The histidine at position 1106 of the C4B isotype of human complement is involved in catalyzing the covalent binding of the thioester to glycerol and water. By replacing the histidine with other residues, it was found that tyrosine is also capable of mediating the reaction. We propose 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 α-macroglobulin, α1I3 of the rat, was also studied. Unlike α2-macroglobulin, which is a tetramer, α1I3 has binding properties similar to those of C4A.
