SIS (Sugar ISomerase) domain repeat 2 found in Glucosamine 6-phosphate synthase (GlmS) and Glucosamine-6-phosphate deaminase (GlmD). The SIS domain is found in many phosphosugar isomerases and phosphosugar binding proteins. GlmS contains a N-terminal glutaminase domain and two C-terminal SIS domains and catalyzes the first step in hexosamine metabolism, converting fructose 6-phosphate into glucosamine 6-phosphate using glutamine as nitrogen source. The glutaminase domain hydrolyzes glutamine to glutamate and ammonia. Ammonia is transferred through a channel to the isomerase domain for glucosamine 6-phosphate synthesis. The end product of the pathway is N-acetylglucosamine, which plays multiple roles in eukaryotic cells including being a building block of bacterial and fungal cell walls. In the absence of glutamine, GlmS catalyzes the isomerization of fructose 6-phosphate into glucose 6- phosphate (PGI-like activity). Glucosamine-6-phosphate deaminase (GlmD) contains two SIS domains and catalyzes the deamination and isomerization of glucosamine-6-phosphate into fructose-6-phosphate with the release of ammonia; in presence of high ammonia concentration, GlmD can catalyze the reverse reaction.
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