Sugar acids
Prof. Sylvester L.B. Kajuna
 Sugar acids are monosaccharides with a carboxyl
group.
Main classes of sugar acids include:
 Aldonic acids, in which the aldehyde functional group of
an aldose is oxidized
 Ulosonic acids, in which the first hydroxyl group of a 2ketose is oxidised creating an α-ketoacid.
 Uronic acids, in which the terminal hydroxyl group of an
aldose or ketose is oxidized
 Aldaric acids, in which both ends of an aldose are
oxidized
Ulosonic acid
Aldonic acid
Uronic acid
Aldaric acid
 Examples of sugar acids include:
 Aldonic acids
 Glyceric acid (3C)
 Xylonic acid (5C)
 Gluconic acid (6C)
 Ascorbic acid (6C, unsaturated lactone)
 Ulosonic acids
 Neuraminic acid (5-amino-3,5-dideoxy-D-glycero
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D-galacto-non-2-ulosonic acid)
Ketodeoxyoctulosonic acid (KDO or 3-deoxy-Dmanno-oct-2-ulosonic acid)
Uronic acids
Glucuronic acid (6C)
Galacturonic acid (6C)
Iduronic acid (6C)
 Aldaric acids
 Tartaric acid (4C)
 meso-Galactaric acid (Mucic acid) (6C)
 D-Glucaric acid (Saccharic acid) (6C)
 Ascorbic acid (Vitamin C)
Amino sugars
 an amino sugar (or more technically a 2-amino-
2-deoxysugar) is a sugar molecule in which a
hydroxyl group has been replaced with an amino
group. More than 60 amino sugars are known, with
one of the most abundant being N-Acetyl-Dglucosamine, which is the main component of chitin.
 Derivatives of amine containing sugars, such as N-
acetylglucosamine and sialic acid, whose nitrogens
are part of more complex functional groups rather
than formally being amines, are also considered
amino sugars. Aminoglycosides are a class of
antimicrobial compounds that inhibit bacterial
protein synthesis. These compounds are conjugates
of amino sugars and aminocyclitols.
Glucosamine
Sialic acid
Neuraminic acid (Sialic Acid)
Sialic acid
Glycosaminoglycans
(mucopolysaccharides)
 Mucopolysaccharides are linear polymers of
repeating disaccharides. The constituent
monosaccharides tend to be modified, with acidic
groups, amino groups, sulfated hydroxyl and amino
groups, etc. Glycosaminoglycans tend to be negatively
charged, because of the prevalence of acidic groups
(glucuronic acid residues).
 Hyaluronate (hyaluronan) is a glycosaminoglycan with
a repeating disaccharide consisting of two glucose
derivatives, glucuronate (glucuronic acid) and Nacetylglucosamine. The glycosidic linkages are b(1→�3)
and b(1→�4).
Proteoglycans
 Proteoglycans are glycosaminoglycans that are
covalently linked to serine residues of specific core
proteins. The glycosaminoglycan chain is synthesized
by sequential addition of sugar residues to the core
protein.
 Some proteoglycans of the extracellular matrix bind
non-covalently to hyaluronate via protein domains
called link modules. For example:
 Multiple copies of the aggrecan proteoglycan associate
with hyaluronate in cartilage to form large complexes.
 Versican, another proteoglycan, binds hyaluronate in
the extracellular matrix of loose connective tissue.
 Heparan sulfate is initially synthesized on a
membrane-embedded core protein as a polymer of
alternating glucuronate and Nacetylglucosamine residues. Later, in segments of
the polymer, glucuronate residues may be converted
to the sulfated sugar iduronic acid, while Nacetylglucosamine residues may be deacetylated
and/or sulfated.
 Heparin, a soluble glycosaminoglycan found in granules
of mast cells, has a structure similar to that of heparan
sulfates, but is relatively highly sulfated. When released
into the blood, it inhibits clot formation by interacting with
the protein antithrombin. Heparin has an extended
helical conformation. Charge repulsion by the many
negatively charged groups may contribute to this
conformation. The heparin molecule includes 10 residues,
alternating IDS (iduronate-2-sulfate) and SGN (N-sulfoglucosamine-6-sulfate).
Oligosaccharides of glycoproteins and glycolipids
 Oligosaccharides that are covalently attached to
proteins or to membrane lipids may be linear or
branched chains. They often include modified
sugars, e.g., acetylglucosamine, etc.
 O-linked oligosaccharide chains of glycoproteins
vary in complexity. They link to a protein via a
glycosidic bond between a sugar residue and a serine
or threonine hydroxyl. O-linked oligosaccharides
have roles in recognition, interaction.
Glycoconjugates
 Glycoconjugates is the general classification for
carbohydrates covalently linked with other chemical
species such as proteins, peptides, lipids and
saccharides. Glycoconjugates are formed in
processes termed glycosylation.
 Glycoconjugates are very important compounds in
biology and consist of many different categories such as
glycoproteins, glycopeptides, peptidoglycans, glycolipids,
glycosides and lipopolysaccharides. They are involved in
cell–cell interactions, including cell–cell recognition; in
cell–matrix interactions; in detoxification processes.
 Generally the carbohydrate part(s) play an integral role
in the function of a glycoconjugate; prominent examples
of this are the blood proteins where fine details in the
carbohydrate structure determine cell binding or not or
lifetime in circulation.
Glycoproteins
 Glycoproteins are proteins that contain
oligosaccharide chains (glycans) covalently attached
to polypeptide side-chains. The carbohydrate is
attached to the protein in a cotranslational or
posttranslational modification. This process is
known as glycosylation.
Types of glycosylation
 There are several types of glycosylation, although the first two
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are the most common.
In N-glycosylation, sugars are attached to nitrogen, typically
on the amide side-chain of asparagine.
In O-glycosylation, sugars are attached to oxygen, typically on
serine or threonine but also on non-canonical amino acids
such as hydroxylysine and hydroxyproline.
In P-glycosylation, sugars are attached to phosphorus on a
phosphoserine.
In C-glycosylation, sugars are attached directly to carbon,
such as in the addition of mannose to tryptophan.
In glypiation, a GPI (Glycosylphosphatidylinositol ) glycolipid
is attached to the C-terminus of a polypeptide, serving as a
membrane anchor.