Glycolysis

• Glycolysis


  Sequence of 10 enzyme-catalysed reactions by which glucose is converted into pyruvate
• Glycolysis


  • Pyruvate can then be metabolised in different ways, depending on availability of O2, mitochondria and tissue
  • Pyruvate can be used as a precursor in biosynthesis
  • Some of free energy is captured by the synthesis of ATP and NADH
• Stage 1 - Priming of glucose

  1. Conversion of glucose into F6P

  2. Phosphorylation of glucose

  3. Isomerisation

  4. Second phosphorylation
• Hexokinase


  Catalyses the phosphorylation of glucose, the reaction is irreversible and under control of regulatory factors
• G6P


  Cannot pass through membrane
• Phosphoglucose isomerase (PGI)


  Catalyses the reversible isomerisation of G6P and F6P
• F6P + ATP → F16P + ADP


  Irreversible reaction that sets the pace of glycolysis and is allosterically controlled
• Stage 2 - Cleavage of F16BP

  1. Aldol cleavage

  2. Isomerisation
• Aldol cleavage


  Splitting of F16BP into 2 different triose phosphates, a reversible reaction
• Triose phosphate isomerase (TPI)


  Catalyses the rapid reversible isomerisation of DHAP and GAP
• Stage 3 - Oxidoreduction reactions and ATP synthesis


  1. Conversion of GAP into 1,3BPG

  2. Substrate level phosphorylation by phosphoglycerate kinase

  3. Interconversion of 3PG and 2PG

  4. Water elimination from 2PG

  5. Substrate level phosphorylation by pyruvate kinase
• 1,3BPG


  An acyl phosphate with a high P-transfer potential
• Glyceraldehyde 3-phosphate dehydrogenase


  Catalyses the conversion of GAP into 1,3BPG, a freely reversible reaction in cells but usually 'drawn' to one side by subsequent metabolic reactions
• Mutase


  An enzyme that catalyses the intramolecular shift of a chemical group, e.g. a phosphoryl group
• Enolase


  Removes a water molecule from 2PG, thereby forming a new double bond to facilitate P-group transfer in a subsequent reaction
• Pyruvate kinase


  Catalyses the irreversible substrate level phosphorylation of ADP to ATP using PEP
• Net reaction of glycolysis: 2GAP + 4ADP + 2NAD+ → 2 pyruvate + 4ATP + 2NADH+ 2H+
• GAPDH


  Catalyses two 'half reactions': 1) Oxidation of the aldehyde group to a carboxylic acid by NAD, 2) Acyl-phosphate formation by joining the carboxylic acid and orthophosphate
• Substrate-level phosphorylation


  The formation of high-energy phosphate bonds by phosphorylation of ADP to ATP (or GDP to GTP) coupled to cleavage of a high-energy metabolic intermediate
• Oxidative phosphorylation


  The formation of high-energy phosphate bonds by phosphorylation of ADP to ATP coupled to the transfer of electrons from reduced coenzymes to molecular oxygen via the electron transport chain
• Glycolysis net reaction: Glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O
• Maintaining redox balance


  1. Alcohol fermentation

  2. Lactic acid fermentation
• Net result of glycolysis with alcohol fermentation: Glc + 2 Pi + 2 ADP + 2 H+ → 2 EtOH + 2 CO2 + 2 ATP + 2 H2O
• Net glycolysis reaction with lactic acid fermentation: Glc + 2 Pi + 2 ADP → 2 lactate + 2 ATP
• Tight control of pathways


  • Enzymes become more or less active in response to allosteric effectors or covalent modification, which is also under hormonal control
  • Amount of enzyme controlled by transcriptional regulation
• Regulation of glycolysis in skeletal muscle

  1. Primary control is cell energy charge, ATP to AMP ratio

  2. Phosphofructokinase (Pfk) is the most important control site

  3. Hexokinase (HK) is also an important control site

  4. Pyruvate kinase (PYK) is also an important control site
• Regulation of glycolysis in liver

  1. Pfk is the most important control site

  2. HK in liver has an isozyme, glucokinase (GK), which is not inhibited by G6P

  3. Pyruvate kinase has a liver-specific isozyme, Pyk-L, which is reversibly phosphorylated
• Pyruvate


  Can undergo anaerobic glycolysis (lactate fermentation), conversion back to glucose in gluconeogenesis, used as a precursor in amino acid metabolism, or oxidative decarboxylation to CO2 and acetyl-CoA in the PDH complex
• Acetyl-CoA links glycolytic pyruvate formation to complete oxidation of glucose to CO2 in the TCA cycle


  1. Pyruvate is transported into the mitochondria and converted to acetyl-CoA by the pyruvate dehydrogenase complex

  2. Acetyl-CoA then enters the TCA cycle in the mitochondrial matrix

  3. Oxidative phosphorylation occurs in the inner membrane of the mitochondria
• Pyruvate dehydrogenase complex (PDC)


  A large multienzyme complex that catalyses the irreversible oxidative decarboxylation of pyruvate to acetyl-CoA, CO2 and NADH
• Cellular processes for efficient extraction of energy by complete oxidation of glucose to CO2: Glycolysis only harvests some of the energy, the rest is harvested by the PDH complex and TCA cycle