Biochemistry Terms Starting With N
Biochemistry Glossary: N
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NAD+
/ N-A-D-PLUS / · Abbreviation from nicotinamide adenine dinucleotide, from Greek nikos (victory) + Latin amid (nitrogen compound) + Arabic al-qaliy (plant ashes, for adenine) + Greek di (two) + Latin nucleo (kernel)
NAD+ is the oxidized form of nicotinamide adenine dinucleotide, a coenzyme that accepts electrons and a proton during cellular metabolism to become NADH, carrying chemical energy to the electron transport chain.
During glycolysis, each glucose molecule reduces 2 NAD+ molecules to NADH, which then carries high-energy electrons to Complex I of the mitochondrial electron transport chain. The bacterium Lactobacillus acidophilus regenerates NAD+ through lactic acid fermentation when oxygen is unavailable, converting pyruvate to lactate so that glycolysis can continue producing ATP. Without continuous NAD+ regeneration, the glycolytic pathway stalls because all available NAD+ molecules remain locked in the reduced NADH form.
Cells maintain a tightly regulated NAD+/NADH ratio, and disruptions to this ratio are linked to metabolic diseases including type 2 diabetes and age-related mitochondrial decline.
Researchers discovered in 2013 that declining NAD+ levels in aging mice could be partially reversed by supplementing with nicotinamide mononucleotide, restoring mitochondrial function in muscle tissue within one week of treatment.
Mitochondria Functions →NAD+ and NADH are entirely separate molecules with different functions. NADH is the reduced form of NAD+ that has accepted two electrons and one proton, and cells continuously interconvert these two forms during metabolic reactions.
Fermentation Biology →Saccharomyces cerevisiae yeast cells regenerate NAD+ through alcoholic fermentation, reducing acetaldehyde to ethanol while oxidizing NADH back to NAD+. This reaction sustains glycolytic ATP production under anaerobic conditions and produces the roughly 5% ethanol concentration typical of fermented beer.
NADH
/NAY-dee-aych/ · Acronym from Nicotinamide Adenine Dinucleotide (reduced form), nicotinamide from nicotinic acid + amide, adenine from Greek 'aden' (gland), dinucleotide from di- (two) + nucleotide
NADH is the reduced form of nicotinamide adenine dinucleotide that carries high-energy electrons from metabolic reactions to the electron transport chain, where their energy drives ATP synthesis during cellular respiration.
During glycolysis and the citric acid cycle, each glucose molecule processed yields 10 NADH molecules that collectively store energy equivalent to approximately 28 ATP molecules when fully oxidized through the electron transport chain. When NADH donates its electrons to Complex I of the electron transport chain, it returns to its oxidized NAD+ form, which can then accept more electrons from subsequent metabolic reactions. Human heart muscle relies on a continuous supply of NADH to generate the approximately 6 kilograms of ATP it produces daily to sustain cardiac contractions.
Disruption of NADH oxidation, as occurs when the poison rotenone blocks Complex I, halts ATP production and causes rapid cell death.
Nicotinamide adenine dinucleotide was first isolated by Arthur Harden and William Young in 1906 while studying yeast fermentation, though its role as an electron carrier was not fully understood until Hans von Euler-Chelpin characterized its structure in work that earned him the Nobel Prize in Chemistry in 1929.
Translation Biology →NADH directly synthesizes ATP inside the mitochondria. NADH donates electrons to the electron transport chain, and the energy released by those electrons pumps protons across the inner mitochondrial membrane; it is the flow of those protons back through ATP synthase that produces ATP.
Carbon Cycle Steps →Escherichia coli regenerates NAD+ from NADH through mixed-acid fermentation when oxygen is unavailable, producing lactate, ethanol, formate, and acetate as byproducts. Under fully anaerobic conditions, E. coli can sustain a glycolytic flux of roughly 10 millimoles of glucose per gram of dry cell weight per hour using this regeneration strategy.
Do Prokaryotes Have Mitochondria? →NADP+
/ N-A-D-P-PLUS / · Acronym from nicotinamide adenine dinucleotide phosphate, with 'phosphate' from Greek phosphoros meaning light-bearing
NADP+ is an oxidized coenzyme that accepts electrons and hydrogen ions during anabolic biosynthetic reactions to form NADPH.
During photosynthesis in plants like spinach, light-dependent reactions generate approximately 12 molecules of NADPH from 12 molecules of NADP+ for every molecule of glucose ultimately produced. This coenzyme differs from NAD+ by a single phosphate group attached to the adenosine ribose, which targets it toward biosynthetic pathways rather than catabolic energy production. Chloroplasts maintain a NADP+/NADPH ratio of about 1:10 during active photosynthesis, reflecting the high demand for reducing power in the Calvin cycle.
Ferredoxin-NADP+ reductase, the enzyme that produces NADPH at the end of the photosynthetic electron transport chain, belongs to the flavoprotein family and uses FAD as an intermediate electron carrier before passing electrons to NADP+.
NADP+ and NAD+ work the same way in cells. They have distinct roles: NADP+ powers reductive biosynthesis reactions while NAD+ mainly works in catabolic oxidation pathways.
In cyanobacteria, ferredoxin-NADP+ reductase transfers electrons from ferredoxin to NADP+ during the final step of photosynthetic electron transport, regenerating the oxidized coenzyme for another round of light-driven reduction. Each molecule of NADP+ reduced in this step carries two electrons and one proton as NADPH into the Calvin cycle.
NADPH
/NAY-dee-pee-aych/ · Acronym from Nicotinamide Adenine Dinucleotide Phosphate (Reduced form); nicotinamide from Greek 'nikotin' (tobacco) + 'amide' (chemical group), adenine from Greek 'adeno' (gland), phosphate from Greek 'phosphoros' (light-bearing)
NADPH is a reduced coenzyme that carries high-energy electrons and donates them to anabolic biosynthetic reactions and antioxidant defense systems within cells.
During photosynthesis in chloroplasts, NADPH provides the reducing power needed to convert carbon dioxide into glucose, with each molecule of glucose requiring 12 molecules of NADPH to be synthesized. Human red blood cells depend on NADPH produced through the pentose phosphate pathway to regenerate glutathione, which protects hemoglobin from oxidative damage. Unlike its oxidized partner NADP+, NADPH maintains a cellular concentration ratio of approximately 100:1 over NADP+ in most tissues, ensuring a constant supply of reducing equivalents for biosynthetic processes.
In patients with glucose-6-phosphate dehydrogenase deficiency, the most common enzyme deficiency affecting over 400 million people worldwide, red blood cells cannot produce adequate NADPH, making them vulnerable to rupture when exposed to certain medications or fava beans.
Building Blocks of Carbohydrates →NADPH and NADH are interchangeable molecules that do the same thing in cells. Each has a distinct metabolic role: NADPH primarily drives biosynthetic reactions and maintains antioxidant defenses, while NADH mainly participates in ATP production through oxidative phosphorylation.
The liver hepatocyte uses NADPH generated from the pentose phosphate pathway to power fatty acid synthesis, with each 16-carbon palmitate molecule requiring 14 NADPH molecules for complete assembly. This demand makes the liver one of the most NADPH-intensive tissues in the human body, consuming a substantial fraction of the roughly 60 grams of glucose the liver processes daily.
Building Blocks of Lipids →Nitrogen Balance
/ NY-troh-jen BAL-ance / · From Greek 'nitron' meaning soda or native soda, and Latin 'balancia' meaning scale with two pans
Nitrogen balance is the difference between the amount of nitrogen consumed through dietary protein and the amount of nitrogen excreted from the body, primarily through urea in urine.
Healthy adults typically maintain nitrogen equilibrium, consuming approximately 10 to 15 grams of nitrogen daily and excreting an equivalent amount. Growing children, pregnant women, and individuals recovering from injury exhibit positive nitrogen balance, where intake exceeds excretion to support new tissue synthesis. Patients with severe burns or cachexia display negative nitrogen balance, losing up to 40 grams of nitrogen per day as the body breaks down muscle protein faster than it can be replaced.
Nutritionists calculate nitrogen balance by multiplying dietary protein grams by 0.16, since protein contains roughly 16 percent nitrogen by mass.
Athletes in heavy training can require up to 2 grams of protein per kilogram of body weight daily to maintain positive nitrogen balance and support muscle growth, nearly double the requirement of sedentary individuals.
Building Blocks of Proteins →Nitrogen balance only reflects how much protein a person eats. It compares nitrogen taken in with nitrogen lost through urine, feces, sweat, skin shedding, and hair growth, making it a measure of whole-body protein turnover rather than dietary intake alone.
A hospitalized patient recovering from major surgery maintains positive nitrogen balance by consuming 120 grams of protein daily while excreting only 12 grams of nitrogen through urine, allowing net tissue repair to proceed at roughly 75 grams of new protein synthesized each day.
Noncompetitive Inhibition
/ non-kom-PET-ih-tiv in-hih-BISH-un / · Latin non (not) + competere (to strive together) + inhibere (to hold back)
Noncompetitive inhibition is a type of enzyme regulation where an inhibitor binds to a site distinct from the active site, reducing the maximum reaction rate regardless of substrate concentration.
In human liver metabolism, the drug valproic acid occupies a noncompetitive inhibitor of certain cytochrome P450 enzymes, binding to allosteric sites and reducing Vmax by approximately 40 to 60 percent while leaving Km unchanged. Unlike competitive inhibitors that block substrate access, noncompetitive inhibitors can bind to both free enzyme and enzyme-substrate complexes simultaneously. Certain pesticides like malathion exhibit noncompetitive inhibition against acetylcholinesterase in insects, disrupting nervous system function by decreasing the enzyme’s catalytic efficiency rather than competing for the active site.
The antibiotic linezolid displays noncompetitive inhibition against bacterial ribosomes with a Ki value of 2.5 micromolar, making it effective even when protein synthesis substrate concentrations are high.
Adding more substrate can overcome noncompetitive inhibition. The inhibitor decreases Vmax because it binds to a separate regulatory site regardless of how much substrate is present, so saturating the active site with substrate does nothing to displace it.
The heavy metal lead inhibits a noncompetitive inhibitor of delta-aminolevulinic acid dehydratase in red blood cells, binding to sulfhydryl groups away from the active site and reducing heme synthesis. Blood lead concentrations above 10 micrograms per deciliter measurably suppress this enzyme's activity, contributing to the anemia seen in lead poisoning.
Nucleic Acid
/ new-KLAY-ik AS-id / · From Latin nucleus meaning kernel or core, and acidus meaning sour
Nucleic acids are large biomolecules composed of nucleotide monomers that store and transmit genetic information in living organisms.
These polymers exist in two primary forms: deoxyribonucleic acid and ribonucleic acid. Humans carry approximately 3.2 billion base pairs of DNA distributed across 46 chromosomes in each cell nucleus, yet that DNA, if fully extended, would stretch roughly 2 meters per cell. Nucleic acids form through phosphodiester bonds linking the sugar of one nucleotide to the phosphate group of the next, creating a sugar-phosphate backbone with nitrogenous bases projecting inward to pair with complementary strands.
The largest known nucleic acid molecule belongs to the marbled lungfish (Protopterus aethiopicus), whose genome spans over 130 billion base pairs, making it more than 40 times larger than the human genome.
RNA Databases →Nucleic acids only exist inside cell nuclei. RNA operates throughout the entire cell, including in the cytoplasm and at ribosomes, and both mitochondria and chloroplasts carry their own DNA molecules entirely separate from nuclear DNA.
Tobacco mosaic virus uses a single strand of RNA roughly 6,400 nucleotides long as its entire genetic material to infect tobacco plants (Nicotiana tabacum) and direct the production of new viral particles. A single infected leaf cell can produce up to 100,000 new virus particles within 24 hours of infection.
Nucleotide
/ NOO-klee-oh-tide / · Latin nucleus meaning kernel or core, combined with -ide chemical suffix
Nucleotide nucleotide is an organic molecule composed of three components: a nitrogenous base, a five-carbon sugar, and one or more phosphate groups.
Nucleotides link together through phosphodiester bonds to form the long chains of DNA and RNA. The human genome contains approximately 3 billion base pairs, each position represented by a paired nucleotide in the DNA double helix. Beyond genetic storage, individual nucleotides like ATP carry chemical energy between reactions, while cyclic AMP relays hormonal signals inside cells by activating protein kinases within milliseconds of a receptor being triggered.
ATP, a single nucleotide molecule, powers nearly every cellular process, and an average human body produces and recycles roughly 50 kilograms of ATP daily despite holding only about 250 grams of it at any one moment.
Nucleotides and nucleosides are the same molecule. A nucleoside contains only a sugar and a nitrogenous base; a nucleotide also carries at least one phosphate group bonded to that sugar, and it is the phosphate that makes nucleotides capable of forming the backbone of DNA and RNA.
Adenosine triphosphate in hummingbird flight muscle releases energy by hydrolyzing its terminal phosphate bond during each wing contraction, with a single bird consuming and regenerating its entire ATP pool in under a second during sustained hovering at roughly 50 wing beats per second.