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Unformatted text preview: Lecture notes 2/5/10 Chapter 5. Nutrition, Laboratory Culture and Metabolism of Microorganisms Cells obey the laws of chemistry and cannot change the direction of a reaction that is dictated by the laws of thermodynamics. They can, however, exert a great deal of control over reaction rates by synthesizing enzymes that are capable of lowering the activation energy required for certain specific reactions. They can also cause unfavorable (endergonic) reactions to proceed by coupling them to favorable (exergonic) reactions. Both of these activities require energy. Lowering the activation energy allows a much larger proportion of molecules to move forward in the reaction Enzymes generally increase reaction rates from 10 8 10 20 times the rate that would occur in the absence of enzyme catalysis Enzyme catalysis: macromolecules (usually proteins) that increase the rate of reaction High specificity: enzyme usually catalyzes a single type of reaction. Reaction specificity and speed mediated by precisely folded enzyme structure. Substrate binding Position substrate relative to catalytically-active amino acids Proper folding of proteins is required to create an active site where substrates can be positioned for catalysis Precise folding of enzyme can make bonds of substrate much more reactive by simulataneously performing multiple modes of catalysis. Oxidation: removal of an electron (or electrons) from a substance Reduction: addition of an electron (or electrons) to a substance Oxidation-reduction Reduction addition of electron(s) Oxidation removal of electron(s) Redox reactions frequently involve transfer of both electron and proton Changes in electron density occur during oxidation The carbon atoms in methane, methanol, formaldehyde, formic acid and carbon dioxide all have the same valence number but each molecule has different electron densities surrounding the carbon atom. Electron donors and acceptors Transfer of electron from donor to acceptor coupling (and balancing) of half-reaction Reduction Potentials Substances vary in their tendencies to become oxidized Reduction potentials calibrated relative to H 2 standard The electron tower represents range in reduction potentials possible for redox couples in nature Redox couples arranged from the strongest reductants (negative reduction potential top) to strongest oxidants (positive reduction potentials bottom). NAD: a redox electron carrier Intermediate electron carrier that allows chemically dissimilar electron donors and acceptors to interact NAD is a freely diffusible intermediate as opposed to electron carriers that are firmly attached to enzymes as prosthetic groups NAD + /NADH (catabolic reactions) NADP + /NADPH (anabolic reaction) Both have same, fairly high, reduction potential (-0.32V)Both have same, fairly high, reduction potential (-0....
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