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The following points highlight the three possibilities of the transformation of pyruvic acid. The possibilities are: 1. Production of Lactic Acid 2. Alcoholic Fermentation 3. Formation of α-Glycerophosphate and Glycerol.
Possibility # 1. Production of Lactic Acid:
In animals, in muscle cells (where anaerobiosis conditions often prevail when oxygen utilization is more rapid than its input in cells) or in lactic bacteria (which live in anaerobiosis), pyruvic acid is reduced to lactic acid while NADH is oxidized to NAD+ (see fig. 4-30). This reaction is catalyzed reversibly by lactate dehydrogenase.
Possibility # 2. Alcoholic Fermentation:
In other microorganisms, especially yeast, pyruvic acid is, in a first step, decarboxylated to CO2 + acetaidehyde, thanks to a pyruvate decarboxylase which has thiamine pyrophosphate (or vitamin B1) as coenzyme and contains zinc (Zn2+). Then in a second step, this acetaidehyde is reduced to ethanol while NADH is oxidized to NAD+, the reaction being catalyzed by an alcohol dehydrogenase (see fig. 4-31).
Possibility # 3. Formation of α-Glycerophosphate and Glycerol:
The fermentation of glucose by yeast is always accompanied by the formation of small quantities of glycerol. As a matter of fact, yeast has a α-glycero- phosphate dehydrogenase which catalyzes the reduction of dihydroxyacetone phosphate to L-α-glycerophosphoric acid, while NADH is oxidized to NAD+ (fig. 4-32).
Then a specific phosphatase can split the ester linkage and give rise to glycerol. This possibility seems to be used by yeast when there is not sufficient acetaldehyde enabling the reoxidation of NADH; in fact, this possibility has been utilized to produce glycerol by fermentation, by blocking acetaidehyde in a combination with disulphite and thus forcing yeast to reduce dihydroxyacetone phosphate for reoxidizing its NADH.
