T2 L2: Mitochondrial respiratory chain & disease

• Electron transport chain

  4 large multi-unit proteins embedded in inner mitochondrial membrane

  Catalyse oxidation of NADH using molecular oxygen, producing water

  energy released isn't in heat form but ATP
• What are the four components of the ETC linked by?

  2 lipid soluble proteins

  • Ubiquinone (Coenzyme Q)
  • Cytochrome C

  they are free to move in membrane by diffusion
• Through which different paths can electrons be accepted to the ETC?

  Complex I (TCA)

  Complex II (TCA)

  Glycerol-3-phosphate shuttle (Glycolysis)

  ETF:Q oxidoreductase (beta-oxidation of fatty acids)
• How are charged molecules/ions transported through mitochondrial membranes?

  ATP, ADP and Pi have negative charges so cant diffuse straight through the membranes, so use proteins:

  1. Adenine nucleotide translocase
  2. Phosphate translocase
  3. ATP synthase
• Adenine nucleotide translocase

  Antiporter - transports ADP into matrix and ATP out into intramembrane space

  passive process, driven by electrochemical gradient

  intramembrane space is highly positively charged due to H+ ions

  exchanging ADP3-, which has a less negative charge, to ATP4- is favoured by the electrochemical gradient

  Inhibited by: Atractyloside

  
• Phosphate translocase

  Symporter - transports both phosphate and hydrogen ions into matrix

  Passive process - driven by electrochemical gradient

  movement is favoured by the transmembrane proton gradient

  
• ATP synthase (as a channel)

  F-type ATP synthase has two functional domains:

  1. F0: oligomycin-sensitive proton channel
  2. F1: ATP synthase

  
• Structure of ATP synthase

  F0 domain: proton channel

  has 3 subunits: a, b, c

  F1 domain: ATP synthase

  has 5 subunits: alpha (x3), beta (x3), gamma, delta, epsilon

  3 beta subunits have catalytic sites for ATP synthesis

  gamma unit rotates in a cycle

  changes properties of beta catalytic units

  at any time, can be:

  • site of ADP + Pi binding
  • site of ATP synthesis
  • site of ATP release

  energy for this rotation provided by difference in charge and proton concentration

  
• Mitochondrial DNA

  closed circular double stranded molecule

  5-10 copies of the genome in each mitochondrion

  >900 different mt proteins encoded by nuclear DNA & assembled in mitochondrion

  mt genome codes for some rRNA, tRNA and parts of the respiratory chain proteins
• Inheritence of mitochondrial genome

  demonstrates maternal inheritance: embryo derives all its mitochondria material from the egg and none from the sperm

  most sperm mitochondria in tail which is not absorbed during fertilisation

  any paternal mitochondria that do enter egg are destroyed
• Mitchondrial DNA damage

  respiratory chain is major producer of reactive oxygen species ROS

  mt genome sufferst greatest exposure & damage by ROS

  mt DNA also less effective at correcting mistakes and repairing mtDNA dmg

  
• How are ROS generated?

  partial reduction of oxygen
• How are ROS removed?

  converted to oxygen and water via antioxidant enzymes
• Mitochondria and aging

  efficiency of mitochondrial function declines with age

  partly due to accumulation of mutations to mtDNA caused by ROS

  OXPHOS enzyme defects strongly implicated in Alzheimer's/Parkinson's and type II diabetes

  involve tissues with very high energy requirements and hence rely on OXPHOS
• Uncoupling reagents

  sever the link between electron flow and ATP synthesis, with the energy being released as heat

  eg UCP1 (thermogenin)
• What is unique about brown adipose tissue (BAT)?

  Brown fat thermogenesis

  specialised for heat generation

  high numbers of mitochondria with thermogenin (UCP1)

  mitochondria generate heat energy instead of ATP

  important in newborns, possible role in obesity/diabetes

  
• What is DNP?

  exogenous uncoupler

  weak acid

  collects a proton from IMS and moves through membrane with it, depositing in matrix

  returns through the membrane to collect another proton

  protons used as heat rather than to make ATP

  toxic, used as slimming agent with disastrous results
• Mitochondrial diseases often involve which systems?

  CNS, musculoskeletal system (also known as mitochondrial myopathies)
• What is the threshold effect regarding mitochondrial disease?


  as long as the percentage of mutated mitochondria doesn't exceed the threshold, the cell can respire sufficiently
• What is mitochondrial genetic bottleneck?


  unpredictable maternal passing down of mutations into offspring cells
• What are mitochondrial myopathies?


  group of neuromuscular diseases

  most occur before age of 20; often beginning with exercise intolerance / muscle weaknesss
• What are the treatments for mitochondrial myopathies?

  no specific treatmetns

  occupational/physical therapy

  vitamin therapies
• What are the preventative strategies available against mitochondrial myopathies?

  IVF strategy designed to replace defective mitochondria inherited from mother

  involves merging DNA from two eggs

  malfunctioning mitochondria replaced by donor healthy ones

  alternative strategy: maternal spindle transfer