Site-directed mutagenesis study of the role of histidine residues in the neutral-to-basic transition of human serum albumin

Site-directed mutagenesis was used to study the role of histidine residues located in domain I in the neutral-to-basic (N-B) transition of human serum albumin (HSA). Based on a previous study of the N-B transition by means of proton NMR, the following recombinant HSA species were synthesized in the yeast species, Pichia pastoris: H9F, H9S, H39F, H39S, H67F, H67S, H105F, H105S, H128F, H128S, H146F, H146S, and wild type HSA. By monitoring the fluorescent intensity of warfarin bound to the above recombinant human serum albumin species as a function of pH, the mutational effect of individual histidine residues on the N-B transition was examined. While H9, H67, H105, H128 and H146 contribute to the transition significantly, H39 appears to have virtually no contribution to the transition. Based on the X-ray crystallographic structure, it is suggested that electrostatic interactions are the principal factor in determining the histidine pK shifts.