Heme sulfuric anhydrides as soybean leghemoglobin structure probes

Mesoheme monosulfuric anhydride reacts at three distinct sites in soybean apoleghemoglobin a, at lysine-6, lysine-19 and lysine-57, the last one being the major site of reaction. The heme peptides obtained from thermolytic and pronase hydrolysates of the anhydride-leghemoglobin a were purified and correlated with the known amino acid sequence of the protein. Mesoheme bissulfuric anhydride also reacts with soybean apoleghemoglobin a giving a complex mixture of hemepeptides after hydrolysis with pronase. The visible spectrum of anhydride leghemoglobin is that of low spin heme. This suggests that anhydride leghemoglobin has a conformation with a covalent attachment via propionic acid side chain to lysine-57 and the sixth coordination position of the heme iron occupied by the distal histidine at position 61. Native leghemoglobin is assumed to exist in a similar type of configuration at low temperature, but with the heme propionate side chain being involved in a salt bridge with lysine-57.