Investigation of the Local Structure and Dynamics of the H Subunit of the Mitochondrial Glycine Decarboxylase Using Heteronuclear NMR Spectroscopy

The lipoate-dependent H protein plays a pivotal role in the catalytic cycle of the glycine decarboxylase complex (GDC), undergoing reducing methylamination, methylene transfer, and oxidation. The local structure and backbone dynamics of the methylamine-loaded H (Hmet), oxidized H (Hox), and H apoprotein (Hapo) have been investigated in solution. Filtered NOESY experiments using a [13C]Hmet as well as comparison of the heteronuclear shifts between the Hox and Hmet proteins demonstrate that the methylamine group is located inside a cleft of the protein. Furthermore, this group appears to be locked in this configuration as indicated by the high value of the activation energy (37 kcal/mol) of the global unloading reaction and by its restricted mobility, deduced from 13C relaxation measurements. Comparisons of the 1H and 15N chemical shifts and 15N relaxation in the three forms suggest that part of the lipoyl−lysine arm interacts with the protein polypeptide in the Hox and Hmet. The major change induced by the...