BMSC230 Mod. 6

• Mitosis produces two genetically identical daughter cells from one parental cell
• Glucose


  Converted to pyruvate

  
• Pyruvate


  Converted to acetyl CoA

  
• Citric acid cycle


  • Complete oxidation of acetyl CoA to CO2
  • Central hub of metabolism
  • High energy electron products come from many possible intermediates

  
• Pyruvate to Acetyl CoA

  Oxygen is available so pyruvate is converted to acetyl CoA by complex pyruvate dehydrogenase

  
• Acetyl CoA to CO2

  Acetyl CoA then moves to the citric acid cycle to be converted to CO2

  
• Pyruvate dehydrogenase complex


  • Large enzyme complex that contains 3 enzymes and 5 coenzymes
  • Performs redox, decarboxylation and transfer of CoA to the acetyl group

  
• Enzymes in pyruvate dehydrogenase complex

  • Pyruvate dehydrogenase
  • Dihydrolipoyl transacetylase
  • Dihydrolipoyl dehydrogenase

  
• Cofactors in pyruvate dehydrogenase complex


  • Thiamine pyrophosphate (TPP)
  • Lipoic acid
  • Coenzyme A
  • Flavin adenine dinucleotide (FAD)
  • Nicotinamide adenine dinucleotide (NAD+)

  
• Pyruvate dehydrogenase complex


  • 3 enzymes in close proximity increases overall reaction rate by keeping intermediates bound to complex - no lost intermediates

  
• Regulation of pyruvate dehydrogenase complex

  1. Covalent modification (phosphorylation)

  2. Allosteric regulation by molecules that reflect high or low energy charge in the cell

  
• Tumours increase rates of glycolysis and glucose uptake, metabolizing glucose to lactate in aerobic conditions to prepare for hypoxic conditions
• Hypoxia inducible factor 1 (HIF-1) promotes aerobic glycolysis by activating pyruvate dehydrogenase kinase, inactivating pyruvate dehydrogenase and leading to more pyruvate being converted to lactate

  
• Citric acid cycle


  • Acetyl CoA (2 carbons) condenses with a 4-carbon molecule to form a 6-carbon one
  • 6-carbon molecule undergoes 2 oxidative decarboxylations to release 2 CO2 and NADH
  • Resulting cycle is oxidized to form NADH and FADH2
  • Cycle condenses with another acetyl CoA to restart

  
• No net gains of carbons in the citric acid cycle and only 2 ATP is produced through every acetyl CoA, but the FADH and NADH2 that are generated aid in ATP production in the next module (oxidative phosphorylation)

  
• Reactions of the citric acid cycle

  1. Acetyl CoA enters to condense with oxaloacetate to form citrate

  2. Citrate is isomerized to isocitrate by aconitase

  3. Isocitrate gets oxidized and decarboxylated to α-ketoglutarate

  4. α-Ketoglutarate undergoes oxidative decarboxylation to form succinyl CoA

  
• Citrate synthase catalyzes the condensation reaction where synthesis of molecules occurs without requiring ATP, unlike synthetases which need energy from ATP hydrolysis

  
• The position of the hydroxyl group on isocitrate needs to be altered by aconitase to allow the oxidative decarboxylations to release CO2 and capture electrons

  
• Hydrolysis of thioester between CoA and the acetyl group


  Reaction #2

  
• Citrate


  Isomerized to isocitrate by catalyst aconitase

  
• Reaction alters position of hydroxy group
• Hydroxyl group is located improperly to allow oxidative decarboxylations to release CO2 and capture electrons

  
• Isocitrate gets oxidized and decarboxylated to α-ketoglutarate


  1. Reaction #3

  2. Catalyst is isocitrate dehydrogenase

  3. NAD+ converted to NADH+H+

  4. CO2 released

  
• α-ketoglutarate converted to succinyl CoA

  1. Reaction #4

  2. Oxidative decarboxylation

  3. Catalyzed by α-ketoglutarate dehydrogenase

  4. NAD+ converted to NADH

  5. CO2 released

  
• Succinyl CoA converted to succinate

  1. Reaction #5

  2. Succinyl CoA Synthetase cleaves thioester bond

  3. Energy released generates ATP

  
• Regeneration of oxaloacetate


  1. Reaction #6

  2. Catalyzed by succinate dehydrogenase, fumarase, and malate dehydrogenase

  3. Electrons captured in FAD and NADH

  
• Net reaction: Acetyl CoA + 3 NAD+ + FAD + ADP + Pi → 2 CO2 + 3 NADH + ATP + 2 H+ + CoA

  
• Only 1 ATP generated, 3 NADH and 1 FADH2 used to create more ATP
• Occurs in aerobic conditions, O2 from electron transport chain and ATP production
• If no oxygen, FAD and NAD+ don't get regenerated
• Regulation of Citric Acid Cycle

  • Controlled by levels of pyruvate dehydrogenase
  • Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase allosterically regulated by NADH, ATP, and ADP

  
• If pyruvate dehydrogenase is inhibited


  Citrate levels increase, can move to cytosol to inhibit PFK, slowing down glucose metabolism
• When cell energy charge is too high

  α-ketoglutarate dehydrogenase is inhibited, α-ketoglutarate increases, used for amino acid and other molecule synthesis

  
• Anaplerotic reactions


  Replenish intermediates of the cycle, e.g. synthesis of oxaloacetate from pyruvate and CO2

  
• Glyoxylate cycle


  Allows plants and bacteria to convert fats to carbohydrates, similar to Citric Acid Cycle but differs in how acetyl CoA enters

  
• Glyoxylate cycle retains all carbons from acetyl CoA, no decarboxylations

  
• Succinate from glyoxylate cycle can be used in Citric Acid Cycle or gluconeogenesis