| Abstract: | The reactions of hemerythrin from Phascolopsis gouldii with the specific sulfhydryl reagents 5,5'-dithiobis(2-nitrobenzoate), 2,2'-dithiodipyridine, and 4,4'-dithiodipyridine were studied at 25 degrees C. Spectrophotometric measurements showed that 1 mol of disulfide reacted per protein subunit consistent with a single cysteine at residue 50. Reaction leads to dissociation of the octameric structure of the native protein to monomers. The first-order rate constants at 25 degrees C and pH 9.0 for reactions of methemerythrin [(1.5 +/- 0.3) X 10(-3) s-1] and metazidohemerythrin [(4.0 +/- 0.3) X 10(-3) s-1] are independent of both the concentration and the nature of the disulfide. The reactions of methemerythrin are strongly inhibited by ClO4-ion, which however has no effect on the rates of those of metazidohemerythrin. The first-order kinetic behavior is ascribed to a conformational change involving the protein controlling the reaction, and this slow change appears to dominate a number of the reactions of hemerythrin. |
