Glycolysis

• Glucose:

  • An excellent fuel
  • Can be efficiently stored in the polymeric form
  • Many organisms can run on glucose only
• Glucose is a versatile biochemical precursor to:

  • Amino acids
  • Membrane lipids
  • Nucleotides in DNA and RNA
  • Cofactors needed for the metabolism
• Glycolysis is a sequence of 10 enzyme- catalysed reaction by which glucose is converted into pyruvate.
• Pyruvate can be metabolised in different ways depending on:

  • Availability of oxygen
  • Mitochondria
  • Tissue
  • Can be used in biosynthesis
• Conversion of glucose into F-6-P:

  1. Phosphorylation of Glc
  2. Isomerization
  3. Second phosphorylation
• Step One: Phosphorylation of Glc

  • Catalysed by hexokinase (traps glucose)
  • Irreversible and under control of regulatory factors
  • Glc6P cannot pass through the membrane
  • The phosphoryl group destabilises glucose= facilitating further metabolism
• Step Two: Isomerisation

  • Glc6P <-> Fru6P
  • Aldose <-> Ketone
  • Catalysed by phosphoglucose isomerase (PGI)
• Process of isomerisation:

  1. Ring opening and binding
  2. Protein abstraction by weak acid residue leads to cis-enediol formation
  3. Keto-enol tautomerism and proton exchange
  4. Ring closed and release from active site
• Step 3: Second Phosphorylation

  • F6P + ATP -> F1,6P + ADP
  • Catalysed by phosphofructokinase (PFK)
  • Irreversible
  • Allosterically controlled
• Stage 2: Cleavage of Fru1,6BP

  • Conversion of 6C sugar into 2x3C fragments
  • Two steps: Aldol cleavage and isomerisation
  • Net reaction: F-1,6BP <-> 2 GAP
• Stage 2, Step 1: Flu1,6BP cleavage by aldolase

  • Splitting of Fru1,6BP into 2 different triose phosphates
  • Reversible reaction
  • Reverse of the reaction is an aldol condensation
• Stage 2, Step 1 reaction mechanism:

  1. Enzyme-substrate intermediate formation through a schiff base
  2. Divalent cation required to stabilise enzyme-substrate complex
• Stage 2, Step 2: DHAP isomerisation

  • DHAP <-> GAP
  • Ketose <-> Aldose
  • Catalysed by triose phosphate isomerase (TPI)
  • Reversible
  • Substrate binding is rate limiting
• Stage 3: Oxidoreduction reaction and ATP synthsis

  • Oxidation of 3C fragments yields ATP and NADH
  • Net reaction: 2 GAP + 4ADP + 2NAD+ -> 2 pyruvate + 4 ATP + 2 NADH + 2H+
• Stage 3 processing include:

  1. Oxidation followed by phosphorylation
  2. ATP production
  3. Intramolecular rearrangement
  4. Dehydration
  5. ATP production
• Stage 3, Step 1: Conversion of GAP into 1,3 BPG

  • 1,3BPG is an acyl phosphate, a compound with a high P-transfer potential
  • Reaction is catalysed by glyceraldehyde 3-phosphate dehydrogenase
  • Reversible in cells
• Stage 3, Step 1: Two half reaction catalysed by GAPDH

  1. Aldehyde group is oxidised to a carboxylic acid by NAD+
  2. Acyl- phosphate formation by joining of carboxylic acid and orthophosphate
  3. Two reactions must be coupled
• ATP generation possible via different independent processes:

  1. Substrate- level phosphorylation
  2. Oxidative phosphorylation in the ETC
• Substrate level phosphorylation

  • Formation of high energy phosphate bonds by phosphorylation of ADP to ATP
  • Coupled to cleavage of a high-energy metabolic intermediate
  • Takes place in cytosol
• Oxidative phosphorylation in ETC

  • 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 ETC
  • Takes place in the mitochondria
• Substrate-level phosphorylation can occur when the P-transfer potential of the substrate is greater than that of ATP
• Stage 3, Step 2: Substrate-level phosphorylation

  • By phosphoglycerate kinase
  • 1,3 Bisphosphoglycerate + ADP + H -> 3-Phosphoglycerate + ATP
• Stage 3, Step 3: Interconversion of 3PG to 2PG

  • Catalysed by phosphoglycerate mutase
  • Mutase is an enzyme that catalyses the intramolecular shift of chemical group
• Stage 3, Step 4: Water Elimination from 2PG

  • 2PG <-> PEP + H2O
  • The enzyme enolase removes a water molecule thereby forming a new double bond to facilitate P-group transfer in a subsequent reaction
• Stage 3, Step 5: Substrate-level phosphorylation

  • By pyruvate kinase
  • Net reaction: PEP + ADP -> Pyruvate + ATP
  • Irreversible reaction
  • Dependent on Mg2+ and K+
• Glycolysis net reaction:

  • Input: Glucose + 2NAD + 2Pi+ 2ADP
  • Gain: 2 pyruvate + 2NADH + 2H + 2ATP + H2O
• Alcoholic fermentation of glucose:

  • Maintaining redox balance
  • NADH produced in the reaction catalyzed by GAPDH is used in the reaction catalyzed byADH
  • NAD+ or NADH do NOT appear in the netreaction but play a crucial role in the overallprocess
• NAD regeneration in anaerobic glycolysis

  • Lactate is formed from pyruvate by a variety of microorganism
  • Reaction catalysed by lactate dehydrogenase
  • Net glycolysis reaction:Glc + 2 Pi + 2 ADP → 2 lactate + 2 ATP