Crystal structure analysis and molecular model of human C3a anaphylatoxin.

Human anaphylatoxin C3a, derived from complement component C3 has been crystallized and its crystal structure determined at 3.2 A resolution based on multiple isomorphous replacement. The electron density map was interpretable in terms of the known chemical sequence and molecular model constructed. The molecule has dimensions of 42 X 22 X 16 A. It resembles a drumstick. It is constructed from two helical segments Tyr15 to Met27 and Gly46 to Ser71 connected by a loop. Residues 1 to 14 are flexible. The C-terminal residues are in irregular conformation. The crystal structure analysis establishes the disulphide linkages as Cys22-Cys49, Cys23-Cys56, Cys36-Cys57.