OXIDATIVE PHOSPHORYLATION
ATP is the single currency of life
• Adenosine triphosphate
• ATP is the most important molecule for capturing and transferring free
energy
• Hydrolysis of ATP to ADP + Pi yields 7.3 kcal/mol energy that can be
used to power e.g. protein synthesis, muscle contraction or transport of
molecules
Oxidative phosphorylation generates ATP
• In aerobic oxidation, sugars and fatty acids are metabolized to C02 and
H20 .
• The released energy is converted to chemical energy of
phoshoanhydride bonds in ATP.
Oxidative phosphorylation is the last stage of
catabolism
NADH and FADH2
NADH and FADH2
• Glycolysis, TCA cycle and fatty acid oxidation generate NADH and
FADH2
• NADH and FADH2 are energy rich molecules because each contains a
pair of electrons that have a high transfer potential
• In oxidative phosphorylation the electon transferring potentila of
NADH and FADH2 is converted to phosphate-transfer potential of
ATP
Mitochondrion
Oxidative phosphorylation
• ATP is formed as electrons are transferred from NADH or FADH2
to 02 by a series of electron carriers.
Proton motive force and chemiosmotic
coupling
• The immediate energy sources that power ATP synthesis are proton
gradient and electric potential (voltage gradient) across the membrane.
• Proton gradient and electric potential are collectively called protonmotive force.
Proton motive force and chemiosmotic
coupling
• The proton motive force is generated by stepwise movement of
electrons by electron carriers that leads to pumping of protons out of
the mitochondrial matrix.
• Oxidation of NADH and phosphorylation of ADP are coupled by a
generation of proton gradient.
Proton motive force and chemiosmotic
coupling
Energy is released gradually in the electron
transfer chain
• Most free energy released when glucose is oxidised to carbon dioxide
is retained in the reduced coenzymes NADH and FADH2
• Respiration: electrons are released from from NADH and FADH2 to
oxygen
• NADH + H+ + 1/2 02 = NAD+ +H20
Energy is released gradually in the electron
transfer chain
• NADH + H+ + 1/2 02 = NAD+ +H20
-52.6 kcal/mol
• ADP + Pi = ATP
+7.6 kcal/mol
• ATP production is maximised by releasing the free energy in small
increments in the electron transfer chain (a.k.a respiratory chain).
• Electron transfer chain contains four multiprotein complexes. Three of
these are electron driven proton pumps that create the proton gradient
The electron transfer chain
The electron transfer chain
The electron transfer chain
Electron transfer is driven by redox potential
Redox potential
• Oxidation-reduction potential
• Oxidant + electron = reductant
• Substance that can exist as a reduced and oxidices form is referred to a
redox couple
• Redox potential of such couple is measured against H+ -> H2 couple.
• Redox potential of H+ -> H2 couple is defined as 0 V (volts).
Redox potential
• A negative redox potential means that a substance has lower affinity
for electrons than hydrogen. Positive redox potential means higher
affinity.
• Strong oxidising agents have positive redox potential
• In the respiratory chain the electrons are transferred to higher redox
potential values, that is, to higher affinity electron carriers.
Electron transfer is driven by redox potential
Prosthetic groups act as electron carriers
Complexes in the chain are transmembrane
proteins
Coenzyme Q and cytochrome c shuttle
electrons
Cytochromes are heme containing proteins
• Cytochromes are covalently linked to heme, an iron-containing
prosthetic group similar to that in hemoglobion or myoglobin.
• Electron transport occurs by by oxidation and reduction of the Fe atom
in the centre of the heme
• Different cytochromes have slightly different heme groups that
generate different ‘environment’ for Fe-ion and thus different tendency
to accept an electron
ATP Synthase
ATP Synthase
• ATP synthase or F0F1 complex has two components that are both itself
multiprotein complexes
• F0 is transmembrane complex that forms a regulated H+ channel
• F1 is protrudes in the matrix and contains the sites for ATP formation
ATP Synthase
• Proton translocation through F0 powers rotation of one subunits of F1
• Three confromations, one binds ADP and Pi so tightly that they
spontaneously form ATP.
Several toxins can block oxidative
phosphorylation
Transporters traffic ATP and ADP
Malate/aspartate shuttle and glycerol
phosphate shuttle are needed for oxidation of
cytosolic NADH
Respiratory control
• Mitochondria can only oxidise FADH and NADH only as long as
there is ADP and Pi available. Electron flow ceases if ATP is not
produced.
• ADP increases when ATP is consumed e.g. in muscle work. Oxidative
phosphorylation is regulated by ATP consumption.
Brown-fat mitochondria contain an uncoupler
of oxidative phosphorylation
• Brown fat specialised to produce heat
• Newborns: brown-fat thermogenesis
• Thermogenin protein, a proton transporter that is not connected to ATP
synthesis.
• Energy released by NADH oxidation converted to heat.
Oxidative Phosphorylation - Summary