They are the building blocks or monomeric units in the nucleic acid (DNA and RNA) structure. Nucleotides are the structural components of several coenzymes of B complex vitamin (examples: FAD, NAD+, NADP+, and coenzyme A). They serve as carriers of high-energy intermediates in the biosynthesis of carbohydrates, lipids, and proteins (example: UDP-glucose, CDP-acylglycerol, and S-adenosylmethionine).…

When a single phosphate moiety is added to a nucleoside, it is termed as a mononucleotide. For example, adenosine monophosphate (AMP) contains adenine + ribose + phosphate. The common nucleotides are given in Table 5.3.   Table 5.3 Common Nucleotides

Nucleotides are the phosphoric acid esters of nucleosides. In ribose moiety of a ribonucleoside, phosphorylation is possible only at three position (C2′, C3′, and C5′) since C1′ and C4′ are involved in the furanose ring formation. In deoxyribosenucleoside, only two positions (C3′ and C5′) are available for phosphorylation.

The nucleosides are generally named for the particular purine or pyrimidine present. Nucleosides containing ribose are ribonucleoside, while those possessing deoxyribose are deoxyribonucleoside. The common nucleosides are given in Table 5.2.   Table 5.2 Common Nucleosides

DEFINITION The nucleotides are compounds in which nitrogenous bases (purine and pyrimidine) are conjugated to the pentose sugars (ribose or deoxyribose) by a β-glycosidic linkage. The β-glycosidic linkage involves C-1 of sugar and the hydrogen atom of N-9 (in the case of purines) or N-1 (in the case of pyrimidine).

The existence of a molecule in keto (lactam) and enol (lactim) form is known as tuatomerism. The heterocyclic rings of purines and pyrimidines with oxo functional groups exhibit tuatomerism as simplified below:

Purine derivatives are derived from the parent compound purine, which is made up of six-membered pyrimidine ring fused to the five-membered imidazole ring. The main purine derivatives found in the nucleic acids are adenine and guanine. Their structures are given below:

These are all derived from their parent heterocyclic compound, pyrimidine. The common pyrimidine derivatives found in nucleic acids are uracil, thymine, and cytosine. Their structures are given below:

Two types of nitrogenous bases are found in all nucleic acids. These are derivatives of pyrimidine and purine.