Glycolysis - A Complete Explanation

What is Glycolysis?

Location

Overall Summary Reaction

Glucose + 2 NAD⁺ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O

• 2 ATP (net; 4 produced, 2 consumed)
• 2 NADH
• 2 Pyruvate

The Two Phases

Phase 1: Energy-Investment (Preparatory) Phase - Steps 1-5

• Consumes 2 ATP to "prime the pump"
• Glucose is phosphorylated twice and split into two 3-carbon triose phosphates
• Goal: activate glucose for cleavage

Phase 2: Energy-Generation (Payoff) Phase - Steps 6-10

• Produces 4 ATP and 2 NADH
• The two triose phosphates are oxidized and dephosphorylated down to pyruvate
• Net gain: 2 ATP, 2 NADH

Glucose Transport Into Cells

1. GLUT transporters (Na⁺/ATP-independent) - Passive facilitated diffusion via a family of 14 isoforms (GLUT-1 to GLUT-14). Each transports one molecule at a time (uniporter).
   • GLUT-1: ubiquitous, most tissues
   • GLUT-2: liver, kidneys, pancreatic β-cells (high Km, low-affinity, high-capacity - acts as glucose sensor)
   • GLUT-4: skeletal muscle and adipose tissue (insulin-dependent - insulin increases GLUT-4 insertion into membrane)
   • GLUT-5: fructose transporter in intestine
2. SGLT (Na⁺-dependent cotransport) - Active transport in intestinal epithelium and renal tubules; couples glucose uptake to the Na⁺ gradient.

The 10 Steps of Glycolysis in Detail

STEP 1: Glucose → Glucose 6-Phosphate (G6P)

• Phosphorylation traps glucose inside the cell (charged phosphate group prevents membrane crossing)
• G6P is a branch-point metabolite - it can enter glycolysis, the pentose phosphate pathway, or glycogen synthesis
• Hexokinase (I-III): Low Km (~0.1 mM) = high affinity for glucose; functions at low glucose. Inhibited by its product G6P (product inhibition). Low Vmax - prevents overconsumption of phosphate
• Glucokinase (HK IV): High Km (~10 mM) = low affinity; only active during hyperglycemia (after a meal). High Vmax - rapidly processes a flood of glucose. NOT inhibited directly by G6P. In liver: regulated by glucokinase regulatory protein (GKRP) - sequestered in nucleus when fructose-6-P is high; released to cytosol when glucose rises. In pancreatic β-cells: acts as a glucose sensor for insulin secretion threshold.

STEP 2: Glucose 6-Phosphate → Fructose 6-Phosphate (F6P)

• Converts an aldose (glucose) to a ketose (fructose) sugar
• Positions the keto group next to carbon 3, which is essential for the next step (cleavage)
• Equilibrium favors G6P, but is pulled forward by consumption of F6P

STEP 3: Fructose 6-Phosphate → Fructose 1,6-Bisphosphate (F1,6-BP)

• This is the rate-limiting, committed step - considered the main control point of glycolysis
• PFK-1 is an allosteric enzyme with a regulatory and catalytic site
• Activators of PFK-1: AMP, ADP, fructose 2,6-bisphosphate (F-2,6-BP), inorganic phosphate
• Inhibitors of PFK-1: ATP (at high concentrations), citrate (signals that TCA cycle intermediates are abundant)
• Fructose 2,6-bisphosphate (F-2,6-BP) is the most potent allosteric activator. It is synthesized by PFK-2 (stimulated by insulin) and degraded by fructose 2,6-bisphosphatase (stimulated by glucagon). This gives hormones indirect control over glycolytic flux.

STEP 4: Fructose 1,6-Bisphosphate → Glyceraldehyde 3-Phosphate + Dihydroxyacetone Phosphate

• The 6-carbon molecule is cleaved into two 3-carbon molecules
• Only G3P continues directly in glycolysis
• DHAP is isomerized to G3P in the next step (so effectively both halves proceed)

STEP 5: Dihydroxyacetone Phosphate → Glyceraldehyde 3-Phosphate

• Converts DHAP into a second G3P
• After this step, all further reactions proceed twice (once for each G3P from one glucose)
• TPI is one of the most catalytically efficient enzymes known (near "catalytic perfection")

From here on, every step happens TWICE per glucose molecule.

STEP 6: Glyceraldehyde 3-Phosphate → 1,3-Bisphosphoglycerate (1,3-BPG)

• This is the only oxidation step in glycolysis - G3P (an aldehyde) is oxidized to a carboxylic acid derivative
• Creates the first high-energy compound (acyl-phosphate bond in 1,3-BPG)
• Mechanism: GAPDH uses a cysteine residue to form a covalent thioester intermediate, conserving energy from the oxidation in the acyl-phosphate bond of 1,3-BPG. This is an example of covalent catalysis. - Basic Medical Biochemistry, p. 818
• This step requires NAD⁺. If NAD⁺ is not regenerated (e.g., no oxygen), glycolysis stops - this is why anaerobic regeneration of NAD⁺ (via lactate) is essential.
• Inhibited by iodoacetate (used in biochemistry research to block glycolysis)

STEP 7: 1,3-Bisphosphoglycerate → 3-Phosphoglycerate (3-PG)

• First ATP-generating step of glycolysis
• "Substrate-level phosphorylation" means phosphate is transferred directly from a metabolic intermediate to ADP (unlike oxidative phosphorylation which uses the ETC)
• This step recoup the 2 ATP invested in steps 1 and 3

STEP 8: 3-Phosphoglycerate → 2-Phosphoglycerate (2-PG)

• A simple isomerization (intramolecular phosphate transfer)
• 2,3-BPG serves as a cofactor in this reaction - it acts as an intermediate in the phosphate transfer mechanism
• Prepares the substrate for the next highly exergonic step

STEP 9: 2-Phosphoglycerate → Phosphoenolpyruvate (PEP)

• Removal of water creates PEP, which contains an extremely high-energy enol phosphate bond (ΔG°' of hydrolysis ≈ -61.9 kJ/mol)
• This stored energy is released in the final step
• Enolase requires Mg²⁺ and Zn²⁺
• Inhibited by fluoride (used in blood collection tubes for glucose testing - gray-top tubes contain NaF to inhibit enolase)

STEP 10: Phosphoenolpyruvate → Pyruvate

• Second and final ATP-generating step; substrate-level phosphorylation
• Allosteric activator: Fructose 1,6-bisphosphate (feedforward activation - when PFK-1 is active, it signals PK to also be active)
• Allosteric inhibitor: ATP, alanine (signals adequate energy and amino acid supply)
• Hormonal regulation (liver PK isoenzyme only):
  • Insulin activates liver PK (dephosphorylation)
  • Glucagon inhibits liver PK (phosphorylation via PKA cascade) - to suppress glycolysis and favor gluconeogenesis during fasting
• Pyruvate kinase deficiency: Autosomal recessive disorder. Most common enzymatic cause of hemolytic anemia. RBCs are entirely dependent on glycolysis for ATP; without adequate PK, ATP is depleted, the Na/K-ATPase fails, and cells lyse.

Summary of All 10 Steps

Regulation of Glycolysis

1. Hexokinase (Step 1)

• Inhibited by G6P (product inhibition) - prevents glucose sequestration when it is not needed
• Glucokinase: regulated by GKRP in response to glucose and fructose-6-P levels

2. PFK-1 (Step 3) - The Main Control Point

• Inhibited by: High ATP (signals energy sufficiency), citrate (TCA cycle backed up)
• Activated by: AMP, ADP (energy deficiency signal), F-2,6-BP (most potent activator), Pi
• The AMP:ATP ratio is the fundamental energy-sensing signal
• F-2,6-BP is the key hormonal messenger:
  • Insulin → activates PFK-2 → increases F-2,6-BP → activates PFK-1 → glycolysis ON
  • Glucagon → activates fructose 2,6-bisphosphatase → decreases F-2,6-BP → glycolysis OFF

3. Pyruvate Kinase (Step 10)

• Activated by: F-1,6-BP (feedforward activation from step 3)
• Inhibited by: ATP, alanine (signals energy surplus and amino acid availability)
• Liver isoenzyme additionally regulated by covalent phosphorylation: glucagon inhibits, insulin activates

AMP as a Sensitive Energy Signal

Tissue-Specific Isoenzymes

Fate of Pyruvate

1. Aerobic conditions (mitochondria present, O₂ available)

• Pyruvate enters the mitochondria via the mitochondrial pyruvate carrier
• Converted to Acetyl-CoA by pyruvate dehydrogenase complex (PDC):
  • Pyruvate + NAD⁺ + CoA → Acetyl-CoA + CO₂ + NADH
• Acetyl-CoA feeds the TCA (Krebs) cycle → oxidative phosphorylation → up to ~30-32 ATP total per glucose

2. Anaerobic conditions (no O₂, or no mitochondria)

• Pyruvate is reduced to lactate by lactate dehydrogenase (LDH):
  • Pyruvate + NADH + H⁺ → Lactate + NAD⁺
• This regenerates NAD⁺, allowing glycolysis to continue
• No additional ATP is gained
• Net yield: only 2 ATP per glucose

3. In yeast and some microorganisms

• Pyruvate → Acetaldehyde + CO₂ (by pyruvate decarboxylase)
• Acetaldehyde → Ethanol + NAD⁺ (by alcohol dehydrogenase)
• This is fermentation

4. In liver (biosynthetic precursor)

• Pyruvate → Oxaloacetate (by pyruvate carboxylase) → gluconeogenesis
• Pyruvate → Acetyl-CoA → fatty acid synthesis

Anaerobic Glycolysis and the Cori Cycle

• Measuring blood lactate is used to detect O₂ debt (inadequate tissue oxygenation)
• Lactic acidosis occurs when circulatory collapse (MI, pulmonary embolism, hemorrhage, septic shock) deprives tissues of O₂, forcing them to rely on anaerobic glycolysis. Lactate accumulates, dropping blood pH. - Biochemistry, 8th ed, Lippincott, p. 318

Energy Yield - Full Accounting

• Malate-aspartate shuttle (heart, liver): each NADH yields ~2.5 ATP
• Glycerol-3-phosphate shuttle (muscle, brain): each NADH yields ~1.5 ATP

Special Topics

The Warburg Effect (Aerobic Glycolysis in Cancer)

HIF-1 and Hypoxia

2,3-Bisphosphoglycerate and RBCs

Dental Caries

Key Diseases/Clinical Correlations

Summary of Key Concepts

1. Glycolysis = 10 steps, cytosol, universal - operates with or without oxygen
2. Phase 1 (steps 1-5) invests 2 ATP to activate glucose; produces 2 G3P
3. Phase 2 (steps 6-10) yields 4 ATP and 2 NADH per glucose
4. Net yield: 2 ATP + 2 NADH + 2 pyruvate per glucose
5. Three irreversible reactions (hexokinase, PFK-1, pyruvate kinase) are the regulatory checkpoints
6. PFK-1 is the primary rate-limiting enzyme; activated by AMP/ADP/F-2,6-BP; inhibited by ATP/citrate
7. Fate of pyruvate: Acetyl-CoA (aerobic) or lactate (anaerobic)
8. Anaerobic glycolysis regenerates NAD⁺ via lactate and sustains ATP production without O₂
9. Warburg effect: cancer cells prefer glycolysis even in O₂ (for biosynthetic precursors + faster ATP rate)
10. Clinical correlations include PK deficiency, lactic acidosis, and the metabolic basis of dental caries