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Factor D (D) is a serine protease essential in the activation of the alternative complement pathway. Only a few of the common serine protease inhibitors inhibit D, binding covalently to the serine hydroxyl of the catalytic triad. 3,4-Dichloroisocoumarin (DCI) is a mechanism-based inhibitor which inhibits most serine proteases and many esterases, including D. The structure of the enzyme:inhibitor covalent adduct of D with DCI, DCI:D, to a resolution of 1.8 Å is described, which represents the first structural analysis of D with a mechanism-based inhibitor. The side chain of the ring-opened DCI moiety of the protein adduct undergoes chemical modification in the buffered solution, resulting in the formation of an α-hydroxy acid moiety through the nucleophilic substitution of both Cl atoms. The inhibited enzyme is similar in overall structure to the native enzyme, as well as to a variety of isocoumarin-inhibited trypsin and porcine pancreatic elastase (PPE) structures, yet notable differences are observed in the active site and binding mode of these small-molecule inhibitors. One region of the active site (residues 189–195) is relatively conserved between factor D, trypsin, and elastase with respect to amino-acid sequence and to conformation. Another region (residues 214–220) reflects the amino-acid substitutions and conformational flexibility between these enzymes. The carbonyl O atom of the DCI moiety was found to be oriented away from the oxyanion hole, which greatly contributes to the stability of the DCI:D adduct. The comparisons of the active sites between native factor D, DCI-inhibited factor D, and various inhibited trypsin and elastase (PPE) molecules are providing the chemical bases directing the design of novel, small-molecule pharmaceutical agents capable of modulating the alternative complement pathway.