SUMMARY PARAGRAPH for PDC5
PDC1, PDC5, and PDC6 encode three different isozymes of pyruvate decarboxylase (2, 1, 4), an enzyme which catalyzes the degradation of pyruvate into acetaldehyde and carbon dioxide, as shown here. As the key enzyme in alcoholic fermentation, pyruvate decarboxylase commits the end product of glycolysis, pyruvate, to ethanol production rather than to its other possible major metabolic fates: the TCA cycle/aerobic respiration (pyruvate converted to acetyl-CoA by the pyruvate dehydrogenase complex) or gluconeogenesis (pyruvate converted to oxaloacetate, by pyruvate carboxylase) (7 and references therein). PDC1, PDC5, and PDC6 can also decarboxylate other 2-oxo acids such as indolepyruvate and 2-keto-3-methyl-valerate, and this activity contributes to the catabolism of the amino acids isoleucine, phenylalanine, tryptophan, and valine (8 and references therein).
Pdc1p is the major isozyme and is strongly expressed in actively fermenting yeast cells. The nearly identical Pdc5p also functions during glycolytic fermentation, but is expressed only in the absence of PDC1 (1) or under thiamine limitation (3). Both PDC1 and PDC5 are under PDC autoregulation such that their promoters are activated in the absence of Pdc1p (10 and references therein). Expression of PDC1 and PDC5 also requires the transcription factor Pdc2p (11 and references therein). The third isozyme, Pdc6p, is not expressed during glucose fermentation and seems to be functional during growth on nonfermentable carbon sources (4). Transcription of PDC6 is dramatically induced under conditions of sulfur limitation, suggesting that it has a role during sulfur-limited growth (12). Consistent with this, the PDC6 gene encodes fewer sulfur-containing amino acids than does either PDC1 or PDC5 (6 codons vs. 17 or 18, respectively) (12).
Pyruvate decarboxylase is conserved among yeast, bacteria and plants and has been well-characterized on both the structural and enzymatic levels. The active enzyme is a homotetramer and requires thiamin diphosphate (ThDP) and Mg++ cofactors (13 and references therein).
Last updated: 2007-06-01