Nutrition & Metabolism - Carbohydrate Metabolism

Carbohydrate Metabolism

• Most dietary carbohydrates are burned as fuel within hours of absorption.
• Oxidative carbohydrate metabolism is glucose catabolism, transferring energy from glucose to ATP.

Glucose Catabolism

• Occurs in small steps controlled by enzymes, transferring energy to ATP.
• Three major pathways:
  • Glycolysis: Glucose splits into two pyruvate molecules.
  • Anaerobic Fermentation: Pyruvate reduces to lactate without oxygen.
  • Aerobic Respiration: Requires oxygen; pyruvate oxidizes to carbon dioxide and water.

Coenzymes

• Enzymes remove electrons (as hydrogen atoms) from intermediate compounds.
• Enzymes transfer hydrogen atoms to coenzymes, which later donate them to other compounds.
• Two key coenzymes: NAD+ and FAD.
  • NAD+ (nicotinamide adenine dinucleotide).
  • FAD (flavin adenine dinucleotide).
• FAD binds two protons and two electrons to become FADH2.
• NAD+ binds two electrons but only one proton to become NADH; the other proton remains a free hydrogen ion.

Glycolysis

• Metabolic pathway that splits glucose into two pyruvate molecules.
• Steps:
  • Phosphorylation: Hexokinase transfers a phosphate from ATP to glucose, forming glucose 6-phosphate (G6P).
    • Keeps intracellular glucose concentration low and prevents sugar from leaving the cell.
  • Priming: G6P rearranges to fructose 6-phosphate, then phosphorylated to fructose 1,6-diphosphate.
    • Consumes two ATPs.
  • Cleavage: Fructose 1,6-diphosphate splits into two three-carbon molecules (PGAL).
  • Oxidation: Each PGAL molecule is oxidized, yielding NADH + H+.
  • Dephosphorylation: Phosphate groups are transferred to ADP, forming ATP, and C3 compound becomes pyruvate
• Net gain: 2 ATP per glucose.
• End products: ATP, NADH, pyruvate.

Anaerobic Fermentation

• Occurs without oxygen; NADH donates electrons to pyruvate, reducing it to lactate & regenerating NAD+.
• Fate of Lactate:
  • Travels to the liver, which oxidizes it back to pyruvate when oxygen is available. The liver can also convert lactate back to G6P
• Limitations:
  • Wasteful because most of the energy remains in lactate.
  • Lactate is toxic.

Aerobic Respiration

• Pyruvate enters mitochondria and oxidizes in the presence of oxygen.
• Most ATP generated this way.
• Two main steps:
  • Matrix Reactions (Citric Acid Cycle): Enzymes in the mitochondrial matrix.
  • Membrane Reactions (Electron Transport Chain): Enzymes bound to mitochondrial cristae membranes.

Matrix Reactions (Citric Acid Cycle)

• Pyruvate prepares to enter the cycle via:
  • Decarboxylation: CO_2 removed from pyruvate to form a C2 compound.
  • C2 compound converts to acetyl group and binds to coenzyme A, forming acetyl-CoA.
• Acetyl-CoA (C2) combines with oxaloacetic acid (C4) to form citric acid (C6).
• Summary:
  • Carbon atoms of glucose are released as CO_2.
  • Energy stored in 8 NADH and 2 FADH2 molecules.

Membrane Reactions (Electron Transport Chain)

• Further oxidizes NADH and FADH2, transferring energy to ATP and regenerating NAD+ and FAD.
• Series of compounds passes electrons:
  • Flavin mononucleotide (FMN): Accepts electrons from NADH.
  • Iron–sulfur (Fe-S) centers: Complexes of iron and sulfur atoms.
  • Coenzyme Q (CoQ): Accepts electrons from FADH2.
  • Copper (Cu) ions: Bound to membrane proteins.
  • Cytochromes: Enzymes with iron cofactors (b, c1, c, a, a3).
• Electrons travel along the chain; oxygen is the final acceptor, forming water.

Chemiosmotic Mechanism

• Electron-transport chain energy fuels respiratory enzyme complexes that act as proton pumps.
• Creates an electrochemical gradient for H+ across the inner mitochondrial membrane.
• H+ current through ATP synthase channels drives ATP synthesis.

Cellular Respiration Stages

• Glycolysis: Glucose to 2 pyruvic acid (2 ATP).
• Krebs Cycle: Pyruvic acid broken down (2 electrons carried by NADH).
• Electron Transport Chain: 32 or 34 ATP.
• Total: 36 or 38 ATP.