Activation of methionine synthase: further characterization of flavoprotein system.

Two homogeneous flavoproteins (R and F components) which, in conjunction with catalytic amounts of NADPH and adenosylmethionine, comprise an efficient system for activation of the B₁₂-containing methionine synthase (M component) from Escherichia coli K-12, have been characterized with respect to oxidation-reduction properties and participation in the activation process. The flavin (FAD) of R component is reduced to FADH₂ by NADPH. Reduced R, in turn, reduces the flavin (FMN) of F component to a blue semiquinone (FMNH ·). Reduction potentials (at pH 7.0) for R and F are ?0.30 and ?0.29 V, respectively. Various other compounds such as ferricyanide, 2,6-dichlorophenol-indophenol, menadione, and -cytochrome c can also serve as electron acceptors for reduced R, but only F can efficiently mediate the NADPH- and R-dependent activation of M component. Activation is assumed, therefore, to involve the sequence: NADPH → R → F → M. During operation of the complete system, the amount of NADPH consumed is less than 2% of the amount of methionine synthesized.