Structural and functional study of D-glucuronyl C5-epimerase [Protein Structure and Folding]

January 7th, 2015 by Qin, Y., Ke, J., Gu, X., Fang, J., Wang, W., Cong, Q., Li, J., Tan, J., Brunzelle, J. S., Zhang, C., Jiang, Y., Melcher, K., Li, J.-p., Xu, H. E., Ding, K.

Heparan sulfate (HS) is a glycosaminoglycan present on the cell surface and in the extracellular matrix which interacts with diverse signal molecules and is essential for many physiological processes including embryonic development, cell growth, inflammation, and blood coagulation. D-glucuronyl C5-epimerase (Glce) is a crucial enzyme in HS synthesis, converting D-glucuronic acid (GlcA) to L-iduronic acid (IdoA) to increase HS flexibility. This modification of HS is important for protein ligand recognition. We have determined the crystal structures of Glce in apo form (unliganded) and in complex with heparin hexasaccharide (product of Glce following O-sulfation), both in a stable dimer conformation. A Glce dimer contains two catalytic sites, each at a positively charged cleft in C-terminal α-helical domains binding one negatively charged hexasaccharide. Based on the structural and mutagenesis studies, three tyrosine residues, Y468, Y528, and Y546 in the active site were found to be crucial for the enzymatic activity. The complex structure also reveals the mechanism of product inhibition, i.e. 2-O- and 6-O-sulfation of HS keeps the C5 carbon of IdoA away from the active-site tyrosine residues. Our structural and functional data advance understanding of the key modification regulation in HS biosynthesis.