Co 2+/Co + redox tuning in methyltransferases induced by a conformational change at the axial ligand

Density functional theory and quantum mechanics/molecular mechanics computations predict cob(I)alamin (Co ⁺Cbx), a universal B ₁₂ intermediate state, to be a pentacoordinated square pyramidal complex, which is different from the most widely accepted viewpoint of its tetracoordinated square planar geometry. The square pyramidality of Co ⁺Cbx is inspired by the fact that a Co ⁺ ion, which has a dominant d ⁸ electronic configuration, forms a distinctive Co ⁺ - H interaction because of the availability of appropriately oriented filled d orbitals. This uniquely H-bonded Co ⁺Cbx may have catalytic relevance in the context of thermodynamically uphill Co ²⁺/Co ⁺ reduction that constitutes an essential component in a large variety of methyltransferases. © 2012 American Chemical Society.