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 2-ketose 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-D-galacto-non-2-ulosonic acid) Ketodeoxyoctulosonic acid (KDO or 3-deoxy-D-manno-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-d-glucosamine, 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 N-acetylglucosamine. 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 N-acetylglucosamine residues. Later, in segments of the polymer, glucuronate residues may be converted to the sulfated sugar iduronic acid, while N-acetylglucosamine 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-sulfo-glucosamine-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 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.