Assignment of a single disulphide bridge in human alpha2-antiplasmin: implications for the structural and functional properties.

Human alpha2-antiplasmin (alpha2AP) is a serpin involved in the regulation of blood coagulation. Most serpins, unlike smaller serine proteinase inhibitors, do not contain disulphide bridges. alpha2AP is an exception from this generalization and has previously been shown to contain four Cys residues organized into two disulphide bridges [Lijnen, Holmes, van Hoef, Wiman, Rodriguez and Collen (1987) Eur. J. Biochem. 166, 565-574]. However, we found that alpha2AP incorporates iodo[14C]acetic acid, suggesting that the protein contains reactive thiol groups. This observation prompted a re-examination of the state of the thiol groups, which revealed (i) a disulphide bridge between Cys43 and Cys116, (ii) that Cys76 is bound to a cysteinyl-glycine dipeptide, and (iii) and Cys125 exists as either a free thiol or in a mixed disulphide with another Cys residue. The disulphide identified between Cys43 and Cys116 appears to be conserved in orthologous proteins since the homologous Cys residues form disulphide bonds in bovine and possibly mouse alpha2AP. The conservation of this disulphide bridge suggests that it is important for functional aspects of alpha2AP. However, the structural and functional analysis described in this study does not support this conclusion.