STRUCTURE-FUNCTION STUDIES OF ALCOHOL DEHYDROGENASES

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Alcohol dehydrogenases from horse liver and yeast have been studied extensively. The three-dimensional structures of the horse liver enzyme and several complexes with substrates and ligands are known, and the genes for four yeast alcohol dehydrogenases have been cloned in plasmids. Thus, answers can now be obtained to several outstanding questions about the catalytic mechanism of the enzyme, the correlation of kinetic characteristics with the structure and function, and the involvement of the tertiary and quaternary structures in activity. "Site-specific mutagenesis" will be used to prepare variants of alcohol dehydrogenases for the following studies. The importance of the proton relay system, which includes His-51 and Ser-48 in the liver enzyme, will be investigated by changing these residues to ones that cannot participate in the proton relay. Amino acid residues that contribute to the environment of the catalytic zinc ion will be substituted, as will amino acids involved in coenzyme binding. The size of the substrate binding pocket will be increased or decreased and the effects on the substrate and rate enhancement specificity will be determined. The kinetics of the enzymes under physiological conditions in vitro will be related to the flux in vivo and to the growth rates of yeast. An attempt will be made to change the specificity of the enzyme for coenzyme and substrate by, for instance, making substitutions that will allow the enzyme to bind NADP as a coenzyme and L-lactate as a substrate. The role of the structural zinc in activity will be examined by removing residues that bind the zinc. Residues in the postulated contact regions between two dimers of the tetrameric yeast enzyme will be altered in an attempt to prepare a dimeric yeast enzyme like the liver form. Extraneous loops or regions of the molecule will be removed in an attempt to make a minimal catalytic unit. A yeast alcohol dehydrogenase will be crystallized for determination of the structure by x-ray crystallography. The cDNA for horse liver alcohol dehydrogenase will be cloned for the same kinds of studies.