Catalytic antibodies: contributions from engineering and expression in Escherichia coli

Antibodies have been raised against the transition state of many reactions and shown to catalyse the relevant reaction. Their moderate catalytic efficiencies can be increased by protein engineering, if ways can be found to express the engineered antibody. We have developed a system by which fully functional Fv and Fab fragments can be expressed in Escherichia coli. The Fv fragment dissociates at low concentrations; we therefore devised methods to stabilize the fragment. We showed that the Fv fragment of the antibody McPC603, a phosphorylcholine-binding immunoglobulin A, binds the antigen with the same affinity as does the intact antibody isolated from mouse ascites. Phosphorylcholine is an analogue of the transition state for the hydrolysis of choline carboxylate ester. The Fv fragment of McPC603 catalysed this hydrolysis. Mutational analysis of the residues in the binding site of the antibody has shown which are essential for binding and for catalysis, and the importance of charged residues in certain positions. The E. coli expression system combined with protein engineering and screening methods will facilitate understanding of enzyme catalysis and the development of new catalytic antibodies.