BROWNIAN DYNAMICS SIMULATIONS OF MOLECULAR RECOGNITION IN AN ANTIBODYANTIGEN SYSTEM

The crystal structure for an antibody-antigen system, that of the anti -hen egg lysozyme monoclonal antibody HyHEL-5 complexed to lysozyme, i s used as the starting point for computer simulations of diffusional e ncounters between the two proteins. The investigation consists of two parts: first, the linearized Poisson-Boltzmann equation is solved to d etermine the long-range electrostatic forces between antibody and anti gen, and then, the relative motion as influenced by these forces is mo deled within Brownian motion theory. The effects of various point muta tions on the calculated reaction rate are considered. It is found that charged residues close to the binding site exert the greatest influen ce in steering the proteins into a configuration favorable for their b inding, while more distant mutations are qualitatively described by th e Smoluchowski model for the mutual diffusion of two uniformly charged spheres. The antibody residues involved in forming salt links with th e lysozyme, Glu-H35 and Glu-H50, appear to be particularly important i n electrostatic steering, as neutralization of both of them yields rea ction rates that are two to three orders of magnitude below those of w ild-type rates. The relative rates obtained from the simulations can b e tested through kinetic measurements on mutant protein complexes. Kin etically efficient partners can also be designed and constructed throu gh directed mutagenesis.