Oxidative Phosphorylation

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University of Massachusetts Amherst
Biochemistry & Molecular Bio.
David Gross

Chapter 15 Notes: Oxidative Phosphorylation • oxidative phosphorylation (OP) is the harvesting of nrg in cofactors from CAC & glycolysis to synthesizeATP • free nrg is converted to transmembrane gradient of protons that drivesATP synthesis 15-1 Thermodynamics of Oxidation-Reduction Rxns • oxidizing agent is reduced as it gains e-, reducing agent is oxidized as it loses e- o AoxidizedreducedA reduced oxidized o in step 6 of CAC FADH2 is oxidized & Q reduced, products are FAD & QH2 • reduction potential indicates a substance’s tendency to accept e- o the affinity of a substance for e- is the standard reduction potential (SRP) o'  symbol is ∈ & has units of volts (V)  a greater SRP means greater tendency of oxidized form of substance to accept e- & become reduced o actual reduction potential depends on concentrations of species o' RT A reduced  ∈=∈ − nF ln(A ) oxidized  R = gas constant  T = temp in K  n = # of e- transferred  F = Faraday constant • free nrg change can b calculated from change in reduction potential o e- flow spontaneously from substance w/ lower reduction potential to substance w/ higher reduction potential o when you reverse the half rxn of a compound you also flip the sign of its SRP o to calculate ∈o' of a system o' o' o'  ∈ =∈ e−acceptor e−donor o'  the bigger the difference in ∈ values, greater tendency for e- to flow from one substance to other and greater change in free nrg o' o' o ∆G =−nF∈ ∨∆G=−nF ∆∈ o each stage of e- transfer during OP, from NADH to O2 (final e- acceptor) occurs w/ negative free nrg change 15-2 Mitochondrial e- Transport • respiration is the reoxidation of NADH & Q by molecular oxygen • e- r shuttle from NADH to O2 in multistep process that offers several opportunities to conserve the free nrg or oxidation • mitochondrial membranes define 2 compartments o outer membrane porous b/c of porin-like proteins o inner membrane is convoluted & encloses mitochondrial matrix o much more selected, matrix has different composition than intermembrane space o the creases in the inner membrane are called cristae o mitochondrial genome contains 13 proteins, all components of respiratory chain complexes o most NADH & QH2 produced by CAC & fatty acid oxidation in the matrix o NADH produced elsewhere can’t cross matrix b/c lack of machinery  reducing equivalents imported by chemical rxns of systems like malate- aspartate shuttle system o ATP exported &ADP imported to matrix by adenine nucleotide translocase o inorganic P imported from cytosol in symport w/ H+ • Complex 1 transfers e- from NADH to ubiquinone o complex 1 called NADH:ubiquinone oxidoreductase or NADH dehydrogenase o catalyzes transfer of e- pair from NADH to ubiquinone o contains prosthetic croups w/ reduction potential btwn that of NADH & Q o 2 e- donated from NADH to flavin mononucleotide (FMN) o then transferred 1 @ a time to iron-sulfur (Fe-S) cluster o Fe-S cluster transfers e- 1 @ a time to Q o as e- transfers from NADH to Q, Complex I transfers 4 H+ from matrix to intermembrane space  move through proton wire, series of H-bonded protein groups + water molecules that form a chain through which a proton can be rapidly relayed  protons released into intermembrane space not the same taken from the matrix • Other oxidation rxns contribute to the Q pool o reduced Q from Comp. I joins pool of Q’s soluble in inner mitochondrial mem. o succinate dehydrogenase (complex II) is one catalyst  it’s embedded in inner membrane  contains several redox centers including FAD group  doesn’t undertake proton translocation, doesn’t contribute to free nrg of ATP synthesis o major source of Q is fatty acid oxidation o glyceraldehyde-3-phosphate can bypass Comp. I and shuttle e- from NADH to Q • Complex III transfers e- from Q to cytochrome C o called ubiquinol:cytochrome c oxidoreductase o cytochromes are proteins with heme prosthetic groups o can undergo reversible 1 e- reduction o net rxn of Comp. III, 2 e- transferred requires 2 cytochromes (c & b) o 2 e- split up  travel through 2Fe-2S clusters & heme groups of the cytochromes o the net result of the Q cycle is that 2 e- from QH2 reduce 2 molecules of
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