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Abstract
The cytoplasmic methyltetrahydrofolate: corrinoid/iron-sulfur protein methyltransferase (MeTr) is a key protein in the Wood-Ljungdahl pathway of CO2 fixation. It reversibly transfers the N5-methyl group from methyltetrahydrofolate (CH3-H4folate) to the Co(I) nucleophilic center of either free cob(I)alamin or its natural acceptor, the corrinoid/iron-sulfur protein in the reductive acetyl-CoA pathway for CO2 fixation. No crystal structure of a methyltetrahydrofolate methyltransferase has been determined to date. The MeTr structure was determined at 2.2 Å resolution by multiwavelength anomalous diffraction methods. The overall architecture of MeTr is a TIM barrel. This represents a new functional class (number 20) of the versatile TIM barrel fold. The MeTr structure is surprisingly similar to the crystal structures of dihydropteroate synthetases despite sharing less than 20% sequence identity. This includes extensive conservation of the pterin ring binding residues (D43, D75, N96, D160) in the bottom of the polar active sites of the methyltransferases and dihydropteroate synthetases. The biggest structural difference between these enzymes is in a loop structure above the active site. It is quite open for MeTr, suggesting a probable cobalamin (or corrinoid) binding site. Such structural solution fits a general trend for cobaimide enzymes. A TIM barrel embeds the relatively unreactive substrate and the cobamide, bound to other protein moiety (subunit), closes the C-terminus top of the barrel forming an isolated reaction cavity. Our results are consistent with either a “front” or “back” side protonation of CH3-H4folate, a key step in the mechanism of MeTr.
