st8308.qxd 03/22/2000 11:36 Page 305 Research Article 305 Crystal structure of cathepsin X: a flip–flop of the ring of His23 allows carboxy-monopeptidase and carboxy-dipeptidase activity of the protease Gregor Guncar1, Ivica Klemencic1, Boris Turk1, Vito Turk1, Adriana Karaoglanovic-Carmona2, Luiz Juliano2 and Dušan Turk1* Background: Cathepsin X is a widespread, abundantly expressed papain-like Addresses: 1Department of Biochemistry and v mammalian lysosomal cysteine protease. It exhibits carboxy-monopeptidase as Molecular Biology, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia and 2Departamento de well as carboxy-dipeptidase activity and shares a similar activity profile with Biofisica, Escola Paulista de Medicina, Rua Tres de cathepsin B. The latter has been implicated in normal physiological events as Maio 100, 04044-020 Sao Paulo, Brazil. well as in various pathological states such as rheumatoid arthritis, Alzheimer’s disease and cancer progression. Thus the question is raised as to which of the *Corresponding author. E-mail:
[email protected] two enzyme activities has actually been monitored. Key words: Alzheimer’s disease, carboxypeptidase, Results: The crystal structure of human cathepsin X has been determined at cathepsin B, cathepsin X, papain-like cysteine 2.67 Å resolution. The structure shares the common features of a papain-like protease enzyme fold, but with a unique active site. The most pronounced feature of the Received: 1 November 1999 cathepsin X structure is the mini-loop that includes a short three-residue Revisions requested: 8 December 1999 insertion protruding into the active site of the protease. The residue Tyr27 on Revisions received: 6 January 2000 one side of the loop forms the surface of the S1 substrate-binding site, and Accepted: 7 January 2000 His23 on the other side modulates both carboxy-monopeptidase as well as Published: 29 February 2000 carboxy-dipeptidase activity of the enzyme by binding the C-terminal carboxyl group of a substrate in two different sidechain conformations.