Rat cathepsin H-catalyzed transacylation: Comparisons of the mechanism and the specificity with papain-superfamily proteases

We found that rat cathepsin H showed strong transacylation activity under physiological conditions. It is a feature of cathepsin H to utilize amino acid amides not only as acylacceptors but also as acyl-donors in the reaction. The pH-dependence of the transacylation activity was distinct from those of other papain-superfamily proteases. The alkaline limb (pK_app=7.5) could be regarded as the pK_a, of the α-amino group of the acyl-donor, which was also involved in the original amino-peptidase activity. The acidic limb (pK_app=5.8) was suggested to be involved in the deacylation step, where amino acid amide attacked the acyl-intermediate as a nucleophile in place of water in the hydrolysis. Although the Nα-deprotonated acyl-acceptor, which is supposed to govern the nucleophilic attack, has a small population in the acidic pH range (above pH 5), the transacylation was detectable even at the acidic pH-range because of the high S1' -site binding ability and suitable nucleophilicity of the acyl-acceptor. In the transacylation between various amino acid amides, the S1 and S1' site appeared to prefer hydrophobic residues without and regardless of a branch at β-carbon, respectively. From these results and the sequence homology in the papain superfamily, we concluded that the reaction was governed by the acyl-donor having a protonated amino group, the acyl-acceptor having a deprotonated amino group and the remarkable hydrophobic character (especially favoring tryptophan amide) of the S1' site, presumably reflecting the good conservation of Trpl77 in papain-superfamily proteases.