UDP-N-acetylgalactosamine biosynthesis (Saccharomyces cerevisiae)

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UDP-N-acetyl-D-glucosamine (UDP-GlcNAc) is a sugar nucleotide utilized for the biosynthesis of chitin, a major structural polysaccharide in the yeast cell wall. UDP-GlcNAc also provides GlcNAc moieties for N-linked glycosylation of proteins and for the biosynthesis of glycosylphosphatidylinositol (GPI), a lipid anchor involved in tethering proteins to the cell membrane. Synthesis of UDP-GlcNAc is an essential process in S. cerevisiae; conditional depletion of several of the enzymes in the pathway (Gna1p, Pcm1p, Qri1p) results in aberrant cell morphology and increased cell lysis.

In S. cerevisiae, UDP-GlcNAc is synthesized in four steps from fructose-6-phosphate (F6P). First, F6P is converted to glucosamine-6-phosphate (GlcN6P) by glutamine:fructose-6-phosphate amidotransferase (GFAT). Second, GlcN6P is N-acetylated by GlcN6P acetyltransferase (Gna1p). This forms GlcNAc-6-P, which in the third step of the pathway is isomerized to GlcNAc-1-P by phosphoacetylglucosamine mutase (Pcm1p). Finally, GlcNAc-1-P is condensed with UTP via UDP-GlcNAc pyrophosphorylase (Qri1p), yielding UDP-GlcNAc. The UDP-GlcNAc biosynthesis pathway described in yeast occurs similarly in other eukaryotes, however in the bacterial pathway GlcN-6-P is initially isomerized to GlcN-1-P prior to N-acetylation.

Description from [YeastPathways](https://pathway.yeastgenome.org/).