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Kinetic analysis of metsulphmyoglobin and metmyoglobin reduction by Fe(EDTA)2-.

ABSTRACT: Metsulphmyoglobin prepared from horse heart myoglobin was purified by ion-exchange chromatography to yield a product that on reduction with Fe(EDTA)2- has an A617/A561 ratio greater than 3.5:1. The kinetics of reduction of this purified metsulphmyoglobin and of native metmyoglobin by Fe(EDTA)2- were studied under various conditions of pH, ionic strength and temperature to compare the relative electron-transfer reactivities of a metallochlorin and a metalloporphyrin in identical protein environments. Although the rate of metsulphmyoglobin reduction is 2-7 times that of metmyoglobin under a variety of conditions, this difference can be more than compensated for by the reported difference in mid-point reduction potential between the two forms of the protein. The electrostatic and activation parameters observed for native metmyoglobin and metsulphmyoglobin are essentially identical, and small differences are found in the pH-dependence of the reduction reaction. These findings lead us to conclude that conversion of the porphyrin prosthetic group into a chlorin has relatively little effect on the electron-transfer reactivity of the central metal atom.

SUBMITTER: Lim AR

PROVIDER: S-EPMC1145122 | biostudies-other | 1985 Aug

REPOSITORIES: biostudies-other

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Project description:Iron ions can be used to degrade tetracycline dispersed in nature. Studies of absorption and fluorescence spectra and quantum chemistry calculations showed that iron is more readily released from Fe(III)-citrate than from Fe(III)-EDTA, so Fe(III)-citrate (Fe(III)-Cit) is more suitable for tetracycline (TC) degradation. At 30 °C, a severe degradation of TC by Fe(III)-Cit occurred as early as after 3 days of incubation in the light, and after 5 days in the dark. In contrast, the degradation of TC by Fe(III)-EDTA proceeded very slowly in the dark. By the fifth day of incubation of TC with Fe(III)-Cit in darkness, the concentrations of the former compound dropped by 55% and 75%, at 20 °C and 30 °C, respectively. The decrease in tetracycline concentrations caused by Fe(III)-EDTA in darkness at the same temperatures was only 2% and 6%, respectively. Light increased the degradation rates of TC by Fe(III)-EDTA to 20% and 56% at 20 °C and 30 °C, respectively. The key role of the light in the degradation of tetracycline by Fe(III)-EDTA was thus demonstrated. The TC degradation reaction showed a second-order kinetics. The rate constants of Fe(III)-Cit-induced TC degradation at 20 °C and 30 °C in darkness were k = 4238 M−1day−1 and k = 11,330 M−1day−1, respectively, while for Fe(III)-EDTA were 55 M−1day−1 and 226 M−1day−1. In light, these constants were k = 15,440 M−1day−1 and k = 40,270 M−1day−1 for Fe(III)-Cit and k = 1012 M−1day−1 and 2050 M−1day−1 at 20 °C and 30 °C; respectively. A possible reason for the higher TC degradation rate caused by Fe(III)-Cit can be the result of its lower thermodynamical stability compared with Fe(III)-EDTA, which we confirmed with our quantum chemistry calculations. Two quantum chemistry calculations showed that the iron complex with EDTA is more stable (the free energy of the ensemble is 15.8 kcal/mol lower) than the iron complex with Cit; hence, Fe release from Fe(III)-EDTA is less effective.

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Kinetic analysis of metsulphmyoglobin and metmyoglobin reduction by Fe(EDTA)2-.

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