Bio

• Glycolysis


  The main pathway of glucose utilization, involving 10 enzymatic reactions
• Glycolysis


  • All enzymes are cytosolic
  • Reactions constitute the main pathway of glucose utilization
• Glycolysis


  1. Phosphorylation of glucose to glucose-6-phosphate by hexokinase

  2. Isomerization of glucose-6-phosphate to fructose-6-phosphate by phosphohexose isomerase

  3. Phosphorylation of fructose-6-phosphate to fructose 1,6-bisphosphate by phosphofructokinase

  4. Cleavage of fructose 1,6-bisphosphate by aldolase to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate

  5. Isomerization of dihydroxyacetone phosphate to glyceraldehyde-3-phosphate by triose phosphate isomerase

  6. Dehydrogenation and phosphorylation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate by glyceraldehyde-3-phosphate dehydrogenase

  7. Phosphoryl group transfer from 1,3-bisphosphoglycerate to ADP by phosphoglycerate kinase to form ATP

  8. Isomerization of 3-phosphoglycerate to 2-phosphoglycerate by phosphoglycerate mutase

  9. Dehydration of 2-phosphoglycerate to phosphoenolpyruvate by enolase

  10. 10. Transfer of phosphoryl group from phosphoenolpyruvate to ADP by pyruvate kinase to form ATP
• Preparatory phase of glycolysis


  Reactions 1-5, where ATP is consumed
• Pay-off phase of glycolysis

  Reactions 6-10, where ATP is produced
• Overall process of glycolysis:
• Pyruvate dehydrogenase complex


  Converts pyruvate to acetyl-CoA, requires thiamin diphosphate (vitamin B1) as a coenzyme
• Pyruvate dehydrogenase complex


  1. Composed of 3 subunits with different enzymatic actions

  2. Pyruvate + NAD+ + CoA → Acetyl-CoA + NADH + H+ + CO2
• Under anaerobic conditions


  Pyruvate is reduced by lactate dehydrogenase to lactate
• Glycolysis in erythrocytes always terminates in lactate
• Exhaustion and pain in skeletal muscles is due to depletion of ATP, glucose, and accumulation of lactate causing acidosis
• Lactate production is increased in septic shock and many cancers
• Regulation of glycolysis


  • Hexokinase, phosphofructokinase, and pyruvate kinase are the major sites of regulation
  • Phosphofructokinase is the most important control element, inhibited by ATP and citrate, but activated by AMP
• Fructose and glucose metabolism


  Converge at the level of the triose-phosphates, bypassing the main regulatory steps, leading to increased lipogenesis
• Regulation of pyruvate dehydrogenase

  • Inhibited allosterically by acetyl-CoA and NADH
  • Regulated by phosphorylation (decreased activity) and dephosphorylation (increased activity)
• Neurological disturbances are common in metabolic defects due to brain's dependence on glucose
• Inherited pyruvate kinase and aldolase deficiency in erythrocytes cause hemolytic anemia
• Arsenic and mercurial ions, and thiamin deficiency, inhibit pyruvate dehydrogenase leading to pyruvic and lactic acidosis
• Inherited pyruvate dehydrogenase deficiency presents with lactic acidosis, particularly after a glucose load
• Tricarboxylic Acid Cycle


  Also known as the Citric Acid Cycle or Krebs Cycle
• Citric Acid Cycle


  The central pathway of carbohydrate, lipid, and amino acid metabolism
• The Citric Acid Cycle

  1. Sequence of reactions in mitochondria

  2. Final common pathway for the oxidation of carbohydrate, lipid, and protein

  3. Central role in gluconeogenesis, lipogenesis, and interconversion of amino acids
• Step 1: Acetyl-CoA and oxaloacetate condensation


  1. Catalyzed by citrate synthase

  2. Carbon-carbon bond formed between methyl carbon of acetyl-CoA and carbonyl carbon of oxaloacetate

  3. Thioester bond of citryl-CoA hydrolyzed, releasing citrate and CoASH

  4. Reaction is irreversible
• Step 2: Citrate isomerization

  1. Catalyzed by aconitase (citrate isomerase)

  2. Occurs in two steps: dehydration to cis-aconitate and rehydration to isocitrate

  3. Citrate is a symmetric molecule but aconitase reacts with it asymmetrically

  4. Channeling of citrate synthase product directly onto aconitase active site

  5. Citrate only available in free solution when aconitase is inhibited by isocitrate accumulation

  6. Fluoroacetate is toxic as it inhibits aconitase, causing citrate accumulation
• Step 3: Isocitrate dehydrogenation and decarboxylation

  1. Catalyzed by isocitrate dehydrogenase

  2. Three isoenzymes, one using NAD+ in mitochondria, two using NADP+ in mitochondria and cytosol
• Step 4: α-Ketoglutarate oxidative decarboxylation

  1. Catalyzed by α-ketoglutarate dehydrogenase complex

  2. Similar to pyruvate dehydrogenase complex

  3. Requires same cofactors

  4. Inhibited by arsenite and high ammonia concentrations
• Step 5: Succinyl-CoA conversion to succinate

  1. Catalyzed by succinate thiokinase (succinyl-CoA synthetase)

  2. Only example of substrate-level phosphorylation in the citric acid cycle

  3. Two isoenzymes, one using GDP and one using ADP, in gluconeogenic tissues

  4. Non-gluconeogenic tissues have only the ADP-using isoenzyme
• Step 6: Succinate dehydrogenation to fumarate

  1. Catalyzed by succinate dehydrogenase

  2. Malonate competitively inhibits this enzyme
• Step 7: Fumarate hydration to L-malate

  Catalyzed by fumarase
• Step 8: Malate dehydrogenation to oxaloacetate


  Catalyzed by malate dehydrogenase
• The citric acid cycle operates only under aerobic conditions
• The citric acid cycle is strictly aerobic, unlike glycolysis which operates in both aerobic and anaerobic conditions
• Energetics of the Citric Acid Cycle

  • 3 NADH and 1 FADH2 produced per acetyl-CoA
  • NADH reoxidation yields ~3 ATP, FADH2 reoxidation yields ~2 ATP
  • 1 ATP (or GTP) formed by substrate-level phosphorylation catalyzed by succinate thiokinase
• Anaplerotic reactions


  Replenish citric acid cycle intermediates extracted for biosynthesis
• The citric acid cycle is an amphibolic pathway, involved in oxidation of acetyl-CoA as well as interconversion of metabolites from amino acids, and providing substrates for biosynthesis
• Regulation of the Citric Acid Cycle

  1. Regulation of acetyl-CoA formation by pyruvate dehydrogenase

  2. Regulation of the cycle's reactions, mainly at citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase

  3. Regulated by energy status ([ATP]/[ADP], [NADH]/[NAD+])

  4. Isocitrate dehydrogenase allosterically inhibited by ATP, leading to citrate accumulation and inhibition of glycolysis

  5. Dehydrogenases activated by Ca2+ during increased energy demand
• Oxaloacetate concentration limits the rate of the citrate synthase reaction
• Electron Transport Chain (ETC)


  A system of electron transport that uses respiratory O2 to finally produce ATP (energy)
• Electron Transport Chain (ETC)

  • Located in the inner mitochondrial membrane
  • Final common pathway of metabolism
  • Electrons from food metabolism are transported to O2
  • Uses maximum amount of body's oxygen
• Metabolic breakdown of energy-yielding molecules

  1. Energy-rich reduced coenzymes

  2. Electrons (e-) lose their free energy

  3. Excess energy generates heat