The cleavage sequence of fibrinopeptide A from fibrinogen fragment E by thrombin, atroxin or batroxobin

Calculations of data from fibrin polymerization and cross-linking experiments infer that thrombin-catalysed release of the second of the two fibrinopeptides A (FpA2) from fibrinogen is concerted, although other data suggest that FpA2 release is random. In the concerted pattern of FpA release, divalent monomer (des AA-fibrin) formation predominates throughout the enzymatic conversion of fibrinogen to fibrin, an effect leading to relatively rapid fibril assembly. Alternatively, random FpA2 release would result in a substantial population of monovalent monomer (des A-fibrin) intermediates during early and intermediate phases of the enzymatic conversion to fibrin. Their formation would cause a delay in fibrin fibril assembly. In order to address the question of the pattern of FpA release directly, we purified plasmic fibrinogen fragment E1 isoforms containing both FpA sequences and studied the sequence of FpA release by thrombin or batroxobin. Des A-fragment E1 intermediates formed by loss of one FpA (FpA1), and des AA-fragment E1 products (lacking both FpA1 and FpA2) were identified by analytical isoelectric focusing and quantified by densitometry. The catalytic rate of release of FpA1 (k1) and FpA2 (k2) by thrombin or batroxobin was similar. The ratio of these rates, k2:k1, was 1.10 +/- 0.42 for thrombin and 1.34 +/- 0.26 for batroxobin. These findings indicate that these enzymes cleave FpA2 randomly from fragment E1.