BIO152: The TCA Cycle and Glucose Metabolism – Comprehensive Study Notes

Glycolysis: Overview

  • A 10-step pathway that converts glucose (a six-carbon sugar) into two molecules of pyruvate (three-carbon molecules).
  • Net products per glucose:
    • 2 ATP (net)
    • 2 NADH
  • Two stages:
    • Stage 1: investment phase — consumes 2 ATP (per glucose).
    • Stage 2: payoff phase — generates 4 ATP and 2 NADH (per glucose).
  • Stage 2 occurs twice for each glucose because glucose is split into two G3P molecules that each proceed through the later steps.
  • Key intermediates in order: glucose → glucose-6-phosphate → fructose-6-phosphate → fructose-1,6-bisphosphate → dihydroxyacetone phosphate (DHAP) + glyceraldehyde-3-phosphate (G3P) → 1,3-bisphosphoglycerate → 3-phosphoglycerate → 2-phosphoglycerate → phosphoenolpyruvate → pyruvate.
  • Energy coupling:
    • ATP consumed in Stage 1: 2 ATP (2 × 1).
    • ATP produced in Stage 2: 4 ATP (2 × 2).
    • NADH produced in Stage 2: 2 NADH.
  • NAD+/NADH role: electron carrier in glycolysis (oxidation of glyceraldehyde-3-phosphate).

Fate of glucose in the presence vs absence of O2

  • With O2 (aerobic conditions): pyruvate proceeds to aerobic respiration via the mitochondrial pathways.
  • Without O2 (anaerobic conditions): pyruvate is fermented to either ethanol or lactate to regenerate NAD+ for glycolysis.
  • Overall, glycolysis provides the initial ATP and NADH supply, which are further processed in downstream pathways depending on oxygen availability.

Fate of glycolysis-derived NADH

  • If O2 is not available:
    • NADH is used to regenerate NAD+ during fermentation, producing either lactate (in animals) or ethanol (in yeast) depending on organism.
  • If O2 is available:
    • Glycolysis-derived cytosolic NADH is re-oxidized via the glycerol phosphate shuttle to transfer electrons into the mitochondrion.
    • This shuttle ultimately yields additional ATP via oxidative phosphorylation (4 ATP per 2 NADH, per the course convention).

Glycerol phosphate shuttle

  • Purpose: re-oxidize cytosolic NADH and transfer reducing equivalents into the mitochondrion.
  • Mechanism (overview):
    • Cytosolic dihydroxyacetone phosphate (DHAP) is reduced to glycerol-3-phosphate (G3P) using NADH.
    • In the mitochondrial membrane, G3P is oxidized back to DHAP by FAD-dependent glycerol-3-phosphate dehydrogenase, producing FADH2 (which donates electrons to the ETC).
  • Net transfer: 2 cytosolic NADH are re-oxidized to NAD+ and 2 FADH2 are produced in the mitochondrion, yielding 4 ATP via oxidative phosphorylation in this shuttle pathway.
  • Diagrammatic flow (per glucose):
    • Cytoplasm: 2 NADH → 2 NAD+ (via DHAP ⇄ G3P conversion)
    • Mitochondrion: 2 FADH2 produced from 2 G3P → ETC
  • Energy implication: each FADH2 yields ATP via the ETC; the shuttle results in 4 ATP per glucose from these glycolysis NADH equivalents.

The TCA cycle (Krebs cycle)

  • Location: mitochondrion (specifically the matrix for the cycle proper).
  • Overall inputs/outputs per acetyl-CoA turn:
    • Input: Acetyl-CoA (2C) + Oxaloacetate (4C) → Citrate (6C)
    • Key products per turn: 3 NADH, 1 FADH2, 1 GTP (equivalent to ATP), and 2 CO2 released.
    • Regenerated: Oxaloacetate (4C) to continue the cycle.
  • Per glucose (two acetyl-CoA turns per glucose):
    • 6 NADH, 2 FADH2, 2 GTP (2 ATP equivalents from GTP), and CO2 released as part of oxidative decarboxylations.
  • Intermediates (in order):
    • Acetyl-CoA (2C) enters the cycle and combines with Oxaloacetate to form Citrate (6C).
    • Citrate → Isocitrate (via aconitase) → α-Ketoglutarate → Succinyl-CoA → Succinate → Fumarate → Malate → Oxaloacetate (cycle restarts).
  • Step-by-step (with key features):
    • Step 1: Pyruvate is decarboxylated to acetyl-CoA; produced NADH and CO2.
    • Step 2: Acetyl-CoA combines with Oxaloacetate to form Citrate.
    • Step 3: Citrate is isomerized to Isocitrate.
    • Step 4: Isocitrate is oxidized to α-ketoglutarate; NAD+ → NADH; CO2 released.
    • Step 5: α-Ketoglutarate is oxidized to Succinyl-CoA; NAD+ → NADH; CO2 released.
    • Step 6: Succinyl-CoA is converted to Succinate; substrate-level phosphorylation yields GTP (GDP + Pi → GTP).
    • Step 7: Succinate is oxidized to Fumarate; FAD → FADH2.
    • Step 8: Fumarate is hydrated to Malate.
    • Step 9: Malate is oxidized to Oxaloacetate; NAD+ → NADH; cycle restarts.
  • Notable points:
    • Step 6 provides substrate-level phosphorylation (GTP produced; energetically equivalent to ATP).
    • Step 7 is the only step that yields FADH2 rather than NADH in the cycle.
    • Per acetyl-CoA turn, CO2 are released in Steps 4 and 5; the PDH step before the cycle also releases CO2 (but that CO2 is from pyruvate decarboxylation prior to TCA).

Acetyl-Coenzyme A (Acetyl-CoA)

  • Structure: 2-carbon acetyl group linked to CoA via a thioester bond; CoA contains pantothenic acid (vitamin B5) as part of its structure.
  • Role: carries the acetate unit into the TCA cycle by combining with oxaloacetate to form citrate.
  • Important reminder: The acetyl group is fully oxidized in the TCA cycle, releasing CO2 and generating reducing equivalents (NADH, FADH2).

Aerobic respiration: two phases

  • Phase I — Tricarboxylic acid (TCA) cycle (Krebs cycle) in the mitochondrial matrix.
  • Phase II — Electron transport chain (ETC) and oxidative phosphorylation on the inner mitochondrial membrane.
  • Final electron acceptor: O2, forming H2O at the end of the ETC.
  • Overall, glucose oxidation proceeds from glycolysis to pyruvate, then to acetyl-CoA, through the TCA cycle, and finally through the ETC to maximize ATP yield.

Energetics: ATP yield from complete oxidation of one mole of glucose

  • Per pyruvate (from glycolysis):
    • 4 NADH → 12 ATP, 1 FADH2 → 2 ATP, 1 GTP → 1 ATP
    • Total per pyruvate: 15 extATP15\ ext{ATP}
  • Per glucose (two pyruvate molecules):
    • From TCA/ETC: 2×15=30 extATP2 \times 15 = 30\ ext{ATP}
    • From glycolysis (substrate-level phosphorylation): 2 ATP
    • From glycerol phosphate shuttle (glycolysis NADH): 4 ATP
    • Total (per glucose, with glycerol phosphate shuttle): 36 extATP36\ ext{ATP}
  • Overall oxidation of glucose to CO2 and H2O:
    • Equation: glucose+6 O<em>2→6 CO</em>2+6 H2O+36 ATP.\text{glucose} + 6\,\mathrm{O<em>2} \rightarrow 6\,\mathrm{CO</em>2} + 6\,\mathrm{H_2O} + 36\,\text{ATP}.
  • Two-half reaction view:
    • Energy release (oxidation): glucose+6 O<em>2→6 CO</em>2+6 H2O.\text{glucose} + 6\,\mathrm{O<em>2} \rightarrow 6\,\mathrm{CO</em>2} + 6\,\mathrm{H_2O}.
    • Energy trapping (ATP synthesis): 36 ADP+36 Pi→36 ATP.36\,\mathrm{ADP} + 36\,\mathrm{Pi} \rightarrow 36\,\mathrm{ATP}.
  • Efficiency of energy trapping:
    • Energy released: 2870 kJ.2870\ \text{kJ}.
    • Energy trapped in ATP: 1098 kJ.1098\ \text{kJ}.
    • Efficiency: η=10982870≈0.38  or  38%.\eta = \frac{1098}{2870} \approx 0.38 \;\text{or} \;38\%.

Quick recap

  • Glycolysis: glucose → 2 pyruvate; net 2 ATP and 2 NADH.
  • Pyruvate enters the TCA cycle (via acetyl-CoA) and generates: 6 NADH, 2 FADH2, 2 GTP per glucose (from two acetyl-CoA turns).
  • NADH and FADH2 feed into ETC to drive ATP synthesis.
  • Glycerol phosphate shuttle converts cytosolic NADH to mitochondrial FADH2, yielding additional ATP.
  • Total ATP yield from one glucose under the course convention: 36 ATP.
  • Ethanol and lactate pathways re-oxidize NADH when O2 is scarce; glycerol phosphate shuttle enables efficient NADH oxidation under aerobic conditions.

Textbook references

  • Hardin, Bertoni and Kleinsmith; Becker's World of the Cell (9th Ed.). Pages 274-279
  • Murdoch University slides (Biology/Physiology course materials)