To determine the active site cysteine residue in aldehyde dehydrogenase, we mutated amino acid residues 49, 162, and 302 of recombinantly expressed rat liver mitochondrial (class 2) aldehyde dehydrogenase. The C49A and C162A mutants were fully active tetrameric enzymes, although the C162A mutant was found to be highly unstable. The C302A mutant was also a tetramer and bound coenzyme, but lacked both dehydrogenase and esterase activities. To test for the role of cysteine 302 as a nucleophile, the residue was mutated to a serine, a poor nucleophile. this C302S mutant was active but was a much poorer catalyst, with a kcat/Km value 7 x 10(5) times lower than that of the recombinant native enzyme. Unlike with native enzyme where deacylation is rate limiting, formation of the serine hemiacetal intermediate appeared to be the rate-limiting step. Cysteine 302 is the only strictly conserved cysteine residue among all the available sequences of the aldehyde dehydrogenase superfamily, supporting the role of this residue as the active site nucleophile of aldehyde dehydrogenase.

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Mutants of Saccharomyces cerevisiae deficient in mitochondrial aldehyde dehydrogenase (ALDH) activity were isolated by chemical mutagenesis with ethyl methanesulfonate. The mutants were selected by their inability to grow on ethanol as the sole carbon source. The ALDH mutants were distinguished from alcohol dehydrogenase mutants by an aldehyde indicator plate test and by immunoscreening. The ALDH gene was isolated from a yeast genomic DNA library on a 5.7-kb insert of a recombinant DNA plasmid by functional complementation of the aldh mutation in S.cerevisiae. An open reading frame which specifies 533 codons was found within the 2.0-kb BamHI-BstEII fragment in the 5.7-kb genomic insert which can encode a protein with a molecular weight of 58,630. The N-terminal portion of the protein contains many positively charged residues which may serve as a signal sequence that targets the protein to the mitochondria. The amino acid sequence of the proposed mature yeast enzyme shows 30% identity to each of the known ALDH sequences from eukaryotes or prokaryotes. The amino acid residues corresponding to mammalian cysteine 302 and glutamates 268 and 487, implicated to be involved at the active site, were conserved. S. cerevisiae ALDH was found to be localized in the mitochondria as a tetrameric enzyme. Thius, that organelle is responsible for acetaldehyde oxidation, as was found in mammalian liver.Saccharomyces cerevisiae, unlike higher eukaryotes, can metabolize as well as produce ethanol. During fermentation, the consumption of sugars results in the accumulation of ethanol. When glucose or any other fermentable sugar is absent from the culture medium, S. cerevisiae can utilize ethanol as the carbon source aerobically. The metabolism of ethanol is well understood in mammals. In the liver, cytosolic alcohol dehydrogenase (ADH) oxidizes ethanol to acetaldehyde and then mitochondrial aldehyde dehydrogenase (ALDH) oxidizes the intermediate to acetate (14). In S. cerevisiae, it appears that a mitochondrial alcohol dehydrogenase (ADH3) is responsible for the initial oxidation (65,67). Little is known about the role of ALDHs in the next step.Various groups have purified and characterized yeast ALDH (12,49,54). It was reported to be a tetrameric enzyme with a subunit molecular mass of 60 kDa and a low Km for acetaldehyde. Unlike the liver enzymes, the yeast enzyme is activated by K+ ions (4, 32). Whereas the mammalian enzymes have been sequenced at the cDNA (16,19,29,34) as well as the protein (24, 33, 60) level, no sequence work has been reported for the yeast enzyme.One of our interests was to study the subcellular localization of acetaldehyde metabolism in S. cerevisiae. In the mammalian liver tissue, this was studied by selectively inhibiting the cytosolic or the mitochondrial isozyme of ALDH (11,52). An alternative approach would be to introduce the enzyme into a cell deficient in ALDH activity. S. cerevisiae would be a suitable model system, as it can metabolize ethanol and could serve as a host for the expression of foreign g...

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