Studies on the effect of reagent and protein charges on reactivity of the β93 sulfhydryl group of human hemoglobin using selected mutations

The reactivity of the β93 sulfhydryl (SH) group of human oxyhemoglobins with the negatively charged 5,5′-dithiobis(2,2′-nitrobenzoate) and the uncharged 2,2′-dithiodipyridine was determined as a function of pH. Selected mutant hemoglobins having increased oxygen affinity and having residue substitutions altering charge near the SH group (Wood, Malmö, Yakima, Kempsey, Andrew-Minneapolis, Osler, and Chesapeake) were compared to hemoglobin (Hb) A. Although both reagents reacted with GSH at the same rate and with the same enthalpies of activation, the rates with Hb were different and the difference showed a pronounced pH dependence. The charged reagent was sensitive to charges near the SH group; a positive charge increased the rate and a negative one decreased the rate. The uncharged reagent which reacted with Hb A with activation enthalpies similar to those for GSH was insensitive to neighboring charges, but was sensitive to tertiary and quaternary structural changes. The rates obtained with the latter reagent did not correlate with oxygen affinity. The evolutionary aspects of the β93 cysteine in relation to structure and function are reviewed.