ETC

• Measuring the rate of fuel oxidation (oxygen consumption/carbon dioxide production) will give us the rate of energy expenditure
• DNP is hydrophobic in both its protonated and deprotonated forms
• Why is DNP hydrophobic in its deprotonated form?

  Negative charge delocalisation around ring structure
• DNP uncoupling mechanism:

  1. Picks up H+ from cytoplasm
  2. Diffuses into matrix
  3. Loses H+
  4. Diffuses back to cytoplasm
• Thermogenin = uncoupling protein 1
• Thermogenin is a natural uncoupler which functions to generate heat under hormonal control
• Where is thermogenin found?

  Brown adipose tissue
• Describe brown adipose tissue compared to white adipose tissue

  Has more mitochondria and smaller lipid droplets
• Thermogenin mechanism

  1. Noradrenaline binds to receptors on surface of BAT
  2. FAs released for beta oxidation
  3. Thermogenin proton channel is opened to dissipate proton gradient produced by beta oxidation
• The ETC contains four complexes embedded in the IMM
• Each complex of the IMM contains structural and prosthetic group proteins
• What do prosthetic group proteins do?

  Transport H/e-
• H+ expelling reactions are on the outside (Complexes I, III, IV)
• H+ consuming reactions are on the matrix side
• NADH delivers electrons directly to complex I
• Complex I pumps protons
• Electrons from Complex I bypass Complex II and go directly to the Q pool
• FADH2 delivers electrons directly to complex II
• Why does NAD accept a H-?

  It already has a positive charge and adding another proton gives a 2+ charge, which requires 2 e- to neutralise
• NADH absorbs strongly at 340nm, but NAD+ does not
• FAD is stuck inside Complex II
• FAD cannot roam around like NAD
• UQ is very hydrophobic due to its ring and tail structure
• UQ is the only mobile component of the ETC
• Reduced UQ becomes UQH2 which transfers protons to Complex III
• Cyt C picks up electrons from Complex III and transfers them to Complex IV
• Cyt C contains a prosthetic group with an iron atom
• Cyt C does not carry protons
• Why doesn't Cyt C carry protons?

  Because iron does not carry protons
• How is iron held in Cyt C to prevent it being dissolved in solution?

  Being placed in the middle of porphyrin rings or iron-sulphur complexes
• Mechanism of H+ pumping:

  1. A carrier of Hs bumps into a carrier of e-
  2. e- are transferred from the H carrier to the e- carrier, and the H+ is released towards the cytoplasm
  3. The e- carrier bumps into a carrier of Hs and takes up a H+, causing the carrier of H to gain a H+
  4. Process repeats
• How can NADH produced from glycolysis enter the mitochondria?

  Glycerol 3-phosphate shuttle or malate aspartate shuttle
• The glycerol 3-phosphate shuttle causes a loss in some proton pumping potential
• Glycerol 3-phosphate shuttle mechanism:

  1. NADH transfers its electrons to DHAP to become glycerol 3-phosphate
  2. Glycerol 3-phosphate is shuttled from the cytoplasm to the intermembrane space
  3. Glycerol 3-phosphate donates its electrons to FAD in the IMM
  4. These electrons continue in the ETC as normal
• The malate aspartate shuttle enables the transfer of NADH without losing proton pumping potential
• Malate aspartate shuttle mechanism

  1. NADH transfers its electrons to oxaloacetate to become malate
  2. Malate is then transported into the matrix
  3. Malate donates its electrons to NAD+ to form NADH
  4. NADH enters the ETC at Complex I
• • Enzymes in the UQ pool can react with oxygen to produce free radicals
• When does the UQ pool create free radicals?

  When Q is stalled due to Complex III being full
• The ETC is the most common place for free radicals to form
• The F0 channel of ATP synthase consists of cylindrical proteins