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β-Nicotinamide adenine dinucleotide More Info

β-Nicotinamide adenine dinucleotide, commonly known as NAD+, is a vital coenzyme found in all living cells that plays a central role in metabolism and cellular physiology. It is a dinucleotide composed of two nucleotides joined through their phosphate groups: one containing an adenine base and the other nicotinamide.

Mechanism of Action

NAD+ primarily functions as an essential coenzyme in redox reactions, acting as an electron carrier. It cycles between an oxidized form (NAD+) and a reduced form (NADH):
Redox Reactions: NAD+ accepts electrons (in the form of a hydride ion) from metabolic substrates during processes like glycolysis, the tricarboxylic acid (TCA) cycle, fatty acid oxidation, and oxidative phosphorylation. This electron transfer is crucial for ATP production and energy metabolism.
Enzymatic Cofactor: NAD+ serves as a cofactor for various dehydrogenase enzymes, facilitating oxidation-reduction reactions essential for cellular respiration and biosynthesis.
Substrate for NAD+-Dependent Enzymes: Beyond redox roles, NAD+ is a substrate for enzymes involved in post-translational modifications, including sirtuins (protein deacetylases), poly(ADP-ribose) polymerases (PARPs), and ADP-ribosyl cyclases, which regulate DNA repair, gene expression, and calcium signaling.
RNA Capping: NAD+ can be incorporated as a non-canonical cap at the 5′ end of RNA transcripts, influencing RNA stability and gene expression regulation.

Biosynthesis and Metabolism

NAD+ levels in cells are maintained through multiple pathways:
De Novo Synthesis: NAD+ can be synthesized from dietary amino acids like tryptophan via the kynurenine pathway.
Preiss-Handler Pathway: Converts dietary nicotinic acid (NA) into NAD+ through intermediate steps.
Salvage Pathway: The predominant pathway in most cells, recycling nicotinamide (NAM), nicotinamide riboside (NR), and nicotinamide mononucleotide (NMN) back into NAD+ to maintain cellular levels. The rate-limiting enzyme in this pathway is nicotinamide phosphoribosyltransferase (NAMPT).
Methylation and Excretion: Excess NAM is methylated by nicotinamide N-methyltransferase (NNMT) and ultimately excreted, regulating NAD+ homeostasis.

Physiological Roles

Energy Metabolism: NAD+ is indispensable for glycolysis, TCA cycle, and oxidative phosphorylation, enabling efficient ATP production.
Redox Homeostasis: Maintains cellular redox balance by cycling between NAD+ and NADH.
DNA Repair and Genomic Stability: Serves as a substrate for PARPs involved in DNA repair mechanisms.
Regulation of Gene Expression: Through sirtuins, NAD+ influences chromatin remodeling and transcriptional regulation.
Immune Response and Inflammation: Modulates immune cell function and inflammatory processes.
Circadian Rhythm: NAD+ levels influence circadian clock regulation via sirtuin activity.
Aging and Disease: Dysregulation or depletion of NAD+ is linked to aging, metabolic disorders, neurodegeneration, cancer, and cardiovascular diseases.

Pharmacokinetics and Cellular Distribution

NAD+ is compartmentalized within cells in the cytoplasm, mitochondria, and nucleus, with distinct pools regulated independently.
It is synthesized and recycled intracellularly, with limited direct uptake from extracellular sources. Oral NAD+ precursors like NR and NMN are metabolized to NAM before contributing to NAD+ pools.
NAD+ has a relatively short half-life and is rapidly turned over due to its consumption by NAD+-dependent enzymes.

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