Lecture 11

• Fermentation and anaerobic respiration enable cells to produce ATP without using oxygen
• Most cellular respiration requires O2 to produce ATP
• Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions)
• In the absence of O2 , glycolysis couples with fermentation or anaerobic respiration to produce ATP
• Anaerobic respiration uses an electron transport chain with an electron acceptor other than O2, for example sulfate (SO42-) or nitrate, fumarate, elemental S
• Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP

  
• Anaerobic Respiration: Sulfate-reducing bacteria

  
• Fermentation consists of glycolysis plus reactions that regenerate NAD+ , the oxidizing agent of glycolysis
• Two common types are alcohol fermentation and lactic acid fermentation

  
• In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2
• Alcohol fermentation by yeast is used in brewing, winemaking, and baking
• Yeast are very interesting because they are capable of growing either aerobically or anaerobically. If there's oxygen around, they're going to grow aerobically. If they find themselves in an oxygen reduced or deprived environment, they can switch to anaerobic respiration or to fermentation.
• Alcohol fermentation

  
• Lactic acid fermentation

  
• In lactic acid fermentation, pyruvate is reduced directly by NADH, forming lactate as an end product, with no release of CO2
• Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt
• Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce
• Fermentative anaerobic organisms mostly use the lactic acid fermentation pathway

  • C6 H12 O6 + 2 ADP + 2 phosphate --> 2 lactic acid + 2 ATP
  • The energy released in this equation is approximately 150 kJ per mole, which is harvested in generating 2 ATP from ADP
  • This is only 5% of the energy per sugar molecule that the typical aerobic reaction captures
• Comparing Fermentation to Aerobic and Anaerobic Respiration

  • All 3 processes use glycolysis to oxidize glucose and other organic fuels to pyruvate
  • In all 3 pathways, NAD+ is oxidizing agent that accepts electrons from food during glycolysis
  • The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation, O2 in aerobic respiration and another molecule, i.e. SO42-, in anaerobic respiration
  • Respiration produces 30-32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule
• Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2
• Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration
• In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes
• Glycolysis with and without oxygen

  
• Glycolysis occurs in nearly all organisms
• Glycolysis probably evolved in ancient prokaryotes before there was oxygen in the atmosphere
• Glycolysis and the citric acid cycle connect to many other metabolic pathways

  • Glycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways
• Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration
• Glycolysis accepts a wide range of carbohydrates
• Proteins must be digested to amino acids; amino acids can feed glycolysis or the citric acid cycle
• Amino acids from proteins are first deaminated and nitrogenous refuse is excreted in the form of ammonia or urea
• Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA)
• Fatty acids are broken down by beta oxidation and yield acetyl CoA
• An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate
• All of these molecules are able to be metabolized into products that can go into the glycolytic pathway

  
• Biosynthesis (Anabolic Pathways)

  • The body uses small molecules to build other substances
  • These small molecules may come directly from food, from glycolysis, or from the citric acid cycle

  
• Feedback inhibition is the most common mechanism for control
• If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down
• Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway
• PFK is an enzyme that is considered the commitment step of glycolysis because it's the enzyme that's putting a second phosphate onto fructose six phosphate to generate this highly charged intermediate that traps it in the cell and really commits it to going through the rest of the steps in glycolysis. This is a great enzyme to regulate because it is an enzyme that if we turn it up, we can increase the flux through the pathway. If we turn it down, we can decrease the flux.

  
• ATP is a natural allosteric inhibitor of PFK, in order to prevent unnecessary production of ATP through glycolysis