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Oxidative Phosphorylation
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CHAPTER 1: LIPIDS
CHAPTER 2: PROTEINS
CHAPTER 3: CARBOHYDRATES
CHAPTER 4 - NUCLEIC ACIDS
CHAPTER 5: BINDING
CHAPTER 6 - TRANSPORT AND KINETICS
CHAPTER 7 - CATALYSIS
CHAPTER 8: OXIDATIVE-PHOSPHORYLATION
CHAPTER 9 - SIGNAL TRANSDUCTION
Chapter 10: METABOLIC PATWAYS
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Overview of the Pathway

Overview of Oxidative Phosphorylation

Enzyme Information
   kDa    Polypeptides
Complex I NADH dehydrogenase (or)
NADH-coenzyme Q reductase
800 25
Complex II Succinate dehydrogenase (or)
Succinate-coenzyme Q reductase
140 4
Complex III Cytochrome C - coenzyme Q oxidoreductase 250 9-10
Complex IV Cytochrome oxidase 170 13
Complex V ATP synthase 380 12-14


Oxidation-Reduction Reactions

The following blue boxes represent the oxidation-reduction reactions that are occuring in each respective complex. Everything within the blue box are those compounds or sites which are tightly bound, constitutive parts of the enzyme. Compounds outside the blue box are mobile electron (ie. hydride ion) carriers.

The following abbreviations are also used below; FMN - Flavin mononucleotide, Fe2+S - reduced iron-sulfur center, Fe3+S - oxidized iron-sulfur center, cyt - cytochrome, CoQ - Coenzyme Q.

Complex I
NADH + H+ FMN Fe2+S CoQ
NAD+ FMNH2 Fe3+S CoQH2


Complex II
Succinate FAD Fe2+S CoQ
Fumarate FADH2 Fe3+S CoQH2


Complex III
CoQH2 cyt b ox Fe2+S cyt c1 ox cyt c red
CoQ cyt b red Fe3+S cyt c1 red cyt c ox


Complex IV
cyt c red cyt a ox cyt a3 red O2
cyt c ox cyt a red cyt a3 ox 2 H2O



Chemistry of Complex V


Complex V Complexes I, II, and IV all "pump" protons (ie. H+) into the mitochondrial space between the inner and outer mitochondrial membrane, establishing a proton gradient across the inner mitochondrial membrane. As the protons pass through Complex V, the osmotic energy of the gradient is converted into chemical energy, in the form of ATP. The use of this transmembrane proton gradient, created by the exergonic reduction reactions occuring between Complex I and Complex IV, to drive the endergonic reaction of ATP synthesis is known as chemiosmotic coupling.

Chemiosmotic coupling is acheived through the unique structure of Complex V. As seen in the schematic image, Complex V is composed of an F1 and an F0 particle. The F1 particle consists of a knob-like structure, which is attached to stalk proteins, linked to the F0 base. The F1 has multiple subunits, three alpha, three beta, one gamma, one delta, and one epsilon. The site of ATP synthesis is the beta subunit.

Recent research conducted by Paul Boyer and John Walker has elucidated the conformational mechanism that occurs during ATP synthesis, awarding them the 1997 Noble Prize in Chemistry. The press release from the Royal Swedish Academy of Sciences explains how the enzyme functions as a type of micro-rotary engine, in which periodic conformational changes occur in the beta subunits of the F1 knob.

Unfortunately, although the Nobel Prize committee awarded Boyer and Walker for "their elucidation of the enzymatic mechanism underlying the synthesis of ATP," neither researcher gave any chemical mechanism by which ATP is synthesized in the enzyme.

Biochemistry - Where molecules become life...