Biosynthesis of UDP- N -acetyl- D -glucosamine (UDP-GlcNAc). The pathways from [U-13C]-glucose to the four biochemical subunits are outlined. The glucose moiety (red) is directly incorporated into UDP-GlcNAc. The acetyl moiety (blue) is incorporated via glycolysis. The ribose moiety (yellow) is incorporated via the pentose phosphate pathway and pyrimidine biosynthesis. The uracil moiety is derived from acetyl-CoA through the Krebs cycle to form aspartate where it is combined with carbamoyl phosphate leading to pyrimidine synthesis (green).