CRYSTAL-STRUCTURE OF THE WILD-TYPE HUMAN PROCATHEPSIN-B AT 2.5 ANGSTROM RESOLUTION REVEALS THE NATIVE ACTIVE-SITE OF A PAPAIN-LIKE CYSTEINEPROTEASE ZYMOGEN

The structure of the wild-type human procathepsin B has been refined t o a crystallographic X-value of 0.18 and X-free of 0.23 exploiting the data obtained from new crystals that diffract beyond 2.5 Angstrom res olution. The structure confirms two previously presented, lower-resolu tion structures. The structure of the propeptide chain folds on the su rface of the enzyme domains and blocks access of substrate to the alre ady formed active site. Abundant solvent molecules fill the cavities b etween the propeptide and the enzyme part of the molecule. The propept ide structure is compared with a substrate model in the S2, S1, S1' an d S2' binding sites. In this crystal form the cathepsin B occluding lo op residues adopt yet another conformation. The structures show that t he occluding loop region between the residues Cys108 and Cys119 behave s quite independently from the rest of she structure and easily adapts to changes in environment. The variety of the observed conformations of the occluding loop is in agreement with Other data showing that the loop is responsible for limiting-cathepsin B activity to that of a ca rboxydipeptidase. The region before Cys108 is essentially the same as in the mature structure, whereas the? region from Cys119 to Thr125 is raised compared to the mature form by the propeptide squeezed between it and the enzyme domains, surface. The structure strongly suggests th at processing of procathepsin B during its autoactivation is not unimo lecular. (C) 1997 Academic Press Limited.