Krebs Cycle and Glycolysis: Steps and Enzymes

Posted on Jan 8, 2025 in Biochemistry

Krebs Cycle (Citric Acid Cycle)

Step 1: Formation of Citrate

  • Substrate: Acetyl CoA
  • Enzyme: Citrate synthase
  • Cofactor: None
  • Product: Citrate

Step 2: Isomerization of Citrate

  • Substrate: Citrate
  • Enzyme: Aconitase
  • Cofactor: None
  • Product: Isocitrate

Step 3: Oxidative Decarboxylation of Isocitrate

  • Substrate: Isocitrate
  • Enzyme: Isocitrate dehydrogenase
  • Cofactor: NAD+
  • Product: Alpha-ketoglutarate, NADH, CO2

Step 4: Oxidative Decarboxylation of Alpha-ketoglutarate

  • Substrate: Alpha-ketoglutarate
  • Enzyme: Alpha-ketoglutarate dehydrogenase complex
  • Cofactors: Thiamine pyrophosphate (TPP), Lipoic acid, Coenzyme A, NAD+
  • Product: Succinyl CoA, NADH, CO2

Step 5: Substrate-level Phosphorylation of Succinyl CoA

  • Substrate: Succinyl CoA
  • Enzyme: Succinyl-CoA synthetase
  • Cofactor: None
  • Product: Succinate, ATP (or GTP)

Step 6: Oxidation of Succinate

  • Substrate: Succinate
  • Enzyme: Succinate dehydrogenase (part of the electron transport chain)
  • Cofactors: FAD, Iron-Sulfur Cluster
  • Product: Fumarate, FADH2

Step 7: Hydration of Fumarate

  • Substrate: Fumarate
  • Enzyme: Fumarase
  • Cofactor: None
  • Product: Malate

Step 8: Oxidation of Malate

  • Substrate: Malate
  • Enzyme: Malate dehydrogenase
  • Cofactor: NAD+
  • Product: Oxaloacetate, NADH

Glycolysis Pathway

Reaction 1: (Irreversible Reaction)

Example: Enzyme example induced fit model

  • Substrate: Glucose
  • Product: Glucose-6-phosphate
  • Enzyme: Hexokinase
  • Cofactor: Magnesium ion (Mg2+)
  • Coenzyme: None

Reaction 2:

  • Substrate: Glucose-6-phosphate
  • Product: Fructose-6-phosphate
  • Enzyme: Phosphoglucose isomerase
  • Cofactor: None
  • Coenzyme: None

Reaction 3: (Irreversible Reaction)

  • Substrate: Fructose-6-phosphate, ATP
  • Product: Fructose-1,6-bisphosphate, ADP
  • Enzyme: Phosphofructokinase-1
  • Cofactor: Magnesium ion (Mg2+)
  • Coenzyme: None

Reaction 4:

  • Substrate: Fructose-1,6-bisphosphate
  • Products: Dihydroxyacetone phosphate (DHAP), Glyceraldehyde-3-phosphate (G3P)
  • Enzyme: Aldolase
  • Cofactor: None
  • Coenzyme: None

Reaction 5:

  • Substrate: Dihydroxyacetone phosphate (DHAP)
  • Product: Glyceraldehyde-3-phosphate (G3P)
  • Enzyme: Triose phosphate isomerase
  • Cofactor: None
  • Coenzyme: None

Reaction 6:

  • Substrate: Glyceraldehyde-3-phosphate (G3P), NAD+, inorganic phosphate (Pi)
  • Products: 1,3-Bisphosphoglycerate, NADH, H+
  • Enzyme: Glyceraldehyde-3-phosphate dehydrogenase
  • Cofactor: None
  • Coenzyme: Nicotinamide adenine dinucleotide (NAD+)

Reaction 7:

  • Substrate: 1,3-Bisphosphoglycerate, ADP
  • Products: 3-Phosphoglycerate, ATP
  • Enzyme: Phosphoglycerate kinase
  • Cofactor: None
  • Coenzyme: None

Reaction 8:

  • Substrate: 3-Phosphoglycerate
  • Product: 2-Phosphoglycerate
  • Enzyme: Phosphoglycerate mutase
  • Cofactor: None
  • Coenzyme: None

Reaction 9:

  • Substrate: 2-Phosphoglycerate
  • Product: Phosphoenolpyruvate (PEP), H2O
  • Enzyme: Enolase
  • Cofactor: None
  • Coenzyme: None

Reaction 10: (Irreversible Reaction)

  • Substrate: Phosphoenolpyruvate (PEP), ADP
  • Product: Pyruvate, ATP
  • Enzyme: Pyruvate kinase
  • Cofactor: None
  • Coenzyme: None
  • End product: Pyruvate + 2ATP + 2NADH