Describe oxidative phosphorylation. Explain the chemiosmotic theory and the structure and working of ATP synthase.

Definition

• Site — inner mitochondrial membrane
• Accounts for about 90% of the ATP made by an aerobic cell
• Distinguished from substrate-level phosphorylation, which needs no oxygen and no membrane

The Chemiosmotic Theory

Electrons pass along the chain → Complexes I, III and IV pump H+ from matrix to intermembrane space → Creates a proton motive force → Two components: a pH gradient and a membrane potential → Protons flow back through the F0 channel of ATP synthase → ATP is synthesised

• The membrane potential is about −150 to −180 mV, matrix side negative
• The membrane must be intact and impermeable to protons — this is why the theory predicts that anything making it leaky will uncouple the process

Evidence supporting the theory

• An intact membrane is essential — disrupted mitochondria oxidise substrate but make no ATP
• Electron transport generates a measurable pH and voltage gradient
• Uncouplers, which carry protons across, abolish ATP synthesis without blocking electron flow
• An artificial pH gradient alone can drive ATP synthesis in the absence of any substrate

ATP Synthase (complex V)

• Binding change mechanism (Boyer and Walker, Nobel Prize 1997) — rotation of the γ subunit drives each β subunit through three states: Loose (binds ADP + Pi), Tight (forms ATP), Open (releases ATP)
• The energy is used to release the ATP, not to form the bond — a counter-intuitive point worth stating
• About 4 protons are needed per ATP, including one for transporting phosphate
• It is a rotary molecular motor, and can run in reverse as an ATPase

P:o Ratio

• The modern values are non-integral because the proton requirement per ATP is not a whole number

Regulation — Respiratory Control

• ADP availability is the chief regulator — oxidation and phosphorylation are tightly coupled
• A rise in ADP (that is, energy use) immediately accelerates respiration; this is acceptor control
• Respiratory control ratio = rate with ADP ÷ rate without; a measure of mitochondrial integrity
• The ATP:ADP ratio is therefore the master signal linking demand to fuel oxidation
• Thyroid hormone increases the number of mitochondria and respiratory chain components, raising basal metabolic rate

Transport Systems of the Inner Membrane

Coupling and its Loss

Applied Aspects

• Uncouplers dissipate the gradient → energy appears as heat. 2,4-dinitrophenol was sold as a slimming drug in the 1930s and caused fatal hyperthermia; it is still bought illicitly and still kills
• Brown adipose tissue uncouples physiologically through thermogenin (UCP-1) to generate heat in the newborn
• Thyrotoxicosis — excess thyroxine partially uncouples and increases chain components, producing heat intolerance, weight loss and a raised metabolic rate
• Aspirin overdose uncouples oxidative phosphorylation, causing hyperthermia and metabolic acidosis alongside the respiratory alkalosis
• Oligomycin blocks ATP synthesis but not electron transport initially — the gradient builds up and then halts the chain, which distinguishes it from an uncoupler