Thermophilic Streptomyces Alkaline Proteinase

The thermostable alkaline proteinase of thermophilic streptomycete, Streptomyces rectus var. proteolyticus, was rapidly and reversibly inactivated by equimolar p-chloromercuribenzoate (p-MB) or HgCl2. The inactivation was prevented by substrates. The enzyme which had been inhibited by diisopropyl phosphofluoridate was still able to bind p-MB with a markedly decreased velocity, while the enzyme whose —SH group had been blocked by p-MB or HgCl2 did not react with diisopropyl phosphofluoridate. A circular dichroism study showed that a slight conformational change was induced by blocking of the —SH group. The binding of p-MB to enzyme resulted in a marked decrease of thermostability, while the enzyme showed a higher resistance to protein-denaturing reagents than mesophilic alkaline proteinases. These results suggest strongly that the —SH group participates in a tight organization of the active site through which a high thermal stability may be expressed.