For the purpose of engineering the antibody combining site, mapping residues that are involved in antigen binding provide us with valuable information. By use of13C NMR spectroscopy with selectively13C-labeled Fv fragments, we have established a general strategy to identify the residues that are perturbed upon binding of small antigen (hapten) molecules [(1990) Biochemistry 30, 6604–6610]. In the present paper, we demonstrate that this strategy can be extended to molecular structural analyses of the complexes of an Fab fragment and a larger antigen molecule such asPseudomonas aeruginosa exotoxin A with a molecular mass of 67 kDa.
