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Chapter 1-7: Introduction, NAD, Enzymes, Cofactors, and Glycolysis

Metabolism Concepts

  • Metabolism: Sum of all chemical processes in an organism.

    • Anabolic: Building complex molecules from simpler ones (requires energy).

    • Catabolic: Breaking down complex molecules into simpler ones (releases energy).

Redox Reactions

  • Redox reactions: Electron transfer between substances; always occur together.

    • Oxidation: Loss of electrons (electron donor).

    • Reduction: Gain of electrons (electron acceptor).

    • Electrons move from one species to another via electron carriers.

Electron Carriers: NAD

  • NAD (nicotinamide adenine dinucleotide): Key electron carrier in glucose catabolism.

    • Exists as **NAD ** (oxidized) and NADH (reduced).

    • **NAD ** accepts electrons and a proton to become NADH ( ext{NAD}^+ + 2e^- + H^+ \rightarrow ext{NADH}).

    • NADH carries electrons to the electron transport chain (ETC) to generate ATP.

    • Derived from niacin (a B vitamin).

Enzymes: Catalysis

  • Enzymes: Proteins that act as biological catalysts.

    • Catalyze reactions: Speed up reactions by lowering activation energy.

    • Activation energy: Energy required to start a reaction.

    • Active site: Region where substrate binds.

    • Substrate: Molecule(s) an enzyme acts upon.

    • Product: Resulting molecule(s) after enzyme action.

    • Enzymes are not consumed in the reaction.

    • Example: Aldolase splits fructose-1,6-bisphosphate in glycolysis.

Cofactors and Coenzymes

  • Cofactor: Non-protein helper required for some enzyme activity.

    • Examples: Magnesium (Mg ), Iron (Fe), Zinc (Zn).

  • Coenzyme: Organic cofactor; often vitamins or derived from vitamins.

    • Example: NAD /NADH (derived from niacin).

  • Vitamins (organic) are precursors to coenzymes; Minerals (inorganic) are cofactors.

Glycolysis: Glucose Breakdown

  • Glycolysis: Ten-step pathway breaking down glucose into pyruvate.

    • Occurs in the cytoplasm.

    • Phases:

    1. Investment Phase: 2 ATP consumed to phosphorylate glucose.

    2. Lysis Phase: 6-carbon sugar splits into two 3-carbon molecules (2 G3P).

    3. Payoff Phase: Energy harvested.

  • Per glucose molecule (Net):

    • 2 Pyruvate

    • 2 net ATP (4 produced, 2 invested)

    • 2 NADH

  • Purpose: Rapid ATP source; generates NADH for ETC (aerobic) or fermentation (anaerobic).

Respiration vs. Fermentation

  • Cellular Respiration (aerobic):

    • Glycolysis followed by Citric Acid Cycle and ETC.

    • Maximizes ATP production using oxygen as final electron acceptor.

  • Fermentation (anaerobic):

    • Occurs without oxygen.

    • Regenerates NAD to allow glycolysis to continue.

Key Terms & Equations Quick Reference

  • Anabolic: Building up.

  • Catabolic: Breaking down.

  • Oxidation: Loss of electrons.

  • Reduction: Gain of electrons.

  • Activation energy: Energy barrier for reactions.

  • ATP accounting in glycolysis: Investment = -2 ATP; Payoff = +4 ATP; Net = +2 AT.

  • NADH yield in glycolysis: 2 NADH per glucose.

  • Pyruvate: 2 molecules per glucose.

  • ext{NAD}^+ + 2e^- + H^+ \rightarrow ext{NADH}