Visualizing molecular juggling within a B[subscript 12]-dependent methyltransferase complex

Abstract

Derivatives of vitamin B[subscript 12] are used in methyl group transfer in biological processes as diverse as methionine synthesis in humans and CO[subscript 2] fixation in acetogenic bacteria. This seemingly straightforward reaction requires large, multimodular enzyme complexes that adopt multiple conformations to alternately activate, protect and perform catalysis on the reactive B[subscript 12] cofactor. Crystal structures determined thus far have provided structural information for only fragments of these complexes inspiring speculation about 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[subscript 12]-dependent methyl transfer, namely the corrinoid iron–sulphur protein and its methyltransferase 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[subscript 12]-dependent methyl transfer. In addition, the structures capture B[subscript 12] at multiple locations between its ‘resting’ and catalytic positions, allowing visualization of the dramatic protein rearrangements that enable methyl transfer and identification of the trajectory for B[subscript 12] movement within the large enzyme scaffold. The structures are also presented alongside in crystallo 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.