STRUCTURAL STUDIES OF THE GLYCINE DECARBOXYLASE COMPLEX FROM PEA LEAFMITOCHONDRIA

The glycine decarboxylase complex consists of four different component enzymes (P-, H-, T-and L-proteins). The 14-kDa lipoamide-containing H -protein plays a pivotal role in the complete sequence of reactions si nce its prosthetic group (lipoic acid) interacts successively with the three other components of the complex and undergoes a cycle of reduct ive methylamination, methylamine transfer and electron transfer. The X -ray crystal structure of different forms of the H-protein has shown a unique conformation of the protein. This leads to the hypothesis of a three-dimensional recognition of the H-protein by the other component s of the system and also by the ligase which lipoylates the H-protein. Striking structural similarities are observed between the H-protein a nd other lipoate domains of 2-oxo acid dehydrogenases and with the bio tin carrier protein of acetyl-CoA carboxylase. In the H-protein, the l ipoamide arm is free to move in the solvent when oxidized but is pivot ed and tightly bound into a cleft at the protein surface when methylam ine-loaded. This implies that the H-protein and the T-component form a stable complex during the catalytic transfer of the methylene unit to the tetrahydrofolate cofactor of the T-protein, This complex has been detected by small angle scattering experiments. In conclusion, in the glycine decarboxylase system, the lipoamide arm does not swing freely from one catalytic site to another as was proposed in other systems.