STRUCTURE OF DIISOPROPYL FLUOROPHOSPHATE-INHIBITED FACTOR-D

Factor D (D) is a serine protease, crucial for the activation of the a lternative complement pathway. Only a limited number of general serine protease inhibitors are known to inhibit D, most of which covalently bind to the serine hydroxyl of the catalytic triad. The structure of t he first enzyme:inhibitor covalent adduct of D with diisopropyl fluoro phosphate (DIP:D) to a resolution of 2.4 Angstrom is described. The in hibited enzyme is similar in overall structure to the native enzyme an d to trypsin, yet exhibits notable differences in the active site. One region of the active site is conserved between D and trypsin with res pect to amino-acid sequence and to conformation. Another reflects the amino-acid substitutions and conformational flexibility between these enzymes. The active-site histidine residue is observed in the gaucheconformation, not the normal gauche- orientation seen in the classic c atalytic triad arrangement required for enzymatic activity in serine p roteases. Comparisons of the active sites between native D, the DIP:D adduct, and DIP-inhibited trypsin have provided fundamental insights c urrently being employed in the design of novel small-molecule pharmace utical agents capable of modulating the alternative complement pathway .