NADH

(noun)

nicotinamide adenine dinucleotide (NAD) carrying two electrons and bonded with a hydrogen (H) ion; the reduced form of NAD

Examples of NADH in the following topics:

  • Outcomes of Glycolysis

    • One glucose molecule produces four ATP, two NADH, and two pyruvate molecules during glycolysis.
    • Glycolysis starts with one molecule of glucose and ends with two pyruvate (pyruvic acid) molecules, a total of four ATP molecules, and two molecules of NADH .
    • Two ATP molecules were used in the first half of the pathway to prepare the six-carbon ring for cleavage, so the cell has a net gain of two ATP molecules and 2 NADH molecules for its use.
    • Glycolysis, or the aerobic catabolic breakdown of glucose, produces energy in the form of ATP, NADH, and pyruvate, which itself enters the citric acid cycle to produce more energy.

  • The Energy-Releasing Steps of Glycolysis

    • In the second half of glycolysis, energy is released in the form of 4 ATP molecules and 2 NADH molecules.
    • The sixth step in glycolysis oxidizes the sugar (glyceraldehyde-3-phosphate), extracting high-energy electrons, which are picked up by the electron carrier NAD+, producing NADH.
    • Thus, NADH must be continuously oxidized back into NAD+ in order to keep this step going.
    • In an environment without oxygen, an alternate pathway (fermentation) can provide the oxidation of NADH to NAD+.
    • The second half of glycolysis involves phosphorylation without ATP investment (step 6) and produces two NADH and four ATP molecules per glucose.

  • Citric Acid Cycle

    • The citric acid cycle is a series of reactions that produces two carbon dioxide molecules, one GTP/ATP, and reduced forms of NADH and FADH2.
    • This step is also regulated by negative feedback from ATP and NADH and by a positive effect of ADP.
    • The enzyme that catalyzes step four is regulated by feedback inhibition of ATP, succinyl CoA, and NADH.
    • Another molecule of NADH is produced.
    • Each turn of the cycle forms three NADH molecules and one FADH2 molecule.

  • Acetyl CoA and the Citric Acid Cycle

    • The cycle provides precursors including certain amino acids as well as the reducing agent NADH that is used in numerous biochemical reactions.
    • The cycle consumes acetate (in the form of acetyl-CoA) and water, reduces NAD+ to NADH, and produces carbon dioxide.
    • The NADH generated by the TCA cycle is fed into the oxidative phosphorylation pathway.
    • CH3C(=O)C(=O)O– (pyruvate) + HSCoA + NAD+ → CH3C(=O)SCoA (acetyl-CoA) + NADH + H+ + CO2
    • In addition, the cycle provides precursors including certain amino acids as well as the reducing agent NADH that is used in numerous biochemical reactions.

  • Electrons and Energy

    • RH (Reducing agent) + NAD+ (Oxidizing agent) → NADH (Reduced) + R (Oxidized)
    • In the above equation, RH is a reducing agent and NAD+ is reduced to NADH.
    • The molecule NADH is critical for cellular respiration and other metabolic pathways.
    • The oxidized form of the electron carrier (NAD+) is shown on the left and the reduced form (NADH) is shown on the right.
    • The nitrogenous base in NADH has one more hydrogen ion and two more electrons than in NAD+.

  • The Entner–Doudoroff Pathway

    • The free energy released in this process is used to form the high-energy compounds ATP (adenosine triphosphate) and NADH (reduced nicotinamide adenine dinucleotide).
    • The Entner–Doudoroff pathway also has a net yield of 1 ATP for every glucose molecule processed, as well as 1 NADH and 1 NADPH.
    • By comparison, glycolysis has a net yield of 2 ATP and 2 NADH for every one glucose molecule processed.

  • Connecting Lipids to Glucose Metabolism

    • Fatty acids are catabolized in a process called beta-oxidation that takes place in the matrix of the mitochondria and converts their fatty acid chains into two carbon units of acetyl groups, while producing NADH and FADH2.
    • The NADH and FADH2 are then used by the electron transport chain.

  • Anaerobic Cellular Respiration

    • Processes that use an organic molecule to regenerate NAD+ from NADH are collectively referred to as fermentation.
    • For example, the group of archaea called methanogens reduces carbon dioxide to methane to oxidize NADH.
    • Similarly, sulfate-reducing bacteria and archaea, most of which are anaerobic, reduce sulfate to hydrogen sulfide to regenerate NAD+ from NADH.
    • Pyruvic acid → CO2 + acetaldehyde + NADH → ethanol + NAD+
    • The second reaction is catalyzed by alcohol dehydrogenase to oxidize NADH to NAD+ and reduce acetaldehyde to ethanol.

  • Control of Catabolic Pathways

    • If either acetyl groups or NADH accumulate, there is less need for the reaction and the rate decreases.
    • The citric acid cycle is controlled through the enzymes that catalyze the reactions that make the first two molecules of NADH .
    • When adequate ATP and NADH levels are available, the rates of these reactions decrease.
    • Enzymes, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase, catalyze the reactions that make the first two molecules of NADH in the citric acid cycle.
    • Rates of the reaction decrease when sufficient ATP and NADH levels are reached.

  • Internal Respiration

    • Oxidative Phosphorylation: Produces ATP from NADH, oxygen, and H+.
    • The carbon dioxide waste is the result of the carbon from glucose (C6H12O6) being broken down to produce the pyruvate and NADH intermediates needed to produce ATP at the end of respiration.