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

1. Garrett & Grisham 2010, p.620.
2. Lippincot biochemistry p.77
3. 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.