1.  Introduction

Nucleotides are essential for many cellular functions, including the storage of genetic information, gene expression, energy metabolism, cell signaling, and biosynthesis. In a cell, nucleotides exist primarily as 5'-triphosphates. ATP is the most prevalent nucleotide, reaching mM concentrations in many cell types, while other nucleotides may be present at much lower concentrations (cAMP).

Biological significance of nucleotides

1. 1.  Building blocks of nucleic acids (DNA and RNA).

2.  Involved in energy storage, muscle contraction, active transport, and maintenance of ion gradients.

3.  Activated intermediates in biosynthesis (e.g. UD P-glucose, S-adenosylmethionine (SAM).

4.  Components of coenzymes (NAD⁺ , NADP⁺ , FAD, FMN, and CoA)

5.  Metabolic regulators:

a.   Second messengers (cAMP, cGMP)

b. Phosphate (PO₃^2- ) donors for phosphorylation of kinases and phosphatases in signal transduction (ATP)

c.  Regulation of some enzymes via adenylation and uridylylation

Each nucleotide contains a purine or pyrimidine base, a ribose or deoxyribose sugar, and a phosphate:

Fig. 25. strcuture of nucleotide

Sources of nucleotides in a cell:

1.  Degradation of nucleic acids (salvage pathways). Free purine bases can be recycled by coupling with the ribose phosphate moiety, 5-phospho-ribosyl-1-pyrophosphate (PRPP), to form nucleotide monophosphates:

Fig. 26 . Soruces of nucleotides (Benjamin cummings, 2008)