NUCLEOTIDE

any member of a class of organic compounds in which the molecular structure comprises a nitrogen-containing unit (base) linked to a sugar and a phosphate group. The nucleotides are of great importance to living organisms, as they are the building blocks of nucleic acids, the substances that control all hereditary characteristics. In the two families of nucleic acids, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), the sequence of nucleotides in the DNA or RNA codes for the structure of proteins synthesized in the cell. The nucleotide adenosine triphosphate (ATP) supplies the driving force of many metabolic processes. Several nucleotides are coenzymes; they act with enzymes to speed up (catalyze) biochemical reactions. The nitrogen-containing bases of nearly all nucleotides are derivatives of three heterocyclic compounds: pyrimidine, purine, and pyridine. The most common nitrogen bases are the pyrimidines (cytosine, thymine, and uracil), the purines (adenine and guanine), and the pyridine nicotinamide. Nucleosides are similar to nucleotides except that they lack the phosphate group. Nucleosides themselves rarely participate in cell metabolism. Adenosine monophosphate (AMP) is one of the components of DNA and RNA and also the organic component of the energy-carrying molecule ATP (adenosine triphosphate). In certain vital metabolic processes, AMP combines with inorganic phosphate to form ADP (adenosine diphosphate) and then ATP. The breaking of the phosphate bonds in ATP releases great amounts of energy that are consumed in driving chemical reactions or contracting muscle fibres. Cyclic AMP, another nucleotide, is involved in regulating many aspects of cellular metabolism, such as the breakdown of glycogen. A dinucleotide, nicotinamide adenine dinucleotide (NAD), participates in many oxidation reactions as an electron carrier, along with the related compound nicotinamide adenine dinucleotide phosphate (NADP). These substances act as cofactors to certain enzymes.