Functional analysis of the ALD gene family of Saccharomyces cerevisiae during anaerobic growth on glucose: the NADP+-dependent Ald6p and Ald5p isoforms play a major role in acetate formation.

In Saccharomyces cerevisiae, acetate is formed by acetaldehyde dehydrogenase (ACDH), a key enzyme of the pyruvate dehydrogenase (PDH) bypass, which fulfils the essential task of generating acetyl-CoA in the cytosol. The role of the five members of the ACDH family (ALD genes) was investigated during anaerobic growth on glucose. Single and multiple aldΔ mutants were generated in the wine-yeast-derived V5 and laboratory CEN.PK strains and analysed under standard (YPD 5 % glucose) and wine (MS 20 % glucose) fermentation conditions. The deletion of ALD6 and ALD5 decreased acetate formation in both strains, demonstrating for the first time that the mitochondrial Ald5p isoform is involved in the biosynthesis of acetate during anaerobic growth on glucose. Acetate production of the ald4Δ mutant was slightly decreased in the CEN.PK strain during growth on YPD only. In contrast, the deletion of ALD2 or ALD3 had no effect on acetate production. The absence of Ald6p was compensated by the mitochondrial isoforms and this involves the transcriptional activation of ALD4. Consistent with this, growth retardation was observed in ald6Δald4Δ, and this effect was amplified by the additional deletion of ALD5. A aldΔ null mutant, devoid of ACDH activity, was viable and produced similar levels of acetate to the ald6Δald4Δald5Δ strain, excluding a role of Ald2p and Ald3p. Thus, acetate is mainly produced by the cytosolic PDH bypass via Ald6p and by a mitochondrial route involving Ald5p. An unknown alternative pathway can compensate for the loss of Ald6p, Ald4p and Ald5p.