Human Biochemistry, Volume 2 - Murray R. 1993

Special Topics
Glycoproteins and Proteoglycans
Glycoproteins - Nucleotide Sugars

The first nucleotide sugar to be described was uridine diphosphate glucose (UDPGlc). Common nucleotide sugars involved in glycoprotein Biosynthesis are listed in Table 54.3; the reasons why some contain UDP while others contain guanosine diphosphate (GDP) or cytidine monophosphate remain unclear. Many, but not all, glycosylation reactions in glycoprotein biosynthesis utilize these compounds (see below). The bond between the phosphate group and the sugars is of an anhydride nature, belonging to the Class of high-energy bonds with a high group-transfer potential. Sugars in such compounds are thus "activated" and, in the presence of appropriate transferases, can be transferred to suitable acceptors.

Nucleotide sugars are formed in the Cytosol, typically with the participation of the corresponding nucleoside triphosphates. The formation of uridine diphosphate galactose

Table 54.3. Major sugars present in human Glycoproteins. The structures of most listed sugars are described in Chap. 14

Sugar

Type

Abbreviation

Note

Galactose

Hexose

Gal

Often occupies a preterminal position before NeuAc in N-linked glycoproteins. Also found in the trisaccharide core of Proteoglycans

Glucose

Hexose

Glc

Present at the biosynthesis stage of N-linked glycoproteins, but not always found in mature glycoproteins

Mannose

Hexose

Man

A common sugar in N-linked glycoproteins

N-Acetylneuraminic

acid

Sialic acid

(9 C atoms)

NeuAc

Often serves as the terminal sugar in N- and O-linked glycoproteins. Other sialic acids are known, but NeuAc is the major species found in humans

Fucose

Deoxyhexose

Fuc

May occupy an outer position in N- and O-linked glycoproteins or be linked to the GlcNAc residue attached to Asn in N-linked glycoproteins

N-Acetylgalactosamine

Amino hexose

GalNAc

Present in both N- and O-linked glycoproteins

N-Acetylglucosamine

Amino hexose

GlcNAc

This sugar is attached to the polypeptide chain of N-linked glycoproteins via an Asn residue; it is also present in other PARTS OF THE oligosaccharide components of these Proteins

Since many glycosylation reactions occur within the lumen of the Golgi apparatus, nucleotide sugars are transported across the Golgi membranes. Systems that transport UDP-Gal, GDP-Man, and CMP-NeuAc into the Golgi cisternae have been described. These are antiport systems, meaning that the influx of one nucleotide sugar molecule is balanced by the efflux of one molecule of the corresponding nucleotide (e.g., UMP, GMP, or CMP) generated from the nucleotide sugars. Such a mechanism ensures an adequate concentration of each nucleotide sugar within the Golgi apparatus. UMP is formed from UDPGal as follows:



Last update: 06/08/2026

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