On the other hand, endergonic reactions are not spontaneous (i.e., ΔG > 0) and must absorb energy to proceed. This is typical of reduction reactions, which in order to proceed, must be coupled with a strongly exergonic reaction through a shared intermediate, as illustrated in Table 6.1. The work accomplished by the absorption of energy corresponds to an unfavorable decrease in entropy that extends from the formation of higher energy molecular bonds or excitation of ion electronic states. In terms of low temperature aqueous geochemistry, most inorganic redox processes are dominated by spontaneous oxidation reactions as the Earth system struggles towards equilibrium through the release of energy. This inevitable march towards equilibrium is accompanied by a progressive increase in entropy. The parade in the opposite direction towards dysequilibrium is sustained entirely by biological activity. A 1 contrasting situation is provided by the surface of Mars, which at one time may have sustained a surface biosphere, but now exists as a highly oxidized desert. Table 6.1: Coupling of an endergonic reaction with a strongly exergonic reaction through a shared intermediate. o Endergonic reaction X + Y = XY K 1 1.0 ΔG > 0 o Exergonic reaction AB = A + B K 2> 1.0 ΔG << 0 Shared intermediate reactions X + AB = XB + A XB + Y = XY + B Overall exergonic reaction o X + Y + AB = A + B + XY K Coupled K 1 2 0 ΔG < 0 A critical point for understanding the impact of biological activity on oxidation- reduction reactions and energy flow is that living organisms are exceptionally proficient at the exercise of coupling exergonic reactions to endergonic reactions. The principle result of this proficiency is that living organisms are capable of synthesizing a bewildering inventory of chemical substances to support their growth and diversification. One of the most important reactions used in biological systems to drive endergonic reactions is the hydrolysis of adenosine triphosphate (ATP); cleavage of either a phosphate (P) ir pyrophosphate (PP) froi ATP to yield adenosine diphosphate (ADP) or adenosine monophosphate (AMP) is highly exergonic o ATP = ADP + P i ΔG = -30.5 kJ/mol ATP = AMP + PP ΔG = -45.6 kJ/mol i When coupled to endergonic reactions, the hydrolysis of ATP gives rise to a shared phosphorylated (i.e., activated) intermediate (cf. Table 6.1 where AB = ATP; A = ADP or AMP; B = P oriPP) thit makes the overall reaction spontaneous. 2 The most fundamental thermodynamic premise that can be applied to energy flow in biological systems is that oxidation reactions are intrinsically spontaneous and yield energy. There is nothing too exceptional about this realization other than the coupling of a half-cell reductant reaction (i.e., electron donor) with an especially low Eh to a half- cell oxidation reaction (i.e., electron acceptor) with an especially high Eh may be o exergonic to the point of being explosive and destructive. In such instances, the release of energy occurs so quickly that it escapes into the surroundings of the system and no manifest work is done, such as the formation of molecular bonds or excitation of ion electronic states. Instead, only a rapid increase in entropy is achieved. A compelling example of this is what can happen when molecular oxygen (i.e., a strong oxidant and o electron acceptor, Eh = 1.23 V) mixes with molecular hydrogen (i.e., a strong reductant and electron donor, Eh = 0 V) under less than optimal conditions, as witnessed by the 1 Hindenburg disaster . The problem of a potentially explosive release of energy from the rapid oxidation o of a highly reduced compound with a low Eh is circumvented biologically by using a series of intermediate redox reactions where electrons are transferred to the coenzyme nicotinamide adenine dinucleotide (NAD + H + 2e = NADH). These reactions occur mainly during the step-wise metabolism of organic compounds in glycolysis and in the citric acid cycle. In this way, the energy released at each oxidation step is captured in reduced NADH, which plays a variety of roles in metabolism as an electron carrier. The most important role of NADH in bioenergetics is that it serves as the primary + electron donor for oxidative phosphorylation. This begins when NAD is regenerated from the oxidation of NADH by the first elements of a membrane bound electron 2 transport chain . As electrons are shuttled sequentially between carriers with ever increasing Eh values, the energy released at each oxidation step is used to pump protons outwards across the membrane. This creates a proton gradient and a source of potential energy that is used to synthesize ATP as protons are transported by ATP synthase back across the membrane. Ultimately, the electrons derived from NADH are transferred to a o terminal electron acceptor characterized by a high Eh value (e.g., molecular oxygen). Example 6.1 The ultimate electron acceptor for the oxidation of NADH in the electron transport chain is oxygen. The two half-cell reactions are + + - o NAD + 2H + 2e = NADH; E = 0.090 V + - o ½ O +22H + 2e = H O; E = 1223 V 1On March 6, 1937 the German airship Hindenburg caught fire and was destroyed within one minute as the buoyant hydrogen gas cells of the craft erupted in flames. 2Electron transport chains are associated with the mitochondrial membranes of Eukaryotes and the plasma membrane of Prokaryotes. 3 The overall reaction is NADH + ½ O + H =2NAD + H O + 2 ΔE for the reaction is ΔE = E o(electron acceptor(electron donor)23 – 0.090 = 1.14 V And ΔG is o ΔG = -nFΔE = -[(2)(96480)(1.14)/1000] = -220 kJ/mol In glycolysis, 6-carbon hexose sugars are converted using ATP into two molecules of glyceraldehyde-3-phosphate. Then NAD and P are used to oxiiize and phosphorylate glyceraldehyde-3-phosphate to form 1,3 Bisphosphoglycerate. This high- energy mixed-acid anhydride is used for substrate level phosphorylation, which yields two molecules of ATP and pyruvate. Under aerobic conditions, pyruvate is eventually metabolized to CO and 2 O via 2he citric acid cycle, and NAD is regenerated by electron transport. Under anaerobic fermentation conditions, NADH is oxidized by + pyruvate to regenerate NAD and form a variety of products, including ethanol and CO , 2 or lactate. This is important for the continuation of glycolysis because without + regeneration of NAD , the reduced coenzyme would accumulate and glycolysis would stop. An especially important bioenergetic process is the use of photo-oxidation for ATP formation (i.e., photophosphorylation), and to reduce a phosphorylated NAD + + molecule, NADP , to NADPH as a source of reducing power. These are the light- dependent reactions of photosynthesis in which light energy from the absorption of photons is used to excite electrons in a light harvesting complex (LHC) comprised of pigment molecules such as the chlorophylls of plants and algae, phycobilins of cyanobacteria, or bacteriochlorphylls of purple bacteria and green sulfur bacteria . For most compounds that absorb light, excited electrons simply return to their ground state and the absorbed energy is given-up as heat; however, excited electrons in a LHC are transferred into electron transport chains that facilitate the generation of ATP and NADPH. The electrons lost from the LHC are subsequently replaced by the 3 oxidation of water in plants, algae and cyanobacteria ; this results in the production of molecular oxygen from which the term oxygenic photosynthesis is derived. In purple bacteria, electrons removed from the excited LHC are recycled after being passed through an electron transport chain to make ATP via ATP synthase. In order to make NADPH, purple bacteria use a wide variety of electron donors (e.g., H , H S, S , organic 0 2 2 3The primary electron donor in oxygenic photosynthesis is referred to as P680, which has an absorption maximum at 680 nm in the red range of the spectrum. P680 is the strongest known biological oxidizing agent with an estimated Eh ~ 1.3 V. This is what makes it possible for P680 to oxidize water
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