Human liver alcohol dehydrogenase. 1. The primary structure of the beta1beta1 isoenzyme

Abstract

Determination of the amino acid sequence of the .beta.1 subunit from the class I (pyrazole-sensitive) human liver alcohol dehydrogenase isoenzyme .beta.1,.beta.1 revealed a 373-residue structure differing at 48 positions (including a gap) from that of the subunit of the well studied horse liver alcohol dehydrogenase EE isoenzyme. The structure deduced was compatible with known differences in composition, UV absorbance, electrophoretic mobility and catalytic properties between the horse and human enzymes. All Zn-liganding residues of the horse E subunit were strictly conserved in the human .beta.1 subunit. This residue remained conserved in all known alcohol dehydrogenase structures. The total cysteine content of the .beta.1 structure was raised from 14 in the subunit of the horse enzyme to 15 by a Tyr .fwdarw. Cys exchange. Most exchanges were on the surface of the molecule and of a well conserved nature. Substitution close to the catalytic center were of interest to explain the altered substrate specificity and different catalytic activity of the .beta.1 homodimer. Functionally, a Ser .fwdarw. Thr exchange at position 48 appeared to be of special importance, since Thr-48 in .beta.1 instead of Ser-48 in the horse enzyme could restrict available space. Four other substitutions also lined the active-site pocket, and appeared to constitute partly compensated exchanges.