The heavy-light chain loop of human cathepsin-L modulates its activity and stability

Differences evident in the sequence alignment of human cathepsin-L with shrimp cathepsin-L and silicatein-alpha suggest the indirect involvement of the heavy to light chain loop (E 286 to E 289) in the function of these enzymes. Deletion of the loop and adjacent residues S 290 to N 293, decreased specific protease activity by 81% and 63%, respectively; complete substitution for the corresponding silicatein-alpha loop decreased activity by 35%. In all cases the Km was largely unchanged. The conformational stability of human procathepsin-L was not altered by deletion of E 286 to E 289 but increased on deletion of S 290 to N 293. Therefore, shortening the loop does not change substrate affinity but does influence activity, in part via conformational change.