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). Nucleotides are intimately involved in the energy reactions of the cell (example: ATP is the energy currency of the cell). Nucleotides control several metabolic reactions by their action as allosteric regulators. Cyclic AMP and cyclic GMP are the second messengers in hormonal function.