ELECTRON-TRANSFER IN SULFITE OXIDASE - EFFECTS OF PH AND ANIONS ON TRANSIENT KINETICS

被引:53
作者
SULLIVAN, EP
HAZZARD, JT
TOLLIN, G
ENEMARK, JH
机构
[1] UNIV ARIZONA, DEPT BIOCHEM, TUCSON, AZ 85721 USA
[2] UNIV ARIZONA, DEPT CHEM, TUCSON, AZ 85721 USA
关键词
D O I
10.1021/bi00097a026
中图分类号
Q5 [生物化学]; Q7 [分子生物学];
学科分类号
071010 ; 081704 ;
摘要
Intramolecular electron transfer (ET) rates in sulfite oxidase (SO) were measured using flavin semiquinone reductants [5-deazariboflavin (dRFH.) and lumiflavin (LFH.)] generated by laser flash photolysis. Rapid bimolecular reduction of the heme by the dRF semiquinone occurred (k = 4 X 10(8) M-1 s-1 at pH 6; 1 X 10(8) M-1 s-1 at pH 9), followed by heme Fe(II) reoxidation due to intramolecular electron transfer to Mo(VI). Flash-induced difference spectra indicated that only spectral processes due to reduction and oxidation of the b-type heme prosthetic group were observed, with no detectable spectral contribution from the Mo cofactor. The extent of reoxidation decreased greatly from pH 6 to 9 (50% to 3%), as expected from the shifts in the redox potentials of the heme and Mo cofactor with pH, consistent with an electron transfer equilibrium between the two redox centers. The observed rate constant for the Fe(II) to Mo(VI) electron transfer decreased from 1650 s-1 at pH 6 to 60 s-1 at pH 9 and showed a maximum of 2400 s-1 at pH 7. Increases in salt concentration greatly decreased intramolecular ET rate constants (direct reduction by flavin semiquinone was unchanged), due to the binding of anions. Titration with the sodium salts of Cl-, SO42-, and H2PO4-/HPO42- resulted in decreases in rate constants of intramolecular ET from 1500 s-1 to <100 s-1 at pH 6 and 7. Similar dissociation constants were measured for the binding of these anions by flash photolysis and by steady-state enzyme kinetics using the inhibition of the sulfite/cytochrome c assay reaction for sulfite oxidase. A mechanism is proposed in which anion binding to the enzyme inhibits the rate of intramolecular electron transfer.
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页码:12465 / 12470
页数:6
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