Crystal Structure Analysis and Molecular Model of Human C3a Anaphylatoxin

Abstract
Human anaphylatoxin C3a [fragment a of complement component 3], derived from C3 was crystallized and its crystal structure determined at 3.2 .ANG. resolution based on multiple isomorphous replacement. The electron density map was interpretable in terms of the known chemical sequence and a molecular model constructed. The molecule has dimensions of 42 .times. 22 .times. 16 .ANG.. It resembles a drumstick. It is constructed from 2 helical segments Tyr15 to Met27 and Gly46 to Ser71 connected by a loop. Residues 1-14 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.

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