A theoretical study of the aminolysis reaction of lysine 199 of human serum albumin with benzylpenicillin: Consequences for immunochemistry of penicillins

Herein, we present results of a computational study on benzylpenicillin att achment to Lys199 of human serum albumin via an aminolysis reaction. The in ternal geometry of the reactive part of the system was taken from previous work at the B3LYP/6-31+G* level on the water-assisted aminolysis reaction o f a penicillin model compound (Diaz, N.; Suarrez, D.; Sordo, T. L. J. Am. C hem. Soc. 2000, 122, 6710-6719). The protein environment around Lys199, the 6-acylamino side chain, and the 2-methyl groups of benzylpenicillin were r elaxed by carrying out geometry optimizations with a hybrid QM/MM method (P M3/AMBER). Two different mechanistic routes were explored: a one-step water -assisted process and a carboxylate and water-assisted route in which the b eta -lactam carboxylate and the ancillary water molecule mediate the proton transfer from the epsilon -amino group of Lys199 to the beta -lactam leavi ng N atom, The corresponding energy profiles in the protein combine the B3L YP/6-31+G* and PM3 energies of the reactive subsystem (benzylpenicillin + L ys199 side chain + the ancillary water molecule) and semiempirical PM3 ener gies of the entire system evaluated with a "divide and conquer" linear-scal ing method. It is observed that penicillin haptenation to HSA can proceed t hrough the water-assisted concerted mechanism which is calculated to have a high energy barrier of similar to 38 kcal/mol, in agreement with the exper imentally observed slow reaction kinetics.