Lecture 10

• The citric acid cycle completes the energy-yielding oxidation of organic molecules
• • In the presence of O2 , pyruvate enters the mitochondrion
  • Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis

  
• Glucose transport into cells by facilitated diffusion
• Glycolysis releases 25% of chemical energy that cells can harvest from glucose
• 1. Pyruvate is actively transported into the mitochondrion by a protein and releases CO2
  2. NAD+ is reduced to NADH
  3. Coenzyme A receives an acetyl group

  
• Mitochondria are in nearly all eukaryotic cells
• Mitochondria: have a smooth outer membrane and an inner membrane folded into cristae
• The inner membrane of mitochondria creates two compartments: intermembrane space and mitochondrial matrix
• Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix
• Cristae present a large surface area for enzymes that synthesize ATP
• The citric acid cycle, also called the Krebs cycle or the tricarboxylic acid (TCA) cycle, takes place within the mitochondrial matrix
• The citric acid cycle oxidizes organic fuel derived from pyruvate, generating 2 CO2, 1 ATP, 3 NADH, and 1 FADH2 per turn
• NADH = reduced form of Nicotinamide Adenine Dinucleotide
• FADH2 = Flavin Adenine Dinucleotide
• The citric acid cycle has eight steps, each catalyzed by a specific enzyme

  • The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate
  • The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle
  • The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain
• Citric Acid Cycle

  
• During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
• Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food

  • These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation
• Glycolysis and citric acid cycle produce 4 ATPs/glucose by substrate-level phosphorylation
• Oxygen is the final electron acceptor of the electron transport chain that functions in aerobic ox-phosphorylation
• The electron transport chain is in the cristae of the mitochondrion
• Most of the electron transport chain’s components are proteins, which exist in multiprotein complexes
• The carriers alternate reduced and oxidized states as they accept and donate electrons
• Electrons drop in free energy as they go down the chain and are finally passed to O2 , forming H2O
• We start off at a higher energy. At each step of the chain, a redox reaction occurs, and you see NADH entering at the top of this electron transport chain, and redox reaction releases NAD. Here's FADH two entering at a slightly lower energy level. Again, redox chemistry, it's oxidized FAD. Each of these Roman numerals 123.4 is a multi protein complex, carrying out these reactions. At the end, we have water being released because oxygen is the final electronic acceptor, and we have the product being released from this chain is water.

  
• Electrons are transferred from NADH or FADH2 to the electron transport chain
• Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O2
• The electron transport chain generates no ATP
• The chain’s function is to break the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts
• Chemiosmosis: The Energy-Coupling Mechanism

  • Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space
  • H+ then moves back across the membrane, passing through channels in ATP synthase
  • ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP
  • This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work
• ATP Synthase: A Nanomachine

  1. H+ ions (protons) enter channel in stator
  2. Protons bind specific sites in the rotor, causing conformational changes that act to spin the rotor within the membrane
  3. Each proton makes 1 complete turn before entering the mitochondrial matrix through another channel in the stator
  4. Rotor spinning causes the internal rod to spin. The rod extends into the stationary catalytic knob
  5. Turning of the rod activates catalytic sites in the knob that produce ATP from ADP and Pi

  
• The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis

  • The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work
• Oxidative phosphorylation

  
• During cellular respiration, most energy flows in this sequence: glucose --> NADH --> electron transport chain --> proton-motive force --> ATP
• About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP molecules
• The rest of the energy stored in glucose is released as heat
• 3 reasons for inexact ATP yield estimate:

  • phosphorylation and redox reactions not directly coupled, so NADH:ATP not a whole number
  • ATP yield varies depending on electron transport system
  • PMF (proton motor force) used to drive other kinds of work
• How do bacteria do it? They lack membrane-bound organelles (i.e. mitochondrion)...but have two cellular membranes! They have a double membrane. They use their intermembrane space that's created as part of the double membrane around the cell to do this kind of chemistry. They have a space where they can build up a concentration of protons that can then be used to drive that same kind of ATP synthase, mechanical motion that leads to the production of ATP. They lack a membrane bound organelle, but they have the ability to create a proton motive force.