Bke2 Biochemistry Lectures

Lecture 16: The citric acid cycle and oxidative phosphorylation
Reading material: Horton, Chapters 13 & 15

Abstract:
The citric acid cycle
Pyruvate, the endproduct of glycolysis, undergoes dehydrogenation and decarboxylation by the pyruvate dehydrogenase complex, which contains three sequentially acting enzymes and requires five coenzymes, to yield acetyl-CoA and CO₂. Acetyl-CoA (a two carbon unit) enters the citric acid cycle, which occurs in the mitochondria of eucaryotes. The enzyme citrate synthase catalyses the condensation of acetyl CoA with oxaloacetate (a four carbon unit) to form citrate. Isocitrate is formed which is further oxidised to a-ketoglutarate by isocitrate dehydrogenase in a reaction that also yields CO₂. a-ketoglutarate then undergoes dehydration and decarboxylation to succinyl-CoA and CO₂. Succinyl-CoA reacts with GDP and P_i to form free succinate and GTP. Succinate is then oxidised to fumarate by succinate dehydrogenase which is further oxidsed to malate by NAD-linked malate dehydrogenase to regenerate a molecule of oxaloacetate.

The overall rate of the cycle is controlled by the rate of conversion of pyruvate to acetyl-CoA and by the flux through three enzymes of the cycle: citrate synthase, isocitrate dehydrogenase and a-ketoglutarate dehydrogenase. The end products ATP and NADH are inhibitory.

Citric acid cycle intermediates are also used as precursors in biosynthesis of amino acids and other biomolecules.

Oxidative phosphorylation
In eucaryotes, oxidative phosphorylation occurs in mitochondria. Oxidative phosphorylation involves the reduction of O₂ to H₂O with electrons donated by NADH and FADH₂.

The conservation of free energy involves the passage of electrones through a chain of membrane-bound oxidation-reduction carriers and the concomitant pumping of protons across the membrane, producing an electrochemical gradient, the proton-motive force. This force drives the synthesis of ATP by membrane-bound enzyme complexes through which protons flow back across the membrane, down their electrochemical gradient.

Shuttle systems, malate-aspartate and the glycerol-3-phosphate shuttle, convey reducing equivalents from cytosolic NADH into the mitochondria. Reducing equivalents from NAD-linked dehydrogenations are transferred to mitochondrial NADH dehydrogenases. They are then passed via a series of cytochromes and further to O₂, reducing it to H₂O. There are alternative paths of entry of electrons into the respiratory chain. Succinate, for example, is oxidised by succinate dehydrogenase, which contains a flavoprotein with FAD.

Oxidative phosphorylation produces most of the ATP required by aerobic cells; 2.5 ATP per oxidised NADH and 1.5 ATP/succinate. Oxidative phosphorylation is regulated by the cellular energy demands. In brown fat tissue, which is specialised for production of metabolic heat, electrontransfer is uncoupled from ATP synthesis.

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