P/O ratio

The Phosphate/Oxygen Ratio, or P/O Ratio, refers to the amount of ATP produced from the movement of two electrons through a defined electron transport chain, donated by reduction of an oxygen atom.^[1]

The P/O ratio is dependent on the quantity of hydrogen atoms transported outward across an electrochemical gradient, and the number of protons which return inward through the membrane via an enzyme such as ATP synthase. The ATP generated will be dependent on the amount of ATP produced per rotation of the ATP synthase rotor, and the number of hydrogen atoms necessary to complete a rotation.

Inward moving hydrogen must not only power rotation of ATP synthase, but may also be used in the transport of products and precursors. Given the net charge differences between ATP and ADP, the enzyme ATP–ADP translocase uses the equivalent of one hydrogen ion when moving ATP (outward) and ADP (inward) across the inner mitochondrial membrane.

Within aerobic respiration, the P/O ratio continues to be debated; however, current figures place it at 2.5 ATP per .5 O₂ reduced to water, though some claim the ratio is 3 ^[2] This figure arises from accepting that 10 H⁺ are transported out of the matrix per 2 e⁻, and 4 H⁺ are required to move inward to synthesize a molecule of ATP.^[3]

Notes

↑ Garrett & Grisham 2010, p.620.
↑ Lippincot biochemistry p.77
↑ Garrett & Grisham 2010, p.620.

References

Garrett RH & Grisham CM (2010). "Biochemistry 4th Ed.". Brooks/Cole.
Nelson DL & Cox MM (2008). "Principles of Biochemistry 5th Ed." WH Freeman and Company.

Metabolism, catabolism, anabolism

General

Energy
metabolism

Aerobic respiration	Glycolysis → Pyruvate decarboxylation → Citric acid cycle → Oxidative phosphorylation (electron transport chain + ATP synthase)

Anaerobic respiration	Electron acceptors are other than oxygen

Fermentation	Glycolysis → Substrate-level phosphorylation ABE Ethanol Lactic acid

Specific
paths

Protein metabolism

Carbohydrate metabolism
(carbohydrate catabolism
and anabolism)

Human

Glycolysis ⇄ Gluconeogenesis

Glycogenolysis ⇄ Glycogenesis

Pentose phosphate pathway Fructolysis Galactolysis

Glycosylation N-linked O-linked

Nonhuman

Photosynthesis Anoxygenic photosynthesis Chemosynthesis Carbon fixation

Xylose metabolism Radiotrophism

Lipid metabolism
(lipolysis, lipogenesis)

Fatty acid metabolism	Fatty acid degradation (Beta oxidation) Fatty acid synthesis

Other	Steroid metabolism Sphingolipid metabolism Eicosanoid metabolism Ketosis Reverse cholesterol transport

Amino acid

Nucleotide
metabolism

Other

Metabolism: Citric acid cycle enzymes

Cycle

Anaplerotic

to acetyl-CoA	Pyruvate dehydrogenase complex (E1, E2, E3) (regulated by Pyruvate dehydrogenase kinase and Pyruvate dehydrogenase phosphatase)

to α-ketoglutaric acid	Glutamate dehydrogenase

to succinyl-CoA	Methylmalonyl-CoA mutase

to oxaloacetate	Pyruvate carboxylase Aspartate transaminase

Mitochondrial
electron transport chain/
oxidative phosphorylation

Primary	Complex I/NADH dehydrogenase Complex II/Succinate dehydrogenase Coenzyme Q Complex III/Coenzyme Q - cytochrome c reductase Cytochrome c Complex IV/Cytochrome c oxidase Coenzyme Q10 synthesis: COQ2 COQ3 COQ4 COQ5 COQ6 COQ7 COQ9 COQ10A COQ10B PDSS1 PDSS2

Other	Alternative oxidase Electron-transferring-flavoprotein dehydrogenase

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