Nicotinamide adenine dinucleotide
Coenzyme central to metabolism, carrying electrons in redox reactions.
Nicotinamide adenine dinucleotide (NAD+) is a coenzyme central to metabolism, found in all living cells. It consists of two nucleotides joined through their phosphate groups, one containing an adenine nucleobase and the other nicotinamide. NAD exists in two forms: an oxidized form (NAD+) and a reduced form (NADH). Its primary function is in redox reactions, carrying electrons from one reaction to another, and it also serves as a substrate for enzymes involved in posttranslational modifications. Because of these critical roles, enzymes involved in NAD metabolism are targets for drug discovery.
- field
- Biochemistry
- known_for
- Central coenzyme in metabolism, redox reactions, and posttranslational modifications
- forms
- NAD+ (oxidized) and NADH (reduced)
- midpoint_potential
- −0.32 volts
- cellular_concentration_rat_liver
- approximately 1 μmole per gram wet weight
Lore & Background
NAD+ is synthesized through two metabolic pathways: de novo from amino acids (tryptophan in animals and some bacteria, or aspartic acid in some bacteria and plants) or via salvage pathways that recycle preformed components such as nicotinamide. In mammals, most tissues use the salvage pathway, but de novo synthesis occurs in the liver from tryptophan, and in the kidney and macrophages from nicotinic acid. Some NAD+ is converted into NADP+ by NAD+ kinase, which phosphorylates NAD+ using ATP or, in some bacteria, inorganic polyphosphate. The balance between NAD+ and NADH, called the NAD+/NADH ratio, is an important component of the redox state of a cell, controlling the activity of several key enzymes. NAD+ and NADH differ in their ultraviolet absorption spectra and fluorescence. These properties allow measurement of conversion between forms in enzyme assays and changes in the redox state of living cells through fluorescence microscopy.
Reader's Guide
Nicotinamide adenine dinucleotide is fundamental to cellular metabolism, acting as a carrier of electrons in redox reactions that are essential for energy production and biosynthesis. Its ability to cycle between oxidized (NAD+) and reduced (NADH) forms without being consumed makes it a reusable coenzyme. Beyond redox chemistry, NAD+ serves as a substrate for enzymes that modify proteins, linking it to cellular signaling and regulation. The importance of these functions has made NAD metabolism a target for drug discovery. The ratio of NAD+ to NADH reflects the metabolic state and health of cells, influencing key enzymes. The existence of both de novo and salvage pathways for NAD+ synthesis underscores its essential nature; a dietary lack of vitamin B3 (niacin) leads to pellagra. The conversion of NAD+ to NADP+ provides a coenzyme specialized for anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses. The distinct ultraviolet absorption and fluorescence properties of NAD+ and NADH enable their study in biochemical assays and live-cell imaging.
Did You Know?
- NAD+ is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups.
- The midpoint potential of the NAD+/NADH redox pair is −0.32 volts, making NADH a moderately strong reducing agent.
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