Visualising molecular juggling within a B ₁₂-dependent methyltransferase complex

Abstract

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.