Sugar in nucleic acids

Catterall H, Davies MJ, Gilbert BC (1992) An EPR study of the transfer of radical-induced damage from the base to sugar in nucleic acid components relevance to the occurrence of strand-breakage. J Chem Soc Perkin Trans 2 1379-1385... 

The sugars are typically ribose (ribonucleic acids, RNA), or 2-deoxyribose (deoxyribonucleic acids, DNA). There are five common bases in nucleic acids adenine (A) thymine (T) uracil (U) cytosine (C) and guanine (G). DNA polymers incorporate the four bases. A, T, C, and G, and RNA, the set A, U, C, and G. 

As is well-known, nucleic acids consist of a polymeric chain of monotonously reiterating molecules of phosphoric acid and a sugar. In ribonucleic acid, the sugar component is represented by n-ribose, in deoxyribonucleic acid by D-2-deoxyribose. To this chain pyrimidine and purine derivatives are bound at the sugar moieties, these derivatives being conventionally, even if inaccurately, termed as pyrimidine and purine bases. The bases in question are uracil (in ribonucleic acids) or thymine (in deoxyribonucleic acids), cytosine, adenine, guanine, in some cases 5-methylcytosine and 5-hydroxymethylcyto-sine. In addition to these, a number of the so-called odd bases occurring in small amounts in some ribonucleic acid fractions have been isolated. 

Figure 1.40 The two forms of sugar residues commonly found in nucleic acids. 3-D-Ribose is the sugar constituent of RNA, while p-D-2-deoxyribose is a component of DNA.

In RNA, the base T found in DNA is replaced by uracil, which is similar in structure to T, but lacks the methyl group. The nucleotides in nucleic acids are linked by phosphodiester bonds between the 3 -hydroxyl of one nucleoside and the 5 -hydroxyl of the sugar of its neighbour in the sequence, as was first shown by Alexander Todd3 in 1952 (Figure 4.13). 

Nucleic acids possess sugar-phosphate backbones, whose net charges do not change over a relatively wide range of pH. Thus, charge densities are nearly constant for different nucleic acids, as their net charge is proportional to the number of residues (i.e. mass). Therefore, the pH of the medium is not as critical in nucleic acid electrophoretic characterization. 

In nucleic acids, the cross-correlation studies were applied to investigation of the sugar conformation [101-103] of the phosphodiester backbone [65] as well as to some more spe-... 

Figure 1-1-3. Five-Carbon Sugars Commonly Found In Nucleic Acids...

The number of possible forms is reduced somewhat by the fact that one of the nitrogens is bonded to the sugar in the nucleic acid it no longer carries a hydrogen to participate in tautomerism. The tautomeric forms indicated are found to predominate in nucleic acids. The oxygen substituents exist almost entirely as carbonyl groups, whereas... 

Riboflavin contains an isoalloxazine ring linked to the reduced sugar ribitol. The sugar unit in riboflavin is the non-cyclic ribitol, so that FAD and FMN differ somewhat from the nucleotides we encounter in nucleic acids. 

Nature is also selective in the geometry involved in nucleic acid synthesis. This specificity involves both the base order and the particular sugar employed. For DNA the employed sugar is (3-2-deoxy-D-ribose, deoxyribose (below left). Deoxyribose has three chiral centers but only one of them is employed in the synthesis of nucleic acids. Ribose, the sugar employed in the synthesis of RNA, has four geometric sites (below right). 

Many kinds of organisms and some mammalian organs, notably liver, possess an alternative pathway for the oxidation of hexoses which results in a pentose phosphate and carbon dioxide. This pentose can be used as a precursor of the ribose found in nucleic acids or other sugars containing from three to seven carbon atoms which are needed in smaller amounts. The first and third reactions in the pentose phosphate pathway generate NADPH which is a major source of reducing power in many cells. 

The structures of sugars and polysaccharides are covered in the appropriate chapters within part 4 just prior to discussing their metabolism. Similarly, the structures of lipids are presented in the lipid metabolism chapters found in part 5. Nucleotide structures are addressed in chapter 23 before considering their metabolism. Finally, nucleic acid and nucleoprotein structures are examined in the first chapter (chapter 25) of part 7 prior to the discussion of the roles these molecules play in nucleic acid and protein metabolism in the six subsequent chapter. ... 

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