glycolysis

• Glycolysis


  My first metabolic pathway

  
• Metabolic Pathway Necessities


  • What organism is it taking place in?
  • What organ/tissue is/are involved?
  • What is the subcellular location of the pathway?
  • What type of pathway is it?
  • What enzymes/cofactors are involved?
  • What are the control points?
  • What are the starting substrates/end products?
  • What is the overall effect/ "purpose" of the pathway?

  
• Where in the cell is energy generated?

  • Animal Cell - Mitochondria, Cytosol
  • Plant Cell - Chloroplasts, Mitochondria, Cytosol
• Mature RBCs lack organelles, so without mitochondria...
• The Cornea of the Eye is poorly served by blood vessels. So what?
• Working muscle – oxygen cannot be supplied to vigorously working muscles as fast as ATP is utilized. Consequently, the muscles must generate some of the required energy in an anaerobic manner.
• Glucose Metabolism – major players

  • Insulin
  • Glucagon
  • Muscle
  • Red blood cells (RBCs)
  • Liver
  • Brain
  • Adipose tissue

  
• Important Questions about Glycolytic Pathway

  • What type of pathway is it?
  • What enzymes/cofactors are involved?
  • Can you classify the enzymes of glycolysis? #ECNumbers?
  • Where are the control points?
  • Which enzymes are involved in regulation?
  • What are the starting substrates/compounds being utilized?
  • What compounds are synthesized by this pathway?
  • What is the overall effect/ "purpose" of the pathway?

  
• Glycolysis


  Also called Embden-Meyerhof pathway (E.M-Pathway), occurs in cytosol, 10 step catabolic pathway, objective is to produce energy (ATP) and the reducing equivalent (NADH) from catabolizing hexose sugars, can occur aerobically or anaerobically

  
• Glycolysis is a major pathway for ATP synthesis in tissues lacking mitochondria or are oxygen starved - erythrocytes, cornea, lens etc.

  
• Glycolysis is very essential for brain tissues which is dependent on glucose for energy.

  
• Glycolysis can catabolize carbs anaerobically unlike many other energy yielding substrates
• Glycolysis is a central metabolic pathway, many of its intermediates are primary substrates or shared intermediates for other pathways e.g. glycogen synthesis, the pentose phosphate pathway, fatty acid synthesis, amino acid synthesis etc.

  
• Phases of Glycolysis


  • Energy investment (and lysis) phase
  • Payoff phase
• Hexokinase Reaction


  Phosphorylated sugar molecules do not readily penetrate cell membranes without specific carriers, this commits glucose to further metabolism in the cell, one of 3 key regulatory enzymes of glycolysis, allosterically inhibited by glucose-6-PO4, allosterically stimulated by Pi, requires Mg2+ for activity, step is irreversible

  
• Hexokinase vs Glucokinase


  Hexokinase has low Km, Glucokinase has high Km
• Energy Investment Phase & Lysis

  2 ATP used
• Phosphofructokinase Reaction


  Fructose 6-phosphate is phosphorylated to Fructose-1, 6-bisphosphate by Phosphofructokinase (PFK-1), the PFK reaction is the rate-limiting step, enzyme allosterically activated by ADP and AMP conc, inhibited by ATP and citrate (high energy), irreversible reaction

  
• The Role of Hormones in Glycolysis Regulation - Insulin

  High glucose conc in blood (fed state) causes the release of insulin from the b-cells of the pancreas, insulin binds to its receptors on liver, muscle and adipose tissue which causes a cascade of reactions inside those cells, protein phosphatase is one of these enzymes activated during this cascade, many enzymes in carbohydrate metabolism are less active when phosphorylated, protein phosphatase dephosphorylates them causing them to become more active – eg PFK-2, glycogen synthase, pyruvate kinase

  
• The Payoff Phase


  • NAD+ in the cytosol is limited and must be regenerated…or else…
  • 4 ATP is synthesized in the payoff phase…but because 2 ATP is used in the Investment Phase the NET production is 2 ATP!
  • ATP is synthesized by substrate level phosphorylation from high energy precursors in this phase
• The Role of Hormones in Glycolysis Regulation - Glucagon


  The effects of insulin are reversed (the starved state) by the hormone glucagon which is made in the a-cells of the pancreas, binding of glucagon to its receptors on liver, muscle and adipose tissues causes a cascade of cellular events intracellularly to activate protein kinase, protein kinase phosphorylates PFK-2, pyruvate kinase and glycogen synthase inactivating them, as such, glycolysis and glycogen synthesis stops and reactions that provide sugars for the body can ensue

  
• Aerobic Fate of Pyruvate – Acetyl CoA


  In the presence of O2, pyruvate enters the mitochondria (outer – aquaporin, inner – pyruvate translocase in symport with H+), where it is oxidatively decarboxylated to acetyl CoA and enters the TCA

  
• Anaerobic Fates of Pyruvate - Lactate

  During glycolysis NAD+ is reduced NADH, NAD+ in the cytosol is limited, problem – without oxygen, NADH cannot be re-oxidized back to NAD+ and glycolysis will stop, solution: pyruvate is reduced to lactate to regenerate NAD+

  
• Anaerobic Fates of Pyruvate - Ethanol

  Alcoholic Fermentation – the production of ethanol and CO2 from the anaerobic metabolism of pyruvate in microbes such as yeasts, the purpose of this reaction is to regenerate NAD+….ethanol is synthesized as a waste product, it is excreted from the cell….and is responsible for the alcoholic content in fermented drinks such as wine, rum, vodka, beer etc