Table of Contents
A nucleotide is regarded as the basic building block of nucleic acid polymers (e.g. DNA and RNA). It is an organic compound made up of three subunits: a nucleobase (either a purine or a pyrimidine), a five-carbon sugar (pentose), and a phosphate group. The sugar component may either be ribose or deoxyribose. The ribose sugar is the sugar component of the nucleotides that make up RNA. The deoxyribose sugar is the sugar component of DNA. Each phosphate group connects the sugar rings of two adjacent nucleotide monomers. The phosphate groups and the sugar moieties form the backbone of a nucleic acid. The directionality of the chain runs from 5′-end to 3′-end. In DNA, the orientation of the two strands is in opposite directions. This is to allow complementary base pairing between nucleobase constituents. Apart from the long chain of nucleic acids, nucleotides also occur in cyclic forms where the phosphate group is linked twice to the sugar moiety. The nucleobase component may either be a purine or a pyrimidine. While both purines and pyrimidines are heterocyclic aromatic compounds, they can be differed from each other based on the chemical structure. A purine has two carbon rings whereas a pyrimidine has one carbon ring. The purine has a pyrimidine ring fused to an imidazole ring. The pyrimidine has only a pyrimidine ring. Thus, the purine has four nitrogen atoms whereas the pyrimidine has two.
Nucleotides may be grouped into mononucleotides, dinucleotides, and trinucleotides. A dinucleotide is a compound comprised of two nucleotides. Therefore, each nucleotide unit is comprised, in turn, of one nucleobase, one pentose moiety, and phosphoric acid.
Flavin adenine dinucleotide
Flavin adenine dinucleotide (FAD) (chemical formula: C27H33N9O15P2) is an example of a dinucleotide. It is a redox cofactor in various metabolic processes. One of the sources of FAD is flavin mononucleotide (FMN), a mononucleotide. FMN may be converted to the dinucleotide FAD through the action of FAD pyrophosphorylase, with ATP. FAD consists mainly of adenine mononucleotide and flavin mononucleotide joined at their phosphate groups. FAD occurs in different redox states: quinone, semiquinone, and hydroquinone. It converts from one state to another either by accepting or donating electrons. Quinone is the fully oxidized form of FAD. When it accepts two electrons (e–) and two protons (H+) it converts to the hydroquinone form, FADH2. FADH2 can convert to FADH when it losses one H+ and one e–, and losing another H+ and one e– reverts it to FAD. FAD can be produced by the reduction and dehydration of flavin-N(5)-oxide.1
Nicotinamide adenine dinucleotide
Nicotinamide adenine dinucleotide (NAD) is another dinucleotide. Its chemical formula is C21H27N7O14P2 It is a ubiquitous coenzyme comprised of two nucleotides (i.e. one with adenine base and the other a nicotinamide). It occurs in two forms: NAD+ (oxidized form) and
NADH (reduced form).
Nicotinamide adenine dinucleotide phosphate
Nicotinamide adenine dinucleotide phosphate (NADP) (chemical formula: C21H29N7O17P3) is a coenzyme made up of ribosylnicotinamide 5′-phosphate (nmn) coupled by pyrophosphatelinkage to the 5′-phosphate adenosine 2′,5′-bisphosphate. It acts as an electron carrier capable of donating electrons to certain biosynthetic reactions, e.g. lipid and nucleic acid syntheses. It is similar to NAD. However, NADP+ has an additional phosphate (particularly on the 2′ position of the ribose ring attached to the adenine).
Dinucleotides are involved in various metabolic processes. For instance, FAD is a redox cofactor similar to FMN is a mononucleotide acts as a cofactor. When contained in a protein, FAD forms certain flavoproteins.
- mono– (“single”) + nucleo– (“nucleus“) + –ide (chemical suffix)
- Devlin, edited by Thomas M. (2011). Textbook of biochemistry: with clinical correlations (7th ed.). Hoboken, NJ: John Wiley & Sons.
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