The mechanism of sugar phosphate isomerization by glucosamine 6-phosphate synthase.
Glucosamine 6-phosphate synthase converts fructose-6P into glucosamine-6P or glucose-6P depending on the presence or absence of glutamine. The isomerase activity is associated with a 40-kDa C-terminal domain, which has already been characterized crystallographically. Now the three-dimensional structures of the complexes with the reaction product glucose-6P and with the transition state analog 2-amino-2-deoxyglucitol-6P have been determined. Glucose-6P binds in a cyclic form whereas 2-amino-2-deoxyglucitol-6P is in an extended conformation. The information on ligand-protein interactions observed in the crystal structures together with the isotope exchange and site-directed mutagenesis data allow us to propose a mechanism of the isomerase activity of glucosamine-6P synthase. The sugar phosphate isomerization involves a ring opening step catalyzed by His504 and an enolization step with Glu488 catalyzing the hydrogen transfer from C1 to C2 of the substrate. The enediol intermediate is stabilized by a helix dipole and the epsilon-amino group of Lys603. Lys485 may play a role in deprotonating the hydroxyl O1 of the intermediate.
Teplyakov
A
A
Genetics and Biochemistry Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA. alext@gm-mv.niddk.nih.gov
Obmolova
G
G
Badet-Denisot
M A
MA
Badet
B
B
D019298:Glucose-6-Phosphate / Q000737:chemistry*; D005945:Glutamine-Fructose-6-Phosphate Transaminase (Isomerizing) / Q000737:chemistry*; D007535:Isomerases / Q000737:chemistry; D007536:Isomerism; D008958:Models, Molecular; D015394:Molecular Structure
D019298:Glucose-6-Phosphate; D005945:Glutamine-Fructose-6-Phosphate Transaminase (Isomerizing); D007535:Isomerases