Crystal Structure of Complement Factor D Complexed with Isatoic Anhydride at 1.5~ Resolution. by Holly Jing(a), Dwight Moore(a), Y.S. Babu(c), John E. Volanakis(b) and Sthanam V. L. Narayana(a). (a)Center for Macromolecular Crystallography, University of Alabama at Birmingham, (b)Division of Clinical Immunology and Rheumatology, Univerisity of Alabama at Birmingham, and (c)BioCryst Pharmaceutical Inc.

Factor D is a good drug design target for modulating human complement activation. Factor D is the rate-limiting enzyme for alternative pathway activation as it is highly specific to its single natural substrate C3bB. A novel mechanism was proposed for the regulation of factor D catalytic activity: substrate binding induces a fully reversible conformational change between active and resting states of factor D[1]. It has been shown that isatoic anhydride inhibits factor D better than inhibiting trypsin, thrombin and C1s. To identify the specific interactions between isatoic anhydride and factor D, inhibitor complex crystals diffracting to 1.5~ resolution were obtained. The crystals belong to P2(1) space group with one molecule per asymmetric unit. Initial phases were obtained from molecular replacement calculations using native factor D structure as the search model. Inhibitor density was clearly visible in both |2Fo-Fc| and |Fo-Fc| electron density maps. The inhibitor fits tightly into a small hydrophobic S1 pocket. The complex structure will serve as the starting template for structure-based drug design.

1. J.E. Volanakis and S.V.L. Narayana. Complement factor D, a novel serine

protease. 1996, Protein Science, 5:553-564.