Friday, July 22, 2011

Fates of pyruvate

The fate of pyruvate depends on cell type and metabolic conditions. There are three main destinations for pyruvate:

(1) aerobic organisms and tissues, under aerobic conditions - pyruvate is oxidized, with loss of the carboxylic group, resulting in the acetyl group from acetyl-CoA, which is then oxidized to CO2 in the Krebs cycle;

(2) Aerobic tissues in conditions of low oxygen (muscle hypoxia, for example), some tissues under aerobic conditions (red cells, for example, because they lack mitochondria), or some anaerobic organisms - pyruvate is reduced to lactate by lactic fermentation . In muscle under conditions of hypoxia, NADH is not reoxidized to NAD+ and NAD+ is required for glycolysis. The reduction of pyruvate to lactate allows the use of as a donor of electrons to regenerate NAD+;

(3) Some tissues of plants, some invertebrates, protists and micro-organisms under anaerobic conditions or hypoxia - pyruvate is converted to ethanol + CO2 (alcoholic fermentation).

While glycolysis can occur in anaerobic conditions, this fact has a price, because it reduces the amount of ATP formed per molecule of glucose (from 30 or 32 it passes for only 2 ATP!), and, therefore, it is needed more glucose oxidation under these conditions.
What happens to pyruvate is directly related to the amount of NAD+ and FAD in the cell. As these amounts are very small, there must be mechanisms to transform the NADH + H+ and FADH2 back into NAD+ and FAD, respectively. This is done by transfer of the electrons from NADH + H+ and FADH2 to other molecules, which can occur by fermentation or respiration. The distinction between these is not (contrary to what is generally thought) that one of the processes uses directly the O2 and the other not! O2 is only required for oxidative phosphorylation, and not for the oxidation of pyruvate. Unlike aerobic metabolism that depends and are limited by the oxygen supply, the anaerobic glycolysis does not depend on the availability of oxygen and can increase speed up to 1000 times the speed at rest, ie, 2 ATP / glucose can represent many ATP/minute.

Main bibliographic sources:
- Quintas A, Freire AP, Halpern MJ, Bioquímica - Organização Molecular da Vida, Lidel
- Nelson DL, Cox MM, Lehninger - Principles of Biochemistry, WH Freeman Publishers

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