factor Va factor Vi factor Va -> factor Vi Activated protein C cleaves peptide bonds in activated factor V (factor Va), converting it to an inactive form (factor Vi). The exact site(s) of cleavage are unknown. Protein S, on the endothelial cell surface, positively regulates this reaction. Although the mechanism of this regulation is unclear, the regulation is physiologically important, as people with reduced amounts of protein S, like people with reduced amounts of protein C, are susceptible to thromboembolism. Authored: D'Eustachio, P, 2004-08-26 13:13:00 EC Number: 3.4.21 Pubmed2538457 Reactome Database ID Release 43141026 Reactome, http://www.reactome.org ReactomeREACT_1071