Figure 9.11 Oxidation products of glucose. Gluconic acid is an aldonic acid formed when the aldehyde group is oxidized. Glucuronic acid, a uronic acid, is a result of oxidation of the primary alcohol group. When both the aldehyde and the primary alcohol groups are oxidized, glucaric acid is formed, which is an aldaric acid.
carbohydrate metabolism and the uronic acids are important constituents of the glycosaminoglycans. Glucuronic acid, formed from glucose, plays a particularly useful role in the detoxification of many compounds before their excretion by the kidneys. It is capable of assisting their removal by forming esters with an alcohol or phenolic group and many drugs and acids are excreted in the urine conjugated with glucuronic acid in this way.
Aldaric acids are formed when stronger oxidizing conditions are employed, such as nitric acid, when both the aldehyde and primary alcohol groups are oxidized.
> Nucleic acids - see Section 13.1.
Members of this group of compounds, which includes the extremely important nucleosides, are formed when the carbon atom of a monosaccharide, or often its deoxy derivative (Figure 9.12), is linked directly to the nitrogen of a nitrogenous base, including the amino group of an amino acid in a peptide chain (Figure 9.13), with the loss of the hydroxyl group. Such A'-glycosidic linkages must not be confused with the bonding in a glycoside, which is through an oxygen atom.
Those nucleosides found in the nucleic acids DNA and RNA involve the joining of ribose of deoxyribose to a purine or a pyrimidine base. One such nucleoside is adenosine, in which a nitrogen of adenine is linked to carbon 1 of the pentose, ribose. In this form it is a component of RNA but as a phosphory-lated derivative of adenosine (e.g. ATP), which is a high energy compound, it fulfils an important role in metabolism. The dinucleotides NAD and NADP are two cofactors necessary for many enzymic transformations and these also contain A'-glycosides of ribose phosphate. Other important nucleosides are found
Figure 9.12 Deoxy derivatives. These contain one less oxygen atom than the monosaccharide from which they are derived. 2-Deoxyribose is a most important deoxy pentose and is a major constituent of deoxyribonucleic acid (DNA). Deoxy hexoses are widely distributed among plants, animals and microorganisms especially as components of complex polysaccharides. Examples are rhamnose (6-deoxymannose), a component of bacterial cell walls, and fucose (6-deoxygalactose), which is often found in glycoproteins and is an important constituent of human blood group substances.
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