Active site yeast alcohol dehydrogenase

Alcohol dehydrogenase is one of the active enzymes in yeast. The active site in alcohol dehydrogenase contains a zinc ion, Zn, that is coordinated to the sulfur atoms of two cysteine residues of the enzyme. The hydride reducing reagent in alcohol dehydrogenase is nicotinamide adenine dinucleotide, NADH, which transfers a hydride ion to a carbonyl compound to yield an alcohol and NAD" ", in a mechanism that is related to the Cannizzaro reaction (Sec. 16.3). 

Twu, J.S., Wold, F. (1973). Butyl isocyanate and active site specific reagent for yeast alcohol dehydrogenase. Biochemistry 12 381-6. 

The results of cysteine modification confirm the similarities in structure and function of the active sites of mammalian and yeast alcohol dehydrogenases (Section II,D). Minor differences are, however, observed. Thus, the nicotinamide-substituted imidazole dinucleotide (137) selectively alkylates one of the two cysteine ligands to the catalytic zinc atom, Cys-43, in the yeast enzyme. In the horse enzyme, on the other hand, the same reagent alkylates a different ligand to the same zinc atom, Cys-174. 

In conclusion, it seems very probable that the three-dimensional structures of the subunits of the yeast and liver alcohol dehydrogenases, including the presence of two zinc atoms, are similar. There must be differences in the subunit contacts since the yeast enzyme is tetrameric and the liver enzyme dimeric. There are also differences in the size of the active site pocket of the subunit and at the carboxyl terminal region of the chain since the yeast enzyme is about 40 residues shorter. 

The use of X-ray techniques to elucidate the three-dimensional structure of enzymes shows that many of them possess a characteristic concave cleft at the active site. Concavities of this type have been observed, for example, in the case of lysozyme [8, 9] trypsin [10], yeast hexokinase [11], liver alcohol dehydrogenase [12] and citrate synthase [13]. It is thus reasonable to assume that the interaction between an enzyme and its substrate, inhibitor or cofactor usually occurs not in bulk water but rather in a shielded proteic cleft whose specific microenvironment is induced by the amino acid residues forming the cleft. Hydrophobicity, electrostatics, solvation and a relatively low dielectric constant prevailing within the cleft no doubt play a decisive role in determining the nature and rate of the reaction catalyzed by the enzyme. 

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