Human cathepsin H: deletion of the mini-chain switches substrate specificity from aminopeptidase to endopeptidase

The mini-chain of human cathepsin H has been identified as the major structural element determining the protease's substrate specificity. A genetically engineered mutant of human cathepsin H lacking the mini-chain, desGlu(-18)-Thr(-11)]-cathepsin H, exhibits endopeptidase activity towards the synthetic substrate Z-Phe-Arg-NH-Mec (kcat = 0.4 s(-1), Km = 92 microM, kcat/Km = 4348 M(-1) s(-1)) which is not cleaved by r-wt cathepsin H. However, the mutant enzyme shows only minimal aminopeptidase activity for H-Arg-NH-Mec (kcat = 0.8 s(-1), Km = 3.6 mM, kcat/Km = 222 M(-1) s(-1)) which is one of the best known substrates for native human cathepsin H (kcat = 2.5 s(-1), Km = 150 microM, kcat/Km = 16666 M(-1) s(-1)). Inhibition studies with chicken egg white cystatin and E-64 suggest that the mini-chain normally restricts access of inhibitors to the active site. The kinetic data on substrates hydrolysis and enzyme inhibition point out the role of the mini-chain as a structural framework for transition state stabilization of free alpha-amino groups of substrates and as a structural barrier for endopeptidase-like substrate cleavage.