Insights into the reactivation of cobalamin-dependent methionine synthase |
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Authors: | Markos Koutmos Supratim Datta Katherine A Pattridge Janet L Smith Rowena G Matthews |
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Institution: | aLife Sciences Institute and ;bDepartment of Biological Chemistry, University of Michigan, Ann Arbor MI 48109 |
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Abstract: | Cobalamin-dependent methionine synthase (MetH) is a modular protein that catalyzes the transfer of a methyl group from methyltetrahydrofolate to homocysteine to produce methionine and tetrahydrofolate. The cobalamin cofactor, which serves as both acceptor and donor of the methyl group, is oxidized once every ≈2,000 catalytic cycles and must be reactivated by the uptake of an electron from reduced flavodoxin and a methyl group from S-adenosyl-L-methionine (AdoMet). Previous structures of a C-terminal fragment of MetH (MetHCT) revealed a reactivation conformation that juxtaposes the cobalamin- and AdoMet-binding domains. Here we describe 2 structures of a disulfide stabilized MetHCT (s-sMetHCT) that offer further insight into the reactivation of MetH. The structure of s-sMetHCT with cob(II)alamin and S-adenosyl-L-homocysteine represents the enzyme in the reactivation step preceding electron transfer from flavodoxin. The structure supports earlier suggestions that the enzyme acts to lower the reduction potential of the Co(II)/Co(I) couple by elongating the bond between the cobalt and its upper axial water ligand, effectively making the cobalt 4-coordinate, and illuminates the role of Tyr-1139 in the stabilization of this 4-coordinate state. The structure of s-sMetHCT with aquocobalamin may represent a transient state at the end of reactivation as the newly remethylated 5-coordinate methylcobalamin returns to the 6-coordinate state, triggering the rearrangement to a catalytic conformation. |
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Keywords: | corrinoid methyltransferase multimodular protein protein conformation cob(II)alamin coordination enzyme catalysis |
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