(427e) Binding of PbDTPA Antigens to Specific Recognition Sequences of Antibody 5B2

The intelligent engineering of antibodies for the specific recognition of heavy metal complex antigens via single site mutations is a key step in the development of antibody based radiotherapeutic reagents and biosensors. The recognition subunit of the antibody is composed of a heavy and light chain sequence forming the binding pocket. The pocket is surrounded by six highly variable loops (3 heavy and 3 light chain loops - natively unfolded) which confer binding specificity to the antibody. Using molecular simulations, we assess the interaction of antibody loops with antigen to determine which amino acids most likely contribute to heavy metal complex binding for antibody 5B2 described in Delehanty et al. (Biochemistry 2003). The radial distribution functions for PbDTPA antigen with the loop side chains varied over extended time blocks, indicating long binding relaxation times. Thus time correlation functions were calculated to get hydrogen-bond time constants between antigen and amino acid side chains. In particular the amino acid sequence TYR-SER-TYR was found to make strong hydrogen-bonds with PbDTPA with a long lifetime. Replica exchange MD simulation of 5B2 was performed while chelate was simultaneously pulled towards the target amino acid sequence to get binding of antigen-antibody. Mutation of SER98 to ALA98 leads to escape of the antigen from the pocket supporting the role of the TYR-SER-TYR motif in binding.