Chemistry and subunit structure of yeast hexokinase isoenzymes

Evidence from ultracentrifugation, sodium dodecyl sulfate electrophoresis, peptide mapping, and carboxypeptidase A digestion allows the conclusion that the two native hexokinases, P-I and P-II, consist of polypeptide chains having molecular weights slightly higher than 50,000. It was demonstrated that some preparations are contaminated with a protease, and that this impurity caused erroneous results in sodium dodecyl sulfate electrophoresis and carboxypeptidase A digestion.Amino acid analyses indicated that both P-I and P-II contain four cysteine, four tryptophan, and eleven methionine residues per mole. In contrast, P-I contains eight, and P-II five, histidine residues per mole. Many of the differences in amino acid composition are small, but reproducible.Peptide mapping indicated that many segments of P-I and P-II have identical sequences. There were about 27 common tryptic peptides, and about 16–19 unique to each form. In addition, both isozymes were found to have the same amino terminus, valine, and the same carboxy terminus, alanine; some evidence for a difference in the penultimate residue at the carboxy terminus was indicated.