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.

  • 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+.

  • 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.

  • Importance of Glycolysis

    • The first phase of glycolysis requires energy, while the second phase completes the conversion to pyruvate and produces ATP and NADH for the cell to use for energy.
    • Overall, the process of glycolysis produces a net gain of two pyruvate molecules, two ATP molecules, and two NADH molecules for the cell to use for energy.

  • Breakdown of Pyruvate

    • The hydroxyethyl group is oxidized to an acetyl group, and the electrons are picked up by NAD+, forming NADH (the reduced form of NAD+).
    • The high-energy electrons from NADH will be used later by the cell to generate ATP for energy.

  • Electron Transport Chain

    • To start, two electrons are carried to the first complex aboard NADH.
    • The enzyme in complex I is NADH dehydrogenase, a very large protein containing 45 amino acid chains.
    • Q receives the electrons derived from NADH from complex I and the electrons derived from FADH2 from complex II, including succinate dehydrogenase.
    • The electron transport chain is a series of electron transporters embedded in the inner mitochondrial membrane that shuttles electrons from NADH and FADH2 to molecular oxygen.