Glycolysis, Krebs Cycle, and Oxidative Phosphorylation: Key Steps and Enzymes

Step 1 of Glycolysis

Substrate → Product: Glucose → Glucose-6-phosphate

Enzyme: Hexokinase

Energy intermediates: ATP → ADP +iP (ATP consumed)

Other molecules: none

Step 2 of Glycolysis

Substrate → Product: Glucose-6-phosphate → Fructose-6-phosphate

Enzyme: Phosphoglucoisomerase

Energy intermediates: none

Other molecules: none

Step 3 of Glycolysis

Substrate → Product: Fructose-6-phosphate → Fructose-1,6-bisphosphate

Enzyme: Phosphofructokinase-1 (PFK-1)

Energy intermediates: ATP → ADP + iP (ATP consumed)

Other molecules: none

Step 4 of Glycolysis

Substrate → Product: Fructose-1,6-bisphosphate → Glyceraldehyde 3 Phosphate (G3P) + Dihydroxyacetone phosphate

Enzyme: Aldolase

Energy intermediates: none

Other molecules none

Notes: Cleavage into two 3-carbon sugars.

Step 5 of Glycolysis

Substrate → Product: Dihydroxyacetone phosphate ↔ Glyceraldehyde-3-Phosphate (G3P)

Enzyme: Triose phosphate isomerase

Energy intermediates: none

Notes: Interconversion maintains equilibrium between two triose phosphates.

Step 6 of Glycolysis

Substrate → Product: Glyceraldehyde-3-Phosphate (G3P) → 1,3-bisphosphoglycerate

Enzyme: G3P dehydrogenase

Energy intermediates: NAD⁺ + 2H⁺ +2e⁻ → NADH + H⁺

Notes: Oxidation and phosphorylation; first energy-yielding step.

Step 7 of Glycolysis

Substrate → Product: 1,3-bisphosphoglycerate → 3-phosphoglycerate

Enzyme: Phosphoglycerate kinase

Energy intermediates: ADP +iP (cytosolic) → ATP

Notes: First substrate-level phosphorylation.

Step 8 of Glycolysis

Substrate → Product: 3-phosphoglycerate → 2-phosphoglycerate

Enzyme: Phosphoglycerate mutase

Energy intermediates: none

Step 9 of Glycolysis

Substrate → Product: 2-phosphoglycerate → Phosphoenolpyruvate (PEP)

Enzyme: Enolase

Energy intermediates: none

Other molecules: H₂O released

Notes: Dehydration reaction producing a high-energy phosphate compound.

Step 10 of Glycolysis

Substrate → Product: Phosphoenolpyruvate (PEP) → Pyruvate

Enzyme: Pyruvate kinase

Energy intermediates: ADP+ iP → ATP

Notes: Second substrate-level phosphorylation; irreversible step.

Pyruvate Oxidation step

Substrate → Product: Pyruvate → Acetyl-CoA + CO₂

Enzyme:

1) Pyruvate dehydrogenase (CO₂ removal)

2) Dihydrolipoamide acetyltransferase/ Dihydrolipoyl transacetylase (adds CoASH)

3) Dihydrolipoamide dehydrogenase/ Dihydrolipoyl dehydrogenase (NADH + H⁺

Energy intermediates: NAD⁺ +2H⁺ + 2e⁻ → NADH + H

Other molecules: CO₂ released

Notes: Link between glycolysis and Krebs cycle; irreversible oxidative decarboxylation.

What is step 1 of the Krebs Cycle?

Substrate → Product: Acetyl-CoA + Oxaloacetate → Citrate

Enzyme: Citrate synthase

Energy intermediates

Other molecules: CoA-SH released

What is step 2 of the Krebs Cycle?

Substrate → Product: Citrate → Isocitrate

Enzyme: Aconitase

Other molecules: H₂O removed then added

Energy intermediates: None

What is step 3 of the Krebs Cycle?

Substrate → Product: Isocitrate → α-Ketoglutarate + CO₂

Enzyme: Isocitrate dehydrogenase

Energy intermediates: NAD⁺ + 2H⁺ +2e⁻ → NADH + H⁺

Other Molecules: CO₂ removed

Notes: First oxidative decarboxylation

What is step 4 of the Krebs Cycle?

Substrate → Product: α-Ketoglutarate → Succinyl-CoA + CO₂

Enzyme: α-Ketoglutarate dehydrogenase

Energy intermediates: NAD⁺ + 2H⁺ +2e⁻ → NADH + H⁺

Other Molecules: CO₂ removed, CoASH added

Notes: Second oxidative decarboxylation; similar to PDH complex.

What is step 5 of the Krebs Cycle?

Substrate → Product: Succinyl-CoA → Succinate

Enzyme: Succinyl-CoA synthetase

Energy intermediates: GDP + Pi → GTP and ADP+ iP → ATP

Other Molecules: CoASH released

Other molecules: CoA-SH released.

What is step 6 of the Krebs Cycle?

Substrate → Product: Succinate → Fumarate

Enzyme: Succinate dehydrogenase

Energy intermediates: FAD⁺ + 2H⁺ +2e⁻ → FADH₂

Notes: Only membrane-bound enzyme (Complex II of ETC).

What is step 7 of the Krebs Cycle?

Substrate → Product: Fumarate → Malate

Enzyme: Fumarase

Other molecules: H₂O added

Energy intermediates: None

Notes: Hydration across double bond.

What is step 8 of the Krebs Cycle?

Substrate → Product: Malate → Oxaloacetate

Enzyme: Malate dehydrogenase

Energy intermediates: NAD⁺ + 2H⁺ +2e⁻ → NADH + H⁺

Notes: Regenerates OAA to restart cycle.

What is Complex I of Oxidative Phosphorylation?

NADH dehydrogenase (Complex I)

Function: Transfers electrons from NADH to ubiquinone (CoQ)

Energy intermediates: NADH + H⁺ → NAD⁺ + 2H⁺ +2e⁻

Protons: 4 H⁺ pumped into intermembrane space.

What is Complex II of Oxidative Phosphorylation?

Succinate dehydrogenase (Complex II)

Function: Transfers electrons from FADH₂ to CoQ

Energy intermediates: FADH₂ oxidized to FAD⁺ + 2H⁺ +2e⁻

Protons: none pumped.

What is Complex III of Oxidative Phosphorylation

Cytochrome bc₁ complex

Function: Transfers electrons from CoQH₂ to cytochrome c

Protons: 4 H⁺ pumped.

What is Complex IV of Oxidative Phosphorylation

Cytochrome c oxidase

Function: Transfers electrons from cytochrome c to O₂

Other molecules: O₂ reduced → H₂O

Protons: 2 H⁺ pumped.

ATP Synthase

Complex V

Function: Uses proton gradient (chemiosmosis) to synthesize ATP from ADP + Pi

Notes: ~3 H⁺ per ATP generated.