Spectroscopically validated density functional theory studies of the B12 cofactors and their interactions with enzyme active sites

Vitamin B12 and its biologically active derivatives, 5′-deoxyadenosylcobalamin (AdoCbl) and methylcobalamin (MeCbl), have long fascinated chemists with their elaborate structures and unusual reactivities in enzymatic systems. Due to their large size and complex electronic structures, these cofactors have posed a major challenge for computational chemists. Yet, recent insights gained from kinetic, spectroscopic, and X-ray crystallographic studies, have established an excellent foundation for the successful completion of density functional theory studies aimed at elucidating the electronic structures of the isolated cofactors and the catalytic cycles of B12-dependent enzymes. This review summarizes important information obtained from experimentally validated computational studies of: (i) the free AdoCbl and MeCbl cofactors in their Co3+, Co2+, and Co1+ oxidation states; (ii) the mechanism by which enzymes involved in the biosynthesis of AdoCbl accomplish the thermodynamically challenging Co2+ → Co1+ reduction; (iii) the strategies employed by AdoCbl-dependent enzymes to achieve a trillion-fold rate acceleration for the homolytic cleavage of the cofactor's CoC(Ado) bond; and (iv) the means by which MeCbl-dependent methyltransferases accelerate the rate of methyl transfer via heterolytic CoC(Me) bond cleavage by as much as six orders of magnitude and reactivate the accidentally oxidized form of the cofactor.