Visualising molecular juggling within a B(12)-dependent methyltransferase complex

Derivatives of vitamin B(12) are used in methyl group transfer in biological processes as diverse as methionine synthesis in humans and CO(2) fixation in acetogenic bacteria(1–3). This seemingly straightforward reaction requires large, multimodular enzyme complexes that adopt multiple conformations to alternately activate, protect, and perform catalysis on the reactive B(12) cofactor. Crystal structures determined thus far have provided structural information for only fragments of these complexes(4–12), 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 B(12)-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 B(12)-dependent methyltransfer. In addition, the structures capture B(12) 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 B(12) 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.