Structures in Tetrahydrofolate Methylation in Desulfitobacterial Glycine Betaine Metabolism at Atomic Resolution

Abstract

Enzymes orchestrating methylation between tetrahydrofolate (THF) and cobalamin (Cbl) are abundant among all domains of life. During energy production in Desulfitobacterium hafniense, MtgA catalyzes the methyl transfer from methylcobalamin (Cbl‐CH3) to THF in the catabolism of glycine betaine (GB). Despite its lack of sequence identity with known structures, we could show that MtgA forms a homodimeric complex of two TIM barrels. Atomic crystallographic insights into the interplay of MtgA with THF as well as analysis of a trapped reaction intermediate (THF‐CH3)+ reveal conformational rearrangements during the transfer reaction. Whereas residues for THF methylation are conserved, the binding mode for the THF glutamyl‐p‐aminobenzoate moiety (THF tail) is unique. Apart from snapshots of individual reaction steps of MtgA, structure‐based mutagenesis combined with enzymatic activity assays allowed a mechanistic description of the methyl transfer between Cbl‐CH3 and THF. Altogether, the THF‐tail‐binding motion observed in MtgA is unique compared to other THF methyltransferases and therefore contributes to the general understanding of THF‐mediated methyl transfer.

Structural and mechanistic basis of the central energy-converting methyltransferase complex of methanogenesis

“…For all sequences tested, MtrH oligomerizes to a homodimer ( SI Appendix , Fig. S12 ) as found for MtgA of Desulfitobacterium hafniense, the nearest relative of MtrH that has been structurally characterized ( 45 ). The Alphafold2 model of MtrA s of M. marburgensis is essentially identical to the X-ray structure of the M. jannaschii protein and the M. fervidus MtrA homolog ( 21 ) ( SI Appendix , Fig.…”

“…For exploring the structural basis of the methyl transfer between methyl-H 4 MPT and cob(I)amide, methyl-H 4 MPT was modeled into MtrH of the MtrA s –MtrH subcomplex by using the methyl-H 4 F binding of MtgA as template ( 45 ). Accordingly, the pterin head sits at the bottom of the TIM barrel of MtrH and the benzoyl, desoxyribose, and ribose moiety extends along the corrinoid ring of MtrA and helix H226-H233 to the protein surface ( Fig.…”