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Conversion of oxy hem erythrin into met hem erythrin in the presence of anions



Conversion of oxy hem erythrin into met hem erythrin in the presence of anions



Journal of Biological Chemistry 252(17): 6069-6075



The interaction of oxyhemerythrin (HrO2) [of Golfingia gouldi] with cyanate and azide ions (X-) is biphasic. The rapid loss of HrO2 is monitored at 500 nm using stopped flow. The dependence of the pseudo-first order rate constant on [HrO2], [X-] and [O2] can be interpreted by the scheme: HrO2 .dblarw. Hr + O2 K1; Hr + X- .dblarw. HrX- k2, k-2, K2. At 25.degree., pH = 6.3 and I = 0.15 M; for X- = CNO-, k2 = 59 M-1 s-1 and k-2 = 0.019 s-1; and for X- = N3-, k2 = 1.6 .times. 102 M-1 s-1 and k-2 = 0.10 s-1. Values of K1K2 from kinetics are in agreement with those obtained from spectral and [O2] determinations of equilibrated mixtures of Hr, CNO- and O2. The 2nd step was analyzed separately by flow observations of the small spectral changes accompanying addition of X- to deoxyhemerythrin. The slower, stoichiometric formation of methemerythrin cyanate or azide was studied and 2nd order rate constants for reaction of HrO2 with X- determined (N3-, 0.34 M-1 s-1 and CNO-, 0.08 M-1 s-1 at 25.degree. and pH = 6.3). These were compared with those for accelerated formation of the methemerythrin ligand adduct in the same conditions in presence of NO2- (0.31 M-1 s-1), HCO2- (8.3 .times. 10-3 M-1 s-1), F- (0.12 M-1 s-1), Cl- (1.4 .times. 10-4 M-1 s-1) and imidazole (4 .times. 10-3 M-1 s-1 at pH 8.0). No rapid pre-equilibria were observed in the case of the latter ligands. The effect of pH is examined.

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