THE EFFECT OF RESIDUE-1106 ON THE THIOESTER-MEDIATED COVALENT BINDINGREACTION OF HUMAN-COMPLEMENT PROTEIN C4 AND THE MONOMERIC RAT ALPHA-MACROGLOBULIN ALPHA(1)I3

The histidine at position 1106 of the C4B isotype of human complement is involved in catalyzing the covalent binding of the thioester to gly cerol and water, By replacing the histidine with other residues, it wa s found that tyrosine is also capable of mediating the reaction, We pr opose that they act as nucleophiles by first attacking the thioester, upon activation, to form acyl intermediates, which subsequently react with the hydroxyl groups of glycerol or water, The monomeric alpha-mac roglobulin, alpha(1)I3 of the rat, was also studied, Unlike alpha(2)-m acroglobulin, which is a tetramer, alpha(1)I3 has binding properties s imilar to those of C4A.