B12-dependent Methyltransferase Research Articles

Visualizing molecular juggling within a B12-dependent methyltransferase complex

Derivatives of vitamin B12 are used in methyl group transfer in biological processes as diverse as methionine synthesis in humans and CO2 fixation in acetogenic bacteria1–3. This seemingly straightforward reaction requires large, multimodular enzyme complexes that adopt multiple conformations to alternately activate, protect, and perform catalysis on the reactive B12 cofactor. Crystal structures determined thus far have provided structural information for only fragments of these complexes4–12, inspiring speculation regarding the overall protein assembly and conformational movements inherent to activity. Here we present X-ray crystal structures of a complete ~220 kDa complex that contains all enzymes responsible for B12-dependent methyltransfer, namely the corrinoid iron-sulfur protein (CFeSP) and its methyltransferase (MeTr) from the model acetogen Moorella thermoacetica. These structures provide the first three-dimensional depiction of all protein modules required for the activation, protection, and catalytic steps of B12-dependent methyltransfer. In addition, the structures capture B12 at multiple locations between its “resting” and catalytic positions, allowing visualisation of the dramatic protein rearrangements that enable methyltransfer and identification of the trajectory for B12 movement within the large enzyme scaffold. The structures are also presented alongside in crystallo UV-vis spectroscopic data, which confirm enzymatic activity within crystals and demonstrate the largest known conformational movements of proteins in a crystalline state. Taken together, this work provides a model for the molecular juggling that accompanies turnover and helps explain why such an elaborate protein framework is required for such a simple, yet biologically essential reaction.

Methyl Transfer from Methanol to Co-cobyrinate: A model for the Coenzyme B12 Dependent Methyltransferase?

With CoI corrins as supernucleophiles, methylation by methanol is achieved when the leaving group is activated by Zn2+ ions at elevated temperatures (see reaction below). This methylation reaction is the first model for the Co-dependent methyltransferase.