Define redox potential. Describe its significance in the electron transport chain.

Definition

• More negative E°′ → greater tendency to donate electrons — a stronger reducing agent
• More positive E°′ → greater tendency to accept electrons — a stronger oxidising agent
• Electrons flow spontaneously from negative to positive

Values Along the Respiratory Chain

• The carriers are arranged in order of increasing redox potential — this is the organising principle of the chain
• Total drop from NADH to oxygen = 1.14 V

Relation to Free Energy

• For NADH to oxygen: ΔG°′ = −2 × 23.06 × 1.14 = about −52.6 kcal/mol
• Since ATP synthesis needs about 7.3 kcal/mol, this could in principle make 7 ATP; in practice 2.5 are made — an efficiency of roughly 35–40%, the rest appearing as heat
• A positive ΔE°′ gives a negative ΔG°′, that is, a spontaneous reaction

The Three Coupling Sites

• FADH2 enters after site 1, which is precisely why it yields less ATP
• The "sites" are a historical description; the modern chemiosmotic account describes them as the three proton-pumping complexes (I, III and IV)

Applied Aspects

• Redox potential explains the order of the chain — each carrier must be more positive than the one before, or electrons would not flow
• The NADH:NAD+ ratio determines the direction of many reactions — in alcoholism the rise in NADH drives pyruvate to lactate and oxaloacetate to malate, producing hypoglycaemia, lactic acidosis and fatty liver
• Redox-active drugs — methylene blue, used in methaemoglobinaemia, works by shuttling electrons; paraquat poisons by redox cycling and generating superoxide in the lung
• Ascorbate–dehydroascorbate and GSH–GSSG couples set the redox environment of the cell, which influences enzyme activity and gene expression