Arrangement of the disulfide bridges in a blood coagulation factor IX/factor X-binding protein from the venom of Trimeresurus flavoviridis.

Blood coagulation factor IX/factor X-binding protein (IX/X-bp) is a two-chain anticoagulant protein that was isolated from the venom of Trimeresurus flavoviridis. The amino acid sequence of IX/X-bp is homologous to the sequences of C-type lectin-like proteins, such as asialoglycoprotein receptor, tetranectin, and the low-affinity Fc epsilon receptor of immunoglobulin E. The amino acid composition and amino acid sequence of cystine-containing peptides, formed as a result of enzymatic digestion of CNBr-generated fragments of IX/X-bp, were analyzed to determine the location of the seven disulfide bridges in the protein. Three disulfide bridges in the A chain link Cys2 to Cys13, Cys30 to Cys127, and Cys102 to Cys119. Three disulfide bridges in the A chain link Cys2 to Cys13, Cys30 to Cys119, and Cys96 to Cys111. An interchain disulfide bond links Cys79 of the A chain and Cys75 of the B chain. The intrachain disulfide-bonding patterns of both the A and B chains of IX/X-bp are similar to those found in other C-type lectin-like proteins. We discuss in this report the sequence homology between IX/X-bp and other two-chain, C-type lectin-like proteins that have been isolated from snake venoms and we compare the S-S bonding patterns of proteins that are homologous to IX/X-bp.