The mechanism of sugar phosphate isomerization by glucosamine 6-phosphate synthase

Glucosamine 6-phosphate synthase converts fructose-6P into glucosamine-6P o r glucose-6P depending on the presence or absence of glutamine. The isomera se activity is associated with a 40-kDa C-terminal domain, which has alread y been characterized crystallographically. Now the three-dimensional struct ures of the complexes with the reaction product glucose-6P and with the tra nsition state analog 2-amino-2-deoxyglucitol-6P have been determined. Gluco se-6P binds in a cyclic form whereas 2-amino-2-deoxyglucitol-6P is in an ex tended conformation. The information on ligand-protein interactions observe d in the crystal structures together with the isotope exchange and site-dir ected mutagenesis data allow us to propose a mechanism of the isomerase act ivity of glucosamine-6P synthase. The sugar phosphate isomerization involve s a ring opening step catalyzed by His504 and an enolization step with Glu4 88 catalyzing the hydrogen transfer from C1 to C2 of the substrate. The ene diol intermediate is stabilized by a helix dipole and the epsilon-amino gro up of Lys603. Lys485 may play a role in deprotonating the hydroxyl O1 of th e intermediate.