The function of the two subunits of thermophilic aminopeptidase I

The thermophilic high-molecular-weight aminopeptidase I (EC 3.4.11.1) from Bacillus stearothermophilus is composed of 12 subunits of two different types (alpha,beta) which can combine in various ratios. Only one type of subunit (alpha) is needed for the degradation of neutral peptides, but dipeptides having amino-terminal aspartic or gultamic acid are substantially hydrolyzed only by enzyme containing the other subunit (beta) as well. Asp-Gly inhibits the enzymatic hydrolysis of glutamic acid 1-(4-nitroanilide) very strongly but hardly affects the hydrolysis of leucine p-nitroanilide. These results indicate that both types of subunit have hydrolytic activity but different specificity. The two subunits have identical molecular weights and their amino-terminal regions are homologous, suggesting that the two chains originate from a single ancestral gene by gene duplication and independent mutation.