Lecture 5

• Mitochondria are a multi-membrane organelle
• Mitochondria are the site of aerobic oxidation, converting nutrients + O2 into energy (ATP) + CO2 + H2O
• Mitochondria are referred to as the cell's combustion engine
• Plant equivalent of mitochondria is chloroplast
• Mitochondria have their own DNA
• mtDNA is inherited cytoplasmically and maternally
• Evidence suggests mitochondria evolved from bacteria through endosymbiosis
• Mitochondria can undergo fission like bacteria
• Mitochondrial membranes consist of outer membrane (smooth), inner membrane (with invaginations called cristae), intermembranous space, and matrix containing mitochondrial DNA and ribosomes
• Mitochondria morphology can vary from individual spheroid/ovoids to long elongated networks
• Mitochondrial morphology is related to the balance of fusion and fission events
• Excess fusion and excess fission can affect mitochondrial morphology
• Mitochondrial fission involves Mitochondrial Fission Factors (MFF) recruiting G-proteins (DRP-1) to constrict or pinch membranes
• Mitochondrial fusion involves Mitofusins (MFN) aiding membrane fusion through GTP hydrolysis
• Mitochondrial fusion is a two-step process: outer membrane fusion followed by inner membrane fusion
• Mitochondrial Transport

  Majority of mitochondrial transport is related to matrix proteins
• Fusion

  1. Outer membrane fusion

  2. Inner membrane fusion
• Why do mitochondria undergo so much fission and fusion?
• Mitochondrial Transport

  1. Matrix proteins

  2. Targeting Sequence

  3. Cargo

  4. Translocons

  5. Import into mitochondrial matrix
• Mitochondrial Transport: Targeting Sequence

  • N-terminally cleaved sequence like ER-targeting signal
  • Sequence is different: 20-50 aa amphipathic alpha-helices
  • Recognized by import receptor on mitochondria outer membrane
• Mitochondrial Transport: Cargo

  • Protein synthesis in cytoplasm but kept unfolded by chaperones (cytosolic Hsc70)
  • This requires energy input (ATP)
  • Folded proteins cannot undergo mitochondrial import
• Mitochondrial Transport: Translocons

  1. Mitochondrial matrix proteins must traverse two membranes requiring two translocons

  2. translocon of the outer membrane (TOM)

  3. translocon of the inner membrane (TIM)

  4. Matrix proteins pass through both translocons simultaneously at points where outer and inner membranes are in close proximity
• Import into mitochondrial matrix

  1. Targeting signal recognized by import receptor and directs cargo to the TOM complex

  2. Unfolded cargo passes through both outer (TOMs) and inner membrane (TIMs) translocons simultaneously

  3. During translocation cargo is bound by matrix chaperone (Hsc70) which hydrolyses ATP to "pull" the cargo into the mitochondria

  4. Targeting sequence processed (cleaved by protease) and cargo folded by chaperonins into mature tertiary state
• Mitochondrial Function: Oxidative Phosphorylation


  1. Coupling a series of oxidation/reduction reactions (electron transport chain) with phosphorylation of ADP (ADP+ Pi) to generate ATP

  2. Goal is to efficiently convert energy stored in hydrocarbon (C-H) bonds of sugars (glucose) and lipids (fatty acids) into ATP

  3. Linked with glycolysis and fatty acid metabolism in cytosol
• Redox Reaction (Oxidation/Reduction)


  1. Transfer of electrons between two molecules

  2. Oxidation - loss of electrons. The molecule was oxidized

  3. Reduction - gain of electrons. The molecule was reduced

  4. In biological systems, Nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD) are high energy electron carriers

  5. Sugars and lipids are converted into intermediate molecules that are then transported into the mitochondrial matrix

  6. Glucose oxidized to pyruvate (Glycolysis). Generates some ATP and NADH in the process

  7. In order for fatty acids to be oxidized, first converted or "activated" with coenzyme A (Fatty acyl CoA)
• Fatty acid oxidation

  Fatty acids first converted or "activated" with coenzyme A (Fatty acyl CoA)
• Generating ATP (Stage 1: Sugar+Fat Metabolism)

  Pyruvate/Fatty acyl CoA oxidized to a common intermediate Acetyl CoA
• Generating ATP (Stage 1: Sugar+Fat Metabolism)


  Electrons donated to electron carriers NAD+ and FAD to generate NADH and FADH2
• Generating ATP (Stage 1: Sugar+Fat Metabolism)

  Acetyl CoA is further oxidized in the Citric Acid Cycle (Krebs or TCA cycle) to generate more NADH and FADH2
• Generating ATP (Stage 1: Sugar+Fat Metabolism)

  Sugars and lipids eventually oxidized to CO2
• Generating ATP (Stage 2: Giving up the electrons)

  NADH and FADH2 donate their electrons to a series of transmembrane inner membrane complexes
• Generating ATP (Stage 2: Giving up the electrons)


  Electrons are shuttled between complexes by mobile electron carriers
• Generating ATP (Stage 2: Giving up the electrons)


  Eventually electrons end up in O2 which is reduced to H2O
• Generating ATP (Stage 2: Giving up the electrons)


  Complex I, III, and IV are also proton (H+) pumps
• Generating ATP (Stage 2: Giving up the electrons)


  Energy released from the passage of electrons through this chain is used to pump H+ from the matrix generating a proton and voltage gradient across inner membrane (-ve charge on matrix side)
• Generating ATP (Stage 3: Pumping Protons)

  Energy released from NADH and FADH2 oxidation stored in form of an electrochemical gradient: Proton Motive Force
• Generating ATP (Stage 3: Pumping Protons)

  This is harnessed by ATP synthase which through chemoosmosis converts H+ movement back into matrix into kinetic energy (rotation/torque)
• Generating ATP (Stage 3: Pumping Protons)

  Rotation of base causes conformational changes in the head domain that bind and fuse ADP with Pi to generate ATP
• Generating ATP (Stage 3: Pumping Protons)


  Smallest known rotary electric motor spins at ~8000 rpm generating 400 ATP molecules per sec
• Genetic basis of Parkinson's Disease