Molecular dynamics study of the IIA binding site in human serum albumin: Influence of the protonation state of Lys195 and Lys199

The IIA binding site of human serum albumin (HSA) preferentially binds hydr ophobic organic anions of medium size (e.g., aspirin, benzylpenicillin, war farin, etc.) and bilirubin. This binding ability is particularly important for the distribution, metabolism, and efficacy of drugs. In addition, HSA c an also covalently link to different IIA substrates owing to the presence o f a highly reactive residue, Lys199, which is strategically located in the IIA site. Herein, we present results of three restrained molecular dynamics (MD) simulations of the IIA binding site on the HSA protein. From these si mulations, we have determined the influence that the ionization state of th e key residue, Lys199, and the nearby Lys195 has on the structure and dynam ics of the IIA binding site. When Lys199 is neutral the computed average di stances for the most significant interresidue contacts are in good agreemen t with those estimated from the X-ray coordinates. The analysis of the solv ent structure and dynamics indicates that the basic form of Lys199 is likel y connected to the acid form of Lys195 through a network of H-bonding water molecules with a donor --> acceptor character. The presence of these, wate r bridges can be important for stabilizing the configuration of the IIA bin ding site and/or promoting a potential Lys195 --> Lys199 proton-transfer pr ocess. These results suggest that both lysine residues located in the IIA b inding site of HSA, Lys195 and Lys199, could play a combined and comparable chemical role. Our simulations also give insight into the binding of bilir ubin to HSA.