Explain glycolysis
Glycolysis is the 10-step metabolic pathway that breaks down one glucose molecule (6 carbons) into two pyruvate molecules (3 carbons each), producing a net gain of 2 ATP and 2 NADH. It occurs in the cytoplasm and does not require oxygen. The pathway has two phases: an energy investment phase (steps 1-5, consuming 2 ATP) and an energy payoff phase (steps 6-10, producing 4 ATP and 2 NADH).
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Glycolysis
Glycolysis is a universal 10-step metabolic pathway that converts one molecule of glucose (6C) into two molecules of pyruvate (3C), generating a net gain of 2 ATP and 2 NADH. It occurs in the cytoplasm of virtually all cells and does not require oxygen, making it the cell's only ATP-producing pathway under anaerobic conditions.
Overview Diagrams
Aerobic vs. Anaerobic Glycolysis:
Two-Phase Summary:
The Two Phases
Phase I: Energy Investment (Preparatory) Phase - Steps 1-5
The cell "primes the pump" by spending 2 ATP to phosphorylate glucose into a highly reactive intermediate.
| Step | Reaction | Enzyme | Notes |
|---|---|---|---|
| 1 | Glucose → Glucose 6-phosphate (G6P) | Hexokinase (HK I-III) or Glucokinase (HK IV in liver/beta cells) | Irreversible, ATP-consuming. G6P traps glucose inside the cell. HK I-III have low Km (high affinity); Glucokinase has high Km - acts as a glucose sensor |
| 2 | G6P → Fructose 6-phosphate (F6P) | Phosphoglucose isomerase | Aldose-ketose isomerization; reversible |
| 3 | F6P → Fructose 1,6-bisphosphate (F1,6BP) | Phosphofructokinase-1 (PFK-1) | Irreversible, ATP-consuming. Rate-limiting and committed step; major regulatory point |
| 4 | F1,6BP → Glyceraldehyde 3-phosphate (G3P) + Dihydroxyacetone phosphate (DHAP) | Aldolase | Cleaves the 6C sugar into two 3C fragments |
| 5 | DHAP → G3P | Triose phosphate isomerase (TPI) | Converts DHAP into G3P; both 3C units now enter the payoff phase |
At this point: 1 glucose has become 2 G3P molecules, at the cost of 2 ATP.
Phase II: Energy Payoff (ATP-Generating) Phase - Steps 6-10
Each step occurs twice per glucose (once for each G3P). All energy is recovered here.
| Step | Reaction | Enzyme | Notes |
|---|---|---|---|
| 6 | G3P + NAD+ → 1,3-Bisphosphoglycerate (1,3-BPG) | Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) | Oxidation coupled to phosphorylation; produces 2 NADH |
| 7 | 1,3-BPG → 3-Phosphoglycerate (3-PG) | Phosphoglycerate kinase (PGK) | Substrate-level phosphorylation - generates 2 ATP (×2) |
| 8 | 3-PG → 2-Phosphoglycerate (2-PG) | Phosphoglycerate mutase | Shifts phosphate group; reversible |
| 9 | 2-PG → Phosphoenolpyruvate (PEP) + H2O | Enolase | Dehydration creates a high-energy compound; inhibited by fluoride (used in lab tubes) |
| 10 | PEP → Pyruvate | Pyruvate kinase (PK) | Irreversible, generates 2 ATP (×2). Third key regulatory enzyme |
Net Energy Yield
| Molecule | Invested | Produced | Net |
|---|---|---|---|
| ATP | 2 | 4 | +2 ATP |
| NADH | 0 | 2 | +2 NADH |
| Pyruvate | - | 2 | 2 pyruvate |
What Happens to Pyruvate?
The fate of pyruvate depends on oxygen availability:
- Aerobic conditions (O2 present): Pyruvate enters the mitochondria, is converted to acetyl-CoA by pyruvate dehydrogenase, and feeds into the TCA cycle and oxidative phosphorylation - generating ~30-32 ATP total per glucose.
- Anaerobic conditions (no O2): Pyruvate is reduced to lactate by lactate dehydrogenase, regenerating NAD+ so glycolysis can continue. This is the only ATP source for red blood cells (no mitochondria), rapidly exercising muscle, and hypoxic tissues.
Regulation - The Three Irreversible Steps
Glycolysis is regulated primarily at its three irreversible (thermodynamically favorable) steps:
1. Hexokinase / Glucokinase (Step 1)
- Hexokinase (I-III): Inhibited by its product G6P (product inhibition). Prevents glucose from being phosphorylated faster than it can be used.
- Glucokinase (liver/beta cells): Not inhibited by G6P; acts as a glucose sensor and responds proportionally to blood glucose.
2. Phosphofructokinase-1 / PFK-1 (Step 3) - The Master Regulator
- Inhibited by: High ATP (energy-rich signal), citrate (signals TCA cycle saturation)
- Activated by: AMP (energy-depleted signal), fructose 2,6-bisphosphate (most potent activator)
- Hormonal control: Insulin (fed state) increases fructose 2,6-bisphosphate via PFK-2, activating glycolysis. Glucagon (fasting) does the opposite.
3. Pyruvate Kinase (Step 10)
- Inhibited by: High ATP, acetyl-CoA, alanine (allosteric)
- Activated by: F1,6BP (feed-forward activation)
- Hormonal control: Glucagon (via cAMP/PKA) phosphorylates and inactivates liver PK during fasting
Clinical Connections
| Condition | Enzyme/Mechanism Involved |
|---|---|
| Pyruvate kinase deficiency | Most common glycolytic enzyme defect; causes hemolytic anemia in RBCs (sole ATP source is glycolysis) |
| Warburg effect (cancer) | Tumor cells preferentially use anaerobic glycolysis even in O2 - generates biosynthetic precursors for rapid growth |
| Ischemia/hypoxia | Switch to anaerobic glycolysis → lactate accumulation → lactic acidosis |
| Arsenic poisoning | Inhibits GAPDH (step 6), blocking ATP production |
| Fluoride in blood tubes | Inhibits enolase (step 9), preventing further glucose metabolism - used to preserve glucose in lab specimens |
Key Summary
- Location: Cytoplasm
- Inputs: 1 glucose + 2 ATP + 2 NAD+ + 2 Pi
- Outputs: 2 pyruvate + 4 ATP (net +2) + 2 NADH + 2 H2O
- Three irreversible steps: Steps 1, 3, 10 (enzymes: hexokinase, PFK-1, pyruvate kinase)
- Rate-limiting step: Step 3 (PFK-1)
- Universal: Occurs in all living cells; the most ancient metabolic pathway
Sources: Lippincott Illustrated Reviews: Biochemistry 8e, pp. 294-310; Basic Medical Biochemistry: A Clinical Approach 6e, pp. 791-800
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