The Respiratory Chain & Oxidative Phosphorylation

BIOMEDICAL IMPORTANCE Capture a far greater proportion of the available free energy – Generation of the high-energy intermediate, ATP A number of drugs inhibit oxidative phosphorylation Several inherited defects have been reported – Myopathy & encephalopathy & often have lactic acidosis.

Energy capture – Substrate level – Oxidative phosphorylation at the respiratory chain level Components – Respiratory chain – Oxidative phosphorylation – NAD-linked dehydrogenase – Flavoprotein- linked dehydrogenase

Mitochondria

Enzymes of Mitochondrial Subcompartments

Foodstuffs - generation of reducing equivalents (2H) - collected by the respiratory chain for oxidation - coupled generation of ATP. The respiratory chain collects & oxidizes reducing equivalents Conversion of food energy to ATP

Sources of reducing equivalents – β-oxidation – the citric acid cycle – Extramitochondrial

Transport of reducing equivalents through the respiratory chain.

Components of the respiratory chain, collecting points

Model of complex I.

Reduction of ubiquinone (UQ)

Oxidation-Reduction Reactions – electron donor (reductant) – electron acceptor (oxidant)

Only electrons are transferred both electrons & protons (hydrogen atoms) are transferred

standard Oxidation-Reduction Potentials for Various Biochemical Reactions

Components of the respiratory chain arranged in order of increasing redox potential – NAD + /NADH to O 2 /2H 2 O – redox span of 1.1 V Redox carriers – NAD-linked dehydrogenase systems, through flavoproteins and cytochromes, to molecular oxygen – iron-sulfur protein (FeS; nonheme iron)

Cytochromes – contain a heme group – Heme a,b,c absorption band type of heme group attached to the protein

Structure of ubiquinone (Q)

Iron-sulfur-protein complex (Fe4S4)

The respiratory chain is a proton pump

The P/O ratio – a measure of the number of ATP molecules formed during the transfer of two electrons through all or part of the electron transport chain.

enough energy to synthesize three molecules of ATP

Four protons are taken into the mitochondrion for each ATP exported

Model for mitochondrial F,Fo-ATP synthase. A rotating molecular motor.

Experimental evidence for rotation of y. and c·subunits

Respiratory Control

States of respiratory control

MANY POISONS INHIBIT THE RESPIRATORY CHAIN Classification – Inhibitors of the respiratory chain – Inhibitors of oxidative phosphorylation Atractyloside, transporter – Uncouplers of oxidative phosphorylation

Action of the uncoupler. 2,4-dinitrophenol,

Activation of UCP-1 by cold adaptation.

Mitochondrial inner membrane contains substrate transport systems

Mitochondrial metabolite transporters.

glycerol-phosphate shuttle malate-aspartate shuttle

Glycerophosphate shuttle for transfer of reducing equivalents

Malate shuttle for transfer of reducing equivalents from the cytosol into the mitochondrion

MITOCHONDRIAL GENES AND DISEASES Subunits of Electron Transport Complexes Encoded by Human Mitochondrial DNA

MITOCHONDRIAL GENES AND DISEASES Leber's Hereditary Optic Neuropathy – Single base changes NADH:ubiquinone oxidoreductase (complex I) sudden-onset blindness in early adulthood

MITOCHONDRIAL GENES AND DISEASES Mutations in Cytochrome b – Exercise Intolerance – lowered activity of the cytochrome bc 1 complex Mutation substituted an aspartate residue for a conserved glycine at position 290. Guanine to adenine transition in the mtDNA, – Missense mutations – Nonsense mutations

Generation of superoxide anions by mitochondrial electron transfer chain.

formation of reactive oxygen species superoxide. hydrogen peroxide. and hydroxyl radical.

Defenses Against Reactive Oxygen Species