Heparan sulfate biosynthesis: A theoretical study of the initial sulfationstep by N-deacetylase/N-sulfotransferase

Heparan sulfate N-deacetylase/N-sulfotransferase (NDST) catalyzes the deace tylation and sulfation of N-acetyl-D-glucosamine residues of heparan sulfat e, a key step in its biosynthesis. Recent crystallographic and mutational s tudies have identified several potentially catalytic residues of the sulfot ransferase domain of this enzyme (Kakuta et al., 1999, J. Biol. Chem. 274:1 0673-10676). We have used the x-ray crystal structure of heparan sulfate N- sulfotransferase with 3'-phosphoadenosine 5'-phosphate to build a solution model with cofactor 3'-phosphoadenosine 5'-phosphosulfate (PAPS) and a mode l heparan sulfate ligand bound, and subsequently performed a 2-ns dynamics solution simulation. The simulation results confirm the importance of resid ues Glu(642), Lys(614), and Lys(833), with the possible involvement of Thr( 617) and Thr(618), in binding PAPS. Additionally, Lys(676) is found in clos e proximity to the reaction site in our solvated structure. This study illu strates for the first time the possible involvement of water in the catalys is. Three water molecules were found in the binding site, where they are co ordinated to PAPS, heparan sulfate, and the catalytic residues.