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.
Highlights
Enzymatic reactions involving the cofactor tetrahydrofolate (THF) are found ubiquitously throughout nature and support various transfer reactions of reduced single carbon species.[1]
In the case of the glycine betaine (GB) pathway, this organism uses a series of enzymes, MtgB, MtgC, and MtgA, to transfer one methyl group via Cbl to THF (Scheme 1).[8]
Site-directed mutagenesis combined with activity assays identified a unique locking mode of the THF glutamyl-p-aminobenzoate moiety (THF tail)
Summary
Enzymatic reactions involving the cofactor tetrahydrofolate (THF) are found ubiquitously throughout nature and support various transfer reactions of reduced single carbon species.[1]. The X-ray structure of the MtgA:THF complex illustrates electron density only for the pterin moiety of the ligand; the THF tail seems to be flexible (Figure 1 C).
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