title: Participation of cob(I) alamin in the reaction catalyzed by methionine synthase from Escherichia coli: a steady-state and rapid reaction kinetic analysis.
abstract: The kinetic mechanism of the reaction catalyzed by cobalamin-dependent methionine synthase from Escherichia coli K12 has been investigated by both steady-state and pre-steady-state kinetic analyses. The reaction catalyzed by methionine synthase involves the transfer of a methyl group from methyltetrahydrofolate to homocysteine to generate tetrahydrofolate and methionine. The postulated reaction mechanism invokes an initial transfer of the methyl group to the enzyme to generate enzyme-bound methylcobalamin and tetrahydrofolate. Enzyme-bound methylcobalamin then donates its methyl group to homocysteine to generate methionine and cob(I)alamin. The key questions that were addressed in this study were the following: (1) Does the reaction involve a sequential or ping-pong mechanism? (2) Is enzyme-bound cob(I)alamin a kinetically competent intermediate? (3) If the reaction does involve a sequential mechanism, what is the nature of the "free" enzyme to which the substrates bind; i.e., is the prosthetic group in the cob(I)alamin or methylcobalamin state? Both the steady-state and rapid reaction studies were conducted at 25 degrees C under anaerobic conditions. Initial velocity analysis under steady-state conditions revealed a family of parallel lines suggesting either a ping-pong mechanism or an ordered sequential mechanism. Steady-state product inhibition studies provided evidence for an ordered sequential mechanism in which the first substrate to bind is methyltetrahydrofolate and the last product to be released is tetrahydrofolate. Pre-steady-state kinetic studies were then conducted to determine the rate constants for the various reactions. Enzyme-bound cob(I)alamin was shown to react very rapidly with methyltetrahydrofolate (with an observed rate constant of 250 s-1 versus a turnover number under maximal velocity conditions of 19 s-1).(ABSTRACT TRUNCATED AT 250 WORDS)
authors: Biophysics Research Division, University of Michigan, Ann Arbor 48109.
authors: R V
authors: RV
authors: Banerjee
authors: V
authors: V
authors: Frasca
authors: D P
authors: DP
authors: Ballou
authors: R G
authors: RG
authors: Matthews
mesh_terms: D013766:5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase / Q000378:metabolism*; D004926:Escherichia coli / Q000201:enzymology*; D007700:Kinetics; D008962:Models, Theoretical; D011485:Protein Binding; D013997:Time Factors; D014805:Vitamin B 12 / Q000031:analogs & derivatives / Q000378:metabolism*
chemical_list: C019476:mecobalamin; D013766:5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase; D014805:Vitamin B 12