Nucleotides

Nucleotides are nitrogen containing organic compounds, which form the monomers of nucleic acids that are involved in the information transfer system of the cells. They are also involved in the mechanism of energy transfer in cells.

A nucleotide is a compound containing carbon, hydrogen, oxygen, nitrogen, and phosphorous. A molecule of a nucleotide is in turn composed of three smaller molecules phosphate (P) sugar (S) and nitrogen base (N).

• The phosphate group is represented by phosphoric acid (H₃PO₄).

• The sugar molecule in the nucleotide is a 5-carbon pentose sugar. It is represented by either ribose sugar (C₅H₁₀O₅) or deoxyribose sugar (C₅H₁₀O₄). Both the sugars have a furanose ring structure.

• The nitrogen base is represented by compounds having nitrogen and carbon in the ring structure. Two types of nitrogen bases occur, namely

a) Purines, which have a double ring structure and

b) Pyrimidines, which have a single ring structure.

fig. 13.14 - Components of Nucleic Acids

Purines are of two types adenine (A) and guanine (G). Pyrimidines are of three types Cytosine (C), Thymine (T), and Uracil (U).

fig. 13.15 - Structures of Nitrogenous Bases in Nucleic Acids

The nitrogen base molecule is attached to the sugar molecule by a glycosidic bond. A combination of nitrogen base with sugar is called as a nucleoside. Nucleosides involving ribose sugars are called ribonucleosides. Similarly, nucleosides involving deoxyribose sugars are called deoxy ribonucleosides.

A nucleoside combines with a phosphate group to form a compound called nucleotide. Nucleotides formed by ribonucleosides are called ribonucleotides. They form the monomers of ribose nucleic acid (RNA). Nucleotides formed by deoxyribonucleosides are called deoxyribonucleotides. They form the monomers of deoxyribose nucleic acid (DNA).

fig. 13.6 - Formation of a Nucleotide

The nucleotides which form nucleic acids have only one phosphate group (monophosphates). Each of them can form a diphosphate and a triphosphate. Linkage of additional phosphate group occurs at the cost of a large amount of energy. The bonds that join the additional phosphate groups are called as high energy or energy rich bonds. Separation of these additional phosphate groups from the nucleotides by enzymatic hydrolysis releases correspondingly large amount of energy. Hence, these higher nucleotides (with one or two additional phosphates) form energy rich compounds.

Adenine + Pentose Sugar - Adenosine (Adenine Nucleoside)

Adenosine + Phosphate - Adenylic Acid or Adenosine Mono Phosphate (AMP)(Adenine Nucleotide)

Adenosine Monophosphate + Phosphate - Adenosine Diphosphate(ADP)

Adenosine Diphosphate + Phosphate - Adenosine Triphosphate (ATP)

Similarly, for other nitrogen bases.

The nucleotides by combining with other organic compounds or molecules form co-enzymes. For example, NAD Nicotinamide Adenine Dinucleotide, FAD-Flavin Adenine Dinucleotide.

Some nucleotides like cyclic AMP function as regulatory chemicals controlling diverse cellular functions.