Electron Transfer Chain Oxidative Phosphorylation

Oxidative phosphorylation occurs in the crista of mitochondria, formed by invaginations of the inner mitochondrial membrane (Figure 5). The hydrogen accepted by NAD+ and FAD during glycolysis and the TCA cycle is oxidized to water by molecular oxygen with accompanying phosphorylation of ADP ! ATP. This is achieved by phosphorylation of ADP coupled with a series of redox reactions whereby the hydrogen ions (H+) and their electrons (e-) are passed along a chain of intermediate carriers in the crista membrane of the mitochondria

A. Representation of a cross section through a mitochondrion

Intermembrane space

Site of transamination of amino acids and nucleotide metabolism

Matrix

Site of the krebs cycle and oxidation of fatty acids

Outer membrane

Relatively permeable, site of enzymes involved in the desaturation and elongation of fatty acids and synthesis of triacylglycerides

Matrix

Intermembrane space

Site of transamination of amino acids and nucleotide metabolism

Site of the krebs cycle and oxidation of fatty acids

Crista

Inner membrane

Regulates transport of substrates into the matrix

Crista

The cristal membrane is the site of the enzymes and coenzymes of the electron transfer chain -See enlarged diagram

Inner membrane

Regulates transport of substrates into the matrix

Crista space

The cristal membrane is the site of the enzymes and coenzymes of the electron transfer chain -See enlarged diagram

Crista space

ATP synthase

Transmembrane stalk

C. Enlarged representation of a primary particle

Primary particles

Site of ATP synthase; a molecular motor'

ATP synthase

Transmembrane stalk

C. Enlarged representation of a primary particle

Primary particles

Site of ATP synthase; a molecular motor'

B. Enlarged representation of a crista

Figure 5 Structure and related functions of a mitochondrion.

(Figure 6), with each intermediate being reduced by the proceeding one and in turn reducing the next one and hence itself being oxidized. The chain consists of a flavoprotein and ubiquinone (coenzyme Q), both hydrogen carriers, followed by a series of cyto-chromes that are carriers of electrons only. Finally, at the end of the chain is cytochrome oxidase, which catalyzes the formation of water from hydrogen ions, electrons, and molecular oxygen. Unlike the other cytochromes, cytochrome oxidase contains copper (Cu2+) in a prosthetic group instead of Fe3+ (in the form of a heme molecule), and this final stage can be inhibited by the irreversible binding of cyanide to Cu2+ preventing it from accepting electrons and therefore terminating the entire hydrogen electron transfer chain and hence all aerobic respiration. This is the basis of the toxicity of cyanide and also several other substances.

Matrix

ATP synthase

NADH+H+

NAD+

Flavin

Flavin-H

ubiquinol ubiquinone ubiquinol ubiquinone

2Fe3+ 2Fe3+

Cyt complex III Y Cyt oxidase 2Fe2+ —2Fe2+

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