Studies on Ca(II) binding to gamma-carboxyglutamic acid. Use of thermal decarboxylation to probe metal ion/gamma-carboxyglutamic acid interactions.

The thermal decarboxylation of N-benzyloxycarbonyl-L-gamma-carboxyglutamic acid alpha-methyl ester [Z)-L-Gla-OMe) has been studied. In the presence of increasing amounts of calcium or magnesium ions, lyophilized powders of (Z)-L-Gla-OMe exhibit a corresponding increase in thermal stability. Both magnesium and calcium form relatively tight, thermally stable complexes with (Z)-L-Gla-OMe at high metal ion concentrations. Differences between Ca(II) and Mg(II) binding are noted at low metal ion concentrations, where (Z)-L-Gla-OMe is in excess. Under these conditions, complex formation with Mg(II) apparently favors a 2:1 Gla-magnesium ion complex in which both Gla residues are unstable to thermal decarboxylation. Calcium ion complexes, however, are found to favor a 3:1 Gla-calcium ion complex in which 1 of the 3 Gla residues is thermally stable.