Question

Intact mitochondrial were isolated, carefully washed with pH 7.0 buffer, and used to measure the P/O ratio for α-ketoglutarate. Included in your assay for the measurements were ADP, phosphate (Pi), NAD+, and malonate in an appropriate buffer. Assume that all the added compounds can get into the mitochondrial matrix and that all enzymes of the citric acid cycle are active.

Note that malonate is a competitive inhibitor or succinate dehydrogenase.

Under these conditions, what would be the P/O ratio for α-ketoglutarate?

P/O ratio = 0.5

P/O ratio = 1.5

P/O ratio = 2.0

P/O ratio = 3.5

P/O ratio = 5.0

Answer 1

NADH enters the oxidative phosphorylation at Complex I, three steps of proton translocation result from electron transport, leading to three equivalents of ATP made by ATP synthase. Substrates oxidized by NAD are said to have a P/O ratio (phosphates fixed per oxygen atom reduced) of three. Substrates oxidized by FAD, primarily succinate, have a P/O ratio of two. This consideration allows a calculation of the energy fixed from the complete oxidation of a mole of glucose:

Two ATPs result from glycolysis, with 2 NADH produced by glyceraldehyde −3 − phosphate dehydrogenase.

Two turns of the TCA cycle, with NADH produced at the pyruvate dehydrogenase, isocitrate dehydrogenase, alpha‐ketoglutarate, and malate dehydrogenase steps and FADH 2 produced at the succinate dehydrogenase step and GTP (equivalent to ATP) produced at the succinyl‐coenzyme A synthetase step.

P/o ratio will be 5