Amino acid sequence of fibrolase, a direct‐acting fibrinolytic enzyme from agkistrodon contortrix contortrix venom

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Abstract
The complete amino acid sequence of fibrolase, a fibrinolytic enzyme from southern copperhead (Agkistrodon contortrix contortrix) venom, has been determined. This is the first report of the sequence of a direct‐acting, non‐hemorrhagic fibrinolytic enzyme found in snake venom. The majority of the sequence was established by automated Edman degradation of overlapping peptides generated by a variety of selective cleavage procedures. The amino‐terminus is blocked by a cyclized glutamine (pyroglutamic acid) residue, and the sequence of this region of the molecule was determined by mass spectrometry. Fibrolase is composed of 203 residues in a single polypeptide chain with a molecular weight of 22,891, as determined by the sequence. Its sequence is homologous to the sequence of the hemorrhagic toxin Ht‐d of Crotalus atrox venom and with the sequences of two metalloproteinases from Trimeresurus flavoviridis venom. Microheterogeneity in the sequence was found at both the amino‐terminus and at residues 189 and 192. All six cysteine residues in fibrolase are involved in disulfide bonds. A disulfide bond between cysteine‐118 and cysteine‐198 has been established and bonds between cysteines‐158/165 and between cysteines‐160/192 are inferred from the homology to Ht‐d. Secondary structure prediction reveals a very low percentage of α‐helix (4%), but much greater β‐structure (39.5%). Analysis of the sequence reveals the absence of asparagine‐linked glycosylation sites defined by the consensus sequence: asparagine‐X‐serine/threonine.