Interaction of Escherichia coli Cobalamin-Dependent Methionine Synthase and Its Physiological Partner Flavodoxin: Binding of Flavodoxin Leads to Axial Ligand Dissociation from the Cobalamin Cof
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- 作者：David M. Hoover ; Joseph T. Jarrett ; Richard H. Sands ; William R. Dunham ; Martha L. Ludwig ; and Rowena G. Matthews
- 刊名：Biochemistry
- 出版年：1997
- 出版时间：January 7, 1997
- 年：1997
- 卷：36
- 期：1
- 页码：127 - 138
- 全文大小：307K
- 年卷期：v.36,no.1(January 7, 1997)
- ISSN：1520-4995
文摘
Cobalamin-dependent methionine synthase from Escherichiacoli catalyzes the last step in denovo methionine biosynthesis. Conversion of the inactivecob(II)alamin form of the enzyme, formed bythe occasional oxidation of cob(I)alamin during turnover, toan active methylcobalamin-containing formrequires a reductive methylation of the cofactor in which an electronis supplied by reduced flavodoxinand the methyl group is derived fromS-adenosyl-l -methionine. E. coliflavodoxin acts specifically inthis activation reaction, and neither E. coli ferredoxin norflavodoxin from the cyanobacteriumSynechococcus will substitute, despite their highly similarmidpoint potentials for one-electron transfer.As assessed by EPR spectroscopy, the binding of flavodoxin tocob(II)alamin methionine synthase resultsin a change in the coordination geometry of the cobalt fromfive-coordinate to four-coordinate. Histidine759 of methionine synthase, which replaces the normal lower liganddimethylbenzimidazole on bindingof methylcobalamin to methionine synthase, is dissociated from thecobalt of the cobalamin by the bindingof flavodoxin. The association of flavodoxin and methioninesynthase depends on ionic strength and pH;the pH dependence corresponds to the uptake of one proton onassociation. The formation of a complexbetween flavodoxin and methionine synthase perturbs the midpointpotentials of the flavin and cobalamincofactors only marginally and without any significant thermodynamicadvantage for electron transfer tothe cobalamin of methionine synthase. No significant binding wasseen between oxidized flavodoxinand methylcobalamin methionine synthase. A model for theinteraction of methionine synthase withflavodoxin is proposed in which flavodoxin binding leads to changes inthe distribution of methioninesynthase conformations.