A dinucleotide is a type of molecule found in living organisms and consists of two nucleotides linked together. Single nucleotides are the subunits that form deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), molecules that contain an organism’s genetic information. Certain types of dinucleotides, such as nicotinamide adenine dinucleotide (NAD+), play an important role in metabolism.
Chemically, a nucleotide consists of several components. It must contain a molecular component called a nitrogenous base, along with a sugar containing five carbon atoms. These two components together are called a nucleoside. The nucleotide must also contain a phosphate group, which is an assembly of phosphorus and oxygen atoms.
The two nucleotides that make up a dinucleotide can be bonded together in different configurations. A part of the sugar component on one nucleotide can bind to the phosphate group on the second nucleotide. Alternatively, it is possible for the phosphate groups of the two nucleotides to link together. NAD+ is formed in the latter way.
NAD+ is an important dinucleotide because it acts as a coenzyme in metabolic reactions. Coenzymes bind to proteins and enable them to function correctly by catalyzing chemical reactions. The main role of NAD+ is to transfer electrons from one compound to another.
Like other dinucleotides, NAD+ consists of two nucleotide structures. One nucleotide contains a nitrogenous base called adenine, which is also found in DNA and RNA. The nitrogenous base of the other nucleotide is nicotinamide, also known as niacin — a B vitamin.
In metabolic reactions, NAD+ accepts electrons from other chemical compounds. When this happens, the NAD+ molecule is reduced, or loses its positive charge, by gaining the negatively charged electron. The modified compound is called NADH. NADH can then contribute an electron to other compounds, acting as a reducing agent. When it donates an electron, it becomes oxidized, turning back into NAD+.
Since NADH can easily transform into NAD+, and vice versa, the two compounds exist in a balanced ratio in these oxidation and reduction, or redox, reactions. They can carry electrons without being consumed or permanently changed in the process. It is, however, possible for the dinucleotide NAD+ to be consumed in other non-metabolic types of reactions. In its role in modifying proteins, for example, NAD+ is consumed. This consumption necessitates the synthesis of new NAD+ and the intake of components of NAD+ in the form of niacin, or vitamin B3 .