Glycolysis
Glycolysis is the anaerobic breakdown of glucose to pyruvate occurring in the cytosol of all cells. It is the central pathway of carbohydrate metabolism and proceeds whether or not oxygen is available.
Overview — 10 Steps, 2 Phases
Preparatory Phase (Steps 1–5): Energy investment — 2 ATP consumed. Glucose is phosphorylated, isomerized, and cleaved into 2 triose phosphates (G3P).
Pay-off Phase (Steps 6–10): Energy recovery — 4 ATP + 2 NADH produced per glucose (×2 for each triose).
Net yield per glucose: 2 ATP, 2 NADH, 2 Pyruvate.
Key Enzymes and Reactions
- Step 1 — Hexokinase/Glucokinase: Glucose → Glucose-6-phosphate (+ ATP consumed). Hexokinase (all cells, low Km, inhibited by G6P). Glucokinase (liver/β-cells, high Km, not inhibited by G6P — acts as a glucose sensor).
- Step 3 — PFK-1 (Rate-limiting step): Fructose-6-P → Fructose-1,6-bisP. Most important regulatory step. Activated by AMP, ADP, fructose-2,6-bisP (major activator). Inhibited by ATP, citrate, H+.
- Step 5 — Aldolase: Fructose-1,6-bisP → DHAP + G3P. DHAP is converted to G3P by Triose phosphate isomerase.
- Step 6 — G3P Dehydrogenase: G3P → 1,3-BPG + NADH (substrate-level oxidation; NAD+ required — limiting in anaerobic conditions).
- Step 7 — Phosphoglycerate Kinase: 1,3-BPG → 3-PG + ATP (substrate-level phosphorylation #1).
- Step 10 — Pyruvate Kinase: PEP → Pyruvate + ATP (#2). Activated by fructose-1,6-bisP (feedforward). Inhibited by ATP, Alanine, glucagon (via phosphorylation).
Fate of Pyruvate
- Aerobic conditions: Pyruvate dehydrogenase complex → Acetyl-CoA → enters TCA cycle
- Anaerobic (muscle): Pyruvate → Lactate (by LDH). Regenerates NAD+ to continue glycolysis.
- Yeast (fermentation): Pyruvate → Ethanol + CO₂ (by pyruvate decarboxylase + alcohol dehydrogenase)
Pyruvate Dehydrogenase Complex (PDC)
Multienzyme complex converting pyruvate → Acetyl-CoA. Requires 5 coenzymes: TPP, Lipoic acid, CoA, FAD, NAD+. Activated by: pyruvate, CoA, NAD+, AMP. Inhibited by: NADH, Acetyl-CoA, ATP. PDC deficiency → Lactic acidosis + neurological damage.
Regulation of Glycolysis
High energy (high ATP/citrate) → inhibits PFK-1 → slows glycolysis. Low energy (high AMP/ADP) → activates PFK-1 → accelerates glycolysis. Fructose-2,6-bisphosphate (made by PFK-2, activated by insulin) is the most potent activator of PFK-1.
Clinical Points
- Pyruvate kinase deficiency → hemolytic anemia (RBCs depend entirely on glycolysis)
- Warburg effect: cancer cells preferentially use glycolysis even in O₂ (aerobic glycolysis)
- Arsenate poisoning: bypasses substrate-level phosphorylation → uncouples glycolysis