Conversion of Pyruvate to Lactic Acid

Oxygen is necessary for oxidative phosphorylation. When oxygen is not present oxidative phosphorylation cannot proceed and NADH does not get oxidized. If NAD+ is not available the ATP producing steps of glycolysis are blocked and no ATP can be produced.

This situation can be remedied for a brief period because lactate dehydrogenase oxidizes NADH by converting pyruvate to lactate in the following reaction: pyruvate + NADH + H+ ® lactate + NAD+

This anaerobic reaction (without O2) cannot continue for long because: 1. it produces only a small fraction (5%) of the ATP that can be produced by aerobic metabolism; 2. lactate produced by the reaction builds up and starts to increase the acidity of the body fluids.

The situation is relieved when oxygen again becomes available and the pyruvate again enters the Krebs citric acid cycle and the reaction that created the lactate reverses: pyruvate + NADH + H+ ¬ lactate + NAD+