Crystal structure of cathepsin X: a flip-flop of the ring of His23 allows carboxy-monopeptidase and carboxy-dipeptidase activity of the protease

Background: Cathepsin X is a widespread, abundantly expressed papain-like m ammalian lysosomal cysteine protease. It exhibits carboxy-monopeptidase as well as carboxy-dipeptidase activity and shares a similar activity profile with cathepsin B. The latter has been implicated in normal physiological ev ents as well as in various pathological states such as rheumatoid arthritis , Alzheimer's disease and cancer progression. Thus the question is raised a s to which of the two enzyme activities has actually been monitored. Results: The crystal structure of human cathepsin X has been determined at 2.67 Angstrom resolution. The structure shares the common features of a pap ain-like enzyme fold, but with a unique active site. The most pronounced fe ature of the cathepsin X structure is the mini-loop that includes a short t hree-residue insertion protruding into the active site of the protease. The residue Tyr27 on one side of the loop forms the surface of the S1 substrat e-binding site, and His23 on the other side modulates both carboxy-monopept idase as well as carboxy-dipeptidase activity of the enzyme by binding the C-terminal carboxyl group of a substrate in two different sidechain conform ations. Conclusions: The structure of cathepsin X exhibits a binding surface that w ill assist in the design of specific inhibitors of cathepsin X as well as o f cathepsin B and thereby help to clarify the physiological roles of both p roteases.