Fig. 1: Structure of human GFAT-1, the key enzyme of the hexosamine pathway.

a Schematic representation of the hexosamine pathway (green box). The enzymes in the pathway are glutamine fructose-6-phosphate amidotransferase (GFAT-1/-2), glucosamine-6-phosphate N-acetyltransferase (GNA-1), phosphoglucomutase (PGM-3), UDP-N-acetylglucosamine pyrophosphorylase (UAP-1), and glucosamine-6-phosphate deaminase (GNPDA-1/−2). UDP-GlcNAc inhibits eukaryotic GFAT (red line). b Overall structure of the human GFAT-1 dimer in cartoon representation. The N-terminal glutaminase domains are colored in light blue and light gray, and the C-terminal isomerase domains in marine and dark gray. Glc6P (yellow sticks) and l-Glu (violet sticks) are highlighted, as well as important loops discussed in this manuscript: R-loop (green), Q-loop (red), and C-loop (orange). c Secondary structure elements of human GFAT-1. β-Sheets are colored in red and α-helices in blue. Glc6P (yellow sticks) and L-Glu (violet sticks) are highlighted. The isomerase domain (left) consists of two sugar isomerase (SIS) sub-domains. The glutaminase domain (right) is composed of a Ntn-hydrolase fold (αββα-core) with two short antiparallel β-sheets covering one side (β9 and β14). d, e Active sites of human GFAT-1. The protein is in cartoon representation; residues involved in substrate binding or catalysis are highlighted as sticks, and dashed lines indicate key interactions. d Frc6P-binding site formed by both isomerase domains. e l-Gln-binding site in one glutaminase domain.