GLUCOSE METABOLISM

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Chapter 14

• GLUCOSE METABOLISM

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OVERVIEW OF GLYCOLYSIS

• Key energy-production pathway
• Glucose? 2 pyruvate 2 ATP
• Pathway common in all organisms-10 common enzymes
• Also known as the Embden-Meyerhof-Parnas
  Pathway after researchers from the 1940s

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OVERVIEW OF GLYCOLYSIS

• Glucose2NAD2ADP 2 Pi?2pyruvate2NADH2ATP
• Stage 1 energy investment rxns 1-5 consumes 2
  ATP
• Stage 2 energy recovery rxns 6-10 generates 4
  ATP and 2 NADH
• NET 2 ATP and 2 NADH

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OVERVIEW OF GLYCOLYSIS

• PATHWAY Figure 14.1

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REACTIONS OF GLYCOLYSIS

• Enzyme 1 Hexokinase
• Glucose ATP Mg2 ? Glucose-6-P ADP
• Mg is essential for shielding ?, ?, and/or ?
  phosphate charges during nucleophilic attack of P
  by Glucose
• Substrate binding closes enzyme active cleft,
  excluding water, moving ATP nearer to glucose
  molecule (catalysis by proximity effect) and
  expediting the nucleophilic reaction

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REACTIONS OF GLYCOLYSIS

• Enzyme 2 Phosphoglucose Isomerase (PGI)
• Glucose-6-P ? Fructose-6-P
• Aldose (pyranose) to Ketose (furanose) with ring
  opening and reclosing
• Classic acid-base enzyme catalysis mechanism
  involving a Lys-H donor and Glu-H acceptor
  (Figure 14.3)

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REACTIONS OF GLYCOLYSIS

• Enzyme 3 Phosphofructokinase (PFK)
• Fructose-6-P ATP Mg2 ? Fructose-1-6-BisP
• Bis versus Di because PO4 not attached to each
  other
• Mechanism very similar to hexokinase
  nucleophilic attack of Mg2-shielded ATP
• Key control point (later - AMP, ATP, Citrate)

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REACTIONS OF GLYCOLYSIS

• Enzyme 4 Aldolase
• Fructose-1-6-BisP ? G3P DHAP
• Glucose atoms 1-3 3, 2, 1 of DHAP Glucose
  atoms 4-6 1, 2, and 3 of G3P
• Aldol cleavage requirements (C2 carbonyl and C4
  hydroxyl) makes clear the need for the
  aldose-ketose isomerization
• Enzyme shows diversity in mechanism. Many have
  Schiff base (sugar keto carbon amine) (covalent
  catalysis) intermediates (fig 14-5) or metal ion
  catalysis

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REACTIONS OF GLYCOLYSIS

• Enzyme 5 Triosphosphate Isomerase (TPI or
  TIM)
• DHAP ? G3P
• Enediol intermediate for isomerization
• Acid-base catalysis like other isomerase, using
  Glu and His
• Stereoelectronic control prevents C3-phosphate
  elimination, which would be favored product in
  non-enzymatic reaction
• Perfect Enzyme rate is diffusion limited
• Equilibrium favors DHAP coupled rxns with
  following steps keep conversion to C3P moving
  forward

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REACTIONS OF GLYCOLYSIS

• Enzyme 6 Glyceraldehyde-3-Phosphate
  Dehydrogenase (GAPDH)
• G3P NAD Pi ? 1,3 BPG NADH H
• Aldehyde oxidation (exergonic) drives synthesis
  of high-energy acyl phosphate (high-energy cpd)
• Cys forms covalent intermediate, thiohemiacetal,
  which is oxidized to a thioester
• Net Rxn is endergonic

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REACTIONS OF GLYCOLYSIS

• Enzyme 7 Phosphoglycerokinase (PGK)
• 1,3 BPG ADP Mg2 ? 3-PG ATP
• Substrate binding cleft swings closed after
  substrate binding to provide a water-free
  environment (proximity effect like hexokinase)
• Couples with G3PDH to pull that reaction forward
• Substrate level phosphorelation directly yields 1
  ATP

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REACTIONS OF GLYCOLYSIS

• Enzyme 8 Phosphoglycerate Mutase (PGM)
• 3PG ? 2PG
• Phosphorelated His on enzyme catalyzes the
  isomeration (proximity effect)
• 2,3 BPG intermediate occasionally diffuses off
  the enzyme this is the factor that binds
  deoxyhemoglobin to lower its binding affinity but
  the majority of cpd produced in RBCs using
  separate enzyme

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REACTIONS OF GLYCOLYSIS

• Enzyme 9 Enolase
• 2PG ? PEP H2O
• Hydrolysis to form second high energy
  intermediate
• Metal Ion Catalysis

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REACTIONS OF GLYCOLYSIS

• Enzyme 10 Pyruvate Kinase (PK)
• PEP ADP ? Pyruvate ATP
• Hydrolysis and tautomerization of second high
  energy intermediate
• Requires both K and 2 Mg2 for ADP nucleophilic
  attack of PEP
• Enolase Rxn was, thus, required to provide high
  energy compound capable of ATP generation

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REACTIONS OF GLYCOLYSIS

• SUMMARY
• Energy investment in stage 1 recaptured in stage
  2
• Products
• ATP net 2 ATP/glucose
• NADH 2 NADH/glucose to be regenerated by
  electron transport pathway with ATP production
• PYRUVATE 2 pyruvates go to Citric Acid Cycle
  (aerobic) or lactate (anaerobic-regenerates NAD)