The covalent binding story of the complement proteins C3 and C4 (I) 1972-1981

Alex Law and Paul Levine recall their work to establish the covalent bond between C3 and target surfaces. It started with a naive experiment by analyzing the membrane polypeptides of sheep erythrocytes bound with ¹²⁵I-labelled C3. They found complexes with molecular weight higher than the individual C3 polypeptides. These complexes survived all conditions designed to disrupt non-covalent interactions. They then showed that the bond was an ester, with an active acyl group on C3 which reacted with a hydroxyl group on the acceptor molecule. With the discovery of an internal thioester by Jim Prahl, Jamila Janatova, Brian Tack and their colleagues, it became clear that the reaction was by an acyl transfer from the thioester of C3 to the target hydroxyl group. Later on they showed that C4 also bound covalently to target molecules. By establishing a fluid phase system to study the kinetics of the binding reactions of C3 and C4, Alex was able to continue the work in the MRC Immunochemistry Unit in Oxford from 1981, to eventually determine the chemical mechanism of the binding reaction. In order to give some sense of reality, this article is written as a narrative from Alex, who did the experiments. Both Alex and Paul are retired. Pauls lives on Martha's Vineyard where he writes occasional articles on science for one of the Island's newspapers. Alex lives in Hong Kong and tries to make some sense of the local politics.