REFINED STRUCTURES AT 2 AND 2.2-ANGSTROM RESOLUTION OF 2 FORMS OF THEH-PROTEIN, A LIPOAMIDE-CONTAINING PROTEIN OF THE GLYCINE DECARBOXYLASE COMPLEX

H-protein, a 14kDa lipoic acid-containing protein is a component of th e glycine decarboxylase complex. This complex which consists of four p rotein components (P-, H-, T- and L-protein) catalyzes the oxidative d ecarboxylation of glycine. The mechanistic heart of the complex is pro vided by the lipoic acid attached to a lysine residue of the II-protei n. It undergoes a cycle of transformations, i.e. reductive methylamina tion, methylamine transfer, and electron transfer. We present details of the crystal structures of the II-protein, in its two forms, H-Pro(O x), with oxidized lipoamide and H-Pro(Met) with methylamine-loaded lip oamide. X-ray diffraction data were collected from crystals of H-Pro(O x) to 2 Angstrom and H-Pro(Met) to 2.2 Angstrom resolution. The final R-factor value for the H-Pro(Ox) is 18.5% for data with F>2 sigma in t he range of 8.0-2.0 Angstrom resolution. The refinement confirmed our previous model, refined to 2.6 Angstrom, of a beta-fold sandwich struc ture with two beta-sheets. The lipoamide arm attached to Lys63, locate d in the loop of a hairpin conformation, is clearly visible at the sur face of the protein. The H-Pro(Met) has been crystallized in orthorhom bic and monoclinic forms and the structures were solved by molecular r eplacement, starting from the H-Pro(Ox) model. The orthorhombic struct ure has been refined with a final R-factor value of 18.5% for data wit h F> 2 sigma in the range of 8.0-2.2 Angstrom resolution. The structur e of the monoclinic form has been refined with a final R-factor value of 17.5% for data with F > 2 sigma in the range of 15.0-3.0 Angstrom. In these two structures which have similar packing, the protein confor mation is identical to the conformation found in the H-Pro(Ox). The ma in change lies in the position of the lipoamide group which has moved significantly when loaded with methylamine. In this case the methylami ne-lipoamide group is tucked into a cleft at the surface of the protei n where it is stabilized ,by hydrogen bonds and hydrophobic contacts. Thus, it is totally protected and not free to move in aqueous solvent. In addition, the II-protein presents some sequence and structural ana logies with other lipoate- and biotin-containing proteins and also wit h proteins of the phosphoenolpyruvate:s ugar phosphotransferase system .