Sequence location of the reactive thiol ester in human α₂‐macroglobulin

IntroductionIt was shown recently that native az-macroglobulin (cQM), Af, 725 000 contains one reactive, labile thiol ester in each of its 4 identical subunits of Iw, 180 000, which is formed by a particular Glx-residue, whose y-carboxyl group is esterified to the sulfhydryl group of a cysteinyl residue [ 13.Complex formation with proteinases involves not only limited proteolysis in the 'bait' region but also the rapid cleavage of this thiol ester as evidenced by the concurrent appearance of the SH-groups (max. 4 mol/mol azM) [ 11. Denaturation of CY,M or treatment with CH3NH2 also leads to cleavage of this thiol ester [I].During 'inactivation' of the proteinase binding capacity with CHaNH2 the latter,is incorporated covalently to form a y-glutamyl methylamide [ 1,2] indicating that this site in ozM may be a site for covalent binding of proteinase.Complement component C3 also reacts covalently with CH3NH2 again leading to the formation of y-glutamyl methylamide f3-51.The recent demonstration that the a-chain of C3 probably also contains a thiol ester [4,5] later shown [6] to involve the Glx-residue and the Cys-residue'in the sequence: -Gly-Cys-Gly~lu~lx-Asn-Met which is identical to the sequence around the CHaNHa-reactive Glx-residue of oZM [ 1,7,8] shows that cuzM and C3 are structurally and functionally