Fate of the activated gamma-carbon-hydrogen bond in the uncoupled vitamin K-dependent gamma-glutamyl carboxylation reaction.

The gamma-glutamyl carboxylation reaction proceeds by an initial vitamin K-dependent gamma-C-H glutamyl bond cleavage and a subsequent carboxylation of the activated glutamyl residue. This system is easily uncoupled such that at low CO2 concentrations which limit the extent of carboxylation there is no effect on the rate of C-H bond cleavage. In an uncoupled system, the fate of activated glutamyl residues is to incorporate a hydrogen as demonstrated by the recovery of only unaltered glutamyl residues from digests of uncoupled reactions. In addition, in reactions carried out in tritiated, deuterated water mixtures, tritium is incorporated into the gamma positions of the glutamyl residues of peptide substrates in a vitamin K-dependent process, indicating that the hydrogen incorporated must ultimately come from solvent. These results, while not proof, put severe restraints on a radical mechanism while favoring a carbanion mechanism.