The contribution of residues 192 and 193 to the specificity of snake venom serine proteinases

Snake venom serine proteinases, which belong to the subfamily of trypsin-like serine proteinases, exhibit a high degree of sequence identity (60-66%). Their stringent macromolecular substrate specificity contrasts with that of the less specific enzyme trypsin. One of them, the plasminogen activator from Trimeresurus stejnegeri venom (TSV-PA), which shares 63% sequence identity with batroxobin, a fibrinogen clotting enzyme from Bothrops atrox venom, specifically activates plasminogen to plasmin like tissue-type plasminogen activator (t-PA), even though it exhibits only 23% sequence identity with t-PA. This study shows that TSV-PA, t-PA, and batroxobin are quite different in their specificity toward small chromogenic substrates, TSV-PA being less selective than t-PA, and batroxobin not being efficient at all. The specificity of TSV-PA, with respect to t-PA and batroxobin, was investigated further by site-directed mutagenesis in the 189-195 segment, which forms the basement of the S(1) pocket of TSV-PA and presents a His at position 192 and a unique Phe at position 193. This study demonstrates that Phe(193) plays a more significant role than His(192) in determining substrate specificity and inhibition resistance. Interestingly, the TSV-PA variant F193G possesses a 8-9-fold increased activity for plasminogen and becomes sensitive to bovine pancreatic trypsin inhibitor.