| Abstract
| - The lipoate containing H-protein plays a pivotal role in the catalytic cycle of the glycinedecarboxylase complex (GDC), undergoing reducing methylamination, methylene transfer, and oxidation.The transfer of the CH2 group is catalyzed by the T-protein, which forms a 1:1 complex with themethylamine-loaded H-protein (Hmet). The methylamine group is then deaminated and transferred to thetetrahydrofolate-polyglutamate (H4FGlun) cofactor of T-protein, forming methylenetetrahydrofolate-polyglutamate. The methylamine group is buried inside the protein structure and highly stable. Experimentaldata show that the H4FGlun alone does not induce transfer of the methylene group, and molecular modelingalso indicates that the reaction cannot take place without significant structural perturbations of the H-protein.We have, therefore, investigated the effect of the presence of the T-protein on the stability of Hmet.Addition of T-protein without H4FGlun greatly increases the rate of the unloading reaction of Hmet, reducingthe activation energy by about 20 kcal mol-1. Differences of the 1H and 15N chemical shifts of the H-proteinin its isolated form and in the complex with the T-protein show that the interaction surface for the H-proteinis localized on one side of the cleft where the lipoate arm is positioned. This suggests that the role of theT-protein is not only to locate the tetrahydrofolate cofactor in a position favorable for a nucleophilicattack on the methylene carbon but also to destabilize the H-protein in order to facilitate the unlocking ofthe arm and initiate the reaction.
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