COENZYME B-12 CHEMICAL PRECEDENT STUDIES - PROBING THE ROLE OF THE IMIDAZOLE BASE-ON MOTIF FOUND IN B-12-DEPENDENT METHYLMALONYL-COA MUTASE

Adenosylcobinamide (AdoCbi(+)) plus N-methylimidazole (N-MeIm), [AdoCb i . N-MeIm]+BF4-, has been studied with the goal of providing a chemic al precedent for the benzimidazole base-off, protein histidine base-on form of adenosylcobalamin (AdoCbl, also coenzyme B-12) found in the r ecent X-ray crystallographic structural study of methylmalonyl-CoA (MM CoA) mutase. Specifically, the axial-base binding K-assoc and associat ed Delta H and Delta S thermodynamic parameters for [AdoCbi . N-MeIm]BF4- have been obtained as well as its Co-C thermolytic cleavage produ cts and kinetic parameters. The thermodynamic studies reveal that imid azole is unique among the aromatic nitrogenous bases tested, with a mo re favorable Delta H = -7.8 +/- 0.4 kcal/mol but a compensatingly less favorable Delta S = -28 +/- 1 eu when binding to AdoCbi(+). A stronge r, shorter Co-N (N-MeIm) bond is implied for [AdoCbi . N-MeIm](+) (i.e ., vs pyridine as the axial base). The product studies reveal that imi dazole changes the mode of Co-C cleavage from greater than or equal to 98% homolysis (for the appended 5,6-dimethylbenzimidazole in AdoCbl) to similar to 50% homolysis and similar to 50% abiological heterolysis for [AdoCbi . N-MeIm](+). The kinetic studies demonstrate that both C o-C homolysis and heterolysis are accelerated by the record amounts of 8- and 350-fold, respectively, vs the reference point of 5,6-dimethyl benzimidazole base-on AdoCbl (and by a record 870- and 30 700-fold, re spectively vs the reference point of the solvent ethylene glycol as th e axial base, [AdoCbi . ethylene glycol](+)). The biological significa nce of these findings is discussed, notably (i) that the MMCoA mutase subclass of B-12-dependent enzymes must either (a) utilize or (b) prev ent Co-C heterolysis and (ii) the expectation that a long, ''ideal'' l ength Co-N(imidazole) is one key way the enzyme can inhibit Co-C heter olysis as well as accelerate Co-C homolysis. Also discussed are the st eric and electronic differences of imidazole vs pyridine axial-ligands , including the literature of imidazole's pi-bonding interactions. Fin ally, a brief summary of the needed [AdoCbi . base](+) and [Co(II)Cbi . base](+) structural, computational, and other additional studies is presented.