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 O2/2H2O
– 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 bc1 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