The Two Major Isozymes of Yeast Alcohol Dehydrogenase

Abstract

The structures of the two cytoplasmic isozymes of alcohol dehydrogenase in a haploid strain of the yeast Saccharomyces cerevisiae have been compared and related to a commercial preparation of the protein that was previously analyzed. The constitutive isozyme I (purified from a petite cell incapable of mitochondrial respiration) is found to correspond closely to the commercially available but taxonomically uncharacterized enzyme. After analysis of 86 % of all positions, two amino acid differences were found among the 298 residues compared. The enzymes from these sources are thus about 99 % identical. The inducible isozyme II (purified from aerobically grown grande cells) differs substantially from isozyme I. After analysis of 82% of all positions, there are 15 amino acid exchanges in relation to isozyme I, and 17 in relation to the commercial preparation among the 284 residues compared. Some of the differences established between isozymes I and II explain heterogeneities previously observed in yeast alcohol dehydrogenase. The amino acid sequences can be matched without insertions or deletions, and 15 of the 17 differences are interpretable as single base changes. The close homology of the two isozymes shows that they were produced from a gene duplication which is unexpectedly recent in view of the functional differences of the isozymes. Differences in patterns of subunit cooperativity and in Km values have been found between the isozymes. Reflecting this, four of the differences between forms I and II appear to correspond to positions close to the interface between subunits in the tertiary structure of the related liver alcohol dehydrogenase, while two apparently correspond to residues near the inter‐domain cleft of that subunit. No residues with known catalytic functions are, however, exchanged. Normally, the amino terminus of the yeast enzyme is blocked by an acyl group, but a significant amount of the isozyme II preparation was found to have a free amino terminus. The presence of unblocked molecules is very rare among acetylated proteins, but may lead to further insight into possible roles of α‐amino‐acetylation. Copyright © 1979, Wiley Blackwell. All rights reserved