[Thermitase--a thermostable serine protease. IV. Kinetic studies on the binding of N-acyl peptide ketones as substrate analog inhibitors].

Peptide methyl ketones were found to be efficient competitive inhibitors of thermitase while the chloromethyl ketone derivatives in general inhibit the enzyme irreversibly. The affinity increases by elongation of the peptide chain up to four residues indicating an interaction of the ligand with an extended binding site in thermitase. The P1 residue is phenylalanine in most of the inhibitors, according to the substrate specificity of thermitase for phenylalanine and other hydrophobic amino acids. Peptide ketones protected with the hydrophobic bencyloxycarbonyl group at the N-terminus are bound to the enzyme more tightly than the corresponding acetyl compounds. As reversible inhibitors peptide methyl ketones form a tetrahedral hemiketale with the oxygen of the active serine. In the case of the chloromethyl ketones this complex is stabilized by covalent binding to histidine-64. The inductive effect of the halogen favours the formation of the hemiketale and is expressed in about twenty times lower inhibition constants of the chloromethyl ketones. Results of the kinetic experiments show a corresponding behaviour in effector binding by thermitase and subtilisin in the S region, and from this point of view also indicate a high similarity of the active centers of both enzymes.