Determination of the positions of the disulfide bonds in aqualysin I (a thermophilic alkaline serine protease) of Thermus aquaticus YT-1.

Aqualysin I is a heat-stable alkaline serine protease produced by Thermus aquaticus YT-1. Aqualysin I comprises 281 amino acid residues and contains four cysteine residues. The cysteine residues seemed to form disulfide bonds in the molecule. Thus, the positions of the disulfide bonds were investigated. Disulfide bond-containing peptides were identified by peptide mapping with HPLC before and after carboxymethylation of chymotryptic peptides of aqualysin I. The disulfide bond-containing peptides were isolated and then carboxymethylated. Carboxymethylcysteine-containing peptides were purified, and their amino acid compositions and sequences were determined. Based on the data obtained and the primary structure of aqualysin I, it was concluded that two disulfide bonds were formed between Cys67 and Cys99, and between Cys163 and Cys194.