Carbohydrate Metabolism
: Glycoproteins, Glycolipids, GPI Anchors, Proteoglycans, and Peptidoglycans
donor sugar and a particular acceptor molecule. This speci-
ficity has led to the hypothesis of a unique glycosyltrans-
ferase for every linkage between two monosaccharides in
an oligosaccharide. Although not strictly true, the con-
cept of “one linkage-one enzyme” is useful in understand-
ing glycoconjugate synthesis. Many glycosyltransferases
are highly specific, using only one particular nucleotide
sugar as donor and requiring as acceptor an oligosac-
charide with the correct terminal and penultimate sugars
in the proper linkage. Other enzymes are less specific.
For example, /3-1,4-galactosyltransferase attaches galac-
tose to different acceptors, provided the terminal sugar is
Synthesis of glycoconjugates, in contrast to that of
DNA, RNA, and protein, is not directed by a template
but depends on the specificity of the enzymes that cat-
alyze addition of each sugar residue. Thus, oligosac-
charide chain biosynthesis may be less predictable than
DNA, RNA, or protein synthesis and more often results
in incomplete oligosaccharide chains. Furthermore, be-
cause there is no template from which to deduce the
oligosaccharide size or sequence in a given glycocon-
jugate, the synthetic reactions can best be described by
sequencing the product. The oligosaccharide reflects its
biosynthetic history, and understanding of the biosynthetic
pathways has hinged on structural analysis of oligosaccha-
The recent advances in the determination of oligosac-
charide structure using high-performance liquid chroma-
tography (HPLC), gas chromatography-mass spectrom-
etry (GC-MS), and nuclear magnetic resonance (NMR)
have increased the capacity to separate and sequence sug-
ars and their linkages in oligosaccharides. Analytical pro-
cedures must overcome several unique sequencing prob-
lems that include:
1. Lack of unique characteristics (such as catalytic
activity) that can be used to follow purification;
2. Presence of more than one type of oligosaccharide
side chain in a particular glycoprotein;
3. Presence of more than one type of glycosidic bond
within an oligosaccharide, in contrast to the peptide
bond that universally links amino acids to
4. Chemical similarity of one sugar to another, which
leads to poor separation in a number of analytical
systems and makes identification of a particular sugar
more difficult. Except for the purification steps,
determination of oligosaccharide structure is now
relatively routine.
or glycosphingolipids,
have a core structure with arms containing terminal sugar
sequences (Figure 16-2). In some cases the arms may
consist of repeating Gal and GlcNAc referred to as poly-
lactosamine. Figure 16-1 shows the structures of several
oligosaccharides found in different types of glycoconju-
gates, together with the abbreviations used in writing these
structures (see also Chapters 10 and 11).
The high degree of specificity of the glycosyltrans-
ferases necessitates a detailed scheme for their nomen-
clature. Criteria used to classify glycosyltransferases are
the nucleotide-sugar, the transferred sugar, the monosac-
charide at the acceptor site, and the linkage. In addition,
transferases can show specificity for the location of the
acceptor site, i.e., the protein or lipid to which the accep-
tor is attached or the exact location of the acceptor on the
This specificity can be indicated in one of two ways:
1. by specifying the sugar acceptor, linkage, and sugar
donor (e.g., GlcNAc-//1,4-galactosyltransferase) or
2. when there are a number of glycosyltransferases with
similar acceptor and linkages, a Roman numeral is
placed after the name derived from the above criteria
(e.g., a family of UDP-N-acetylglucosaminyltrans-
ferases are numbered I-VI based on the particular
acceptor sites to which they transfer GlcNAc
(Figure 16-3).
Uridine diphosphate (UDP) is the most common nu-
cleotide carrier for sugars, although cytidine monophos-
phate (CMP)) and guanosine diphosphate (GDP) are also
used. The most abundant nucleotide-sugar (and the first
discovered) is UDP-glucose. Its structure and a list of
other nucleotide-sugar types are given in Figure 16-4. See
Chapter 15 for a discussion of the synthesis of UDP-
glucose and its role in glycogen synthesis. Figure 16-5
summarizes biosynthetic pathways for UDP-glucose and
other nucleotide-sugars important for glycoconjugate syn-
thesis. The sugar donors can be synthesized from glu-
cose, provided that the requisite enzymes are present,
and the interconversion of fucose to other sugars does
not occur. Similarly, N-acetylmannosamine
in vivo
to be entirely metabolized to N-acetylneuraminic acid
(sialic acid), despite the existence of alternative reaction
pathways, as shown in Figure 16-5). As a result, radioac-
tively labeled fucose and N-acetylmannosamine, when ad-
ministered experimentally in animals, can mark specific
glycosylation sites within cells.
Several sugar-nucleotide pool defects have been iden-
Type I carbohydrate-deficient glycoprotein syn-
(CDGs type I), a defect in phosphomannomu-
tase, one of the enzymes responsible for converting
glucose to GDP-mannose, results in an absence of
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