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. Author manuscript; available in PMC: 2022 Dec 1.
Published in final edited form as: Trends Endocrinol Metab. 2021 Oct 29;32(12):980–993. doi: 10.1016/j.tem.2021.09.006

Figure 2. Monosaccharide metabolism is metabolically channeled within a cell.

Figure 2.

The most common extracellular monosaccharides of the brain are glucose, glucosamine, fructose, mannose, fucose, and galactose, and they can directly enter cells through the GLUT family of membrane transporters. Additionally, monosaccharides can be utilized for the biosynthesis of N-linked glycan substrates also known as sugar-nucleotides that include: UDP-GlcNAc, CMP-Sialic acid, GDP-mannose, UDP-glucose, GDP-fucose, and UDP-galactose. For example, UDP-GlcNAc can be synthesized from glucose, glucosamine, fructose, and mannose based on monosaccharides availability. Brain glycogen provides an additional intracellular pool of glucose and glucosamine. UDP-glucose is the substrate for glycogen but also the substrate for UDP-galactose, further adding to the complexity of sugar-nucleotide metabolism. Sugar-nucleotides are synthesized in the cytoplasm, yet N-linked glycan assembly occurs in the ER and Golgi. Early N-linked glycan biosynthesis occurs in the ER and primarily utilizes GlcNAc and mannose as the basic building blocks. N-linked glycan maturation occurs in the Golgi, additional branching occurs in the cis Golgi, galactose additions occur in the medial Golgi, and fucose and sialic acid are linked in the trans Golgi. Collectively, monosaccharide metabolism and N-linked glycan biosynthesis are complex processes traversing multiple cellular compartments.