ELECTRON
ELECTRON TRANSPORT CHAITRANSPORT CHAIN:N:
-Biological oxidations are catalyzed by intracellular enzymes. The purpose of oxidation is to obtain energy. -Biological oxidations are catalyzed by intracellular enzymes. The purpose of oxidation is to obtain energy. -Electron Transport: Electrons carried by reduced coenzymes (NADH or FADH2) are
-Electron Transport: Electrons carried by reduced coenzymes (NADH or FADH2) are passed sequentiallypassed sequentially through a chain of
through a chain of proteins and coenzymes (so called electron transport chain)to O2.proteins and coenzymes (so called electron transport chain)to O2. -Oxidative Phosphorylation: Coupling e-Transport (Oxidation) and ATP
-Oxidative Phosphorylation: Coupling e-Transport (Oxidation) and ATP synthesis (Phosphorylation) .synthesis (Phosphorylation) . -It all happens in mitochondrionor at the inner mitochondrial membrane(eukaryotic cells).
-It all happens in mitochondrionor at the inner mitochondrial membrane(eukaryotic cells).
Mitochondria: Mitochondria:
•outer membrane relatively permeable •outer membrane relatively permeable •inner membrane permeable only to
•inner membrane permeable only to those things with specific transportersthose things with specific transporters –
– Impermeable to NADH and FADH2Impermeable to NADH and FADH2 –
– Permeable to pyruvatePermeable to pyruvate •Compartmentalization •Compartmentalization –Kreb's and β
–Kreb's and β-oxidation in matrix-oxidation in matrix –
– Glycolysis in cytosolGlycolysis in cytosol
the mitochondrion contained the enzymes responsible for electron transport and oxidativethe mitochondrion contained the enzymes responsible for electron transport and oxidative
phosphorylation phosphorylation
REDOX POTENTIALS:
Redox potential is a capacity to hold electrons within itself.Molecules with lower standard redox potential have higher capacity to donate electrons.Higher standard redox potential have capacity to accept electrons from molecules with lower
standard redox potential. Nernst equation:
=Standard redox potential
= Standard free energy change
REDOX POTENTIALS:
Redox potential is a capacity to hold electrons within itself.Molecules with lower standard redox potential have higher capacity to donate electrons.Higher standard redox potential have capacity to accept electrons from molecules with lower
standard redox potential. Nernst equation:
=Standard redox potential
= Standard free energy change
n= number of electrons transferred
F= faraday constant (96485 J/volt/mole)
Electrons move from higher to lower standard free energy change.
-Removal of H across a C-C bond is not sufficiently exergonic to reduce NAD+,but it does yield enough energy to reduce FAD.
-That’s why succinate dehydrogenase uses FAD other than NAD+as coenzyme. Most energy from Redox:
•electrons during metabolic reactions sent to NAD and FAD – Glycolysis
•In cytosol
•produces 2 NADH
– Pyruvate dehydrogenase reaction •In mitochondrial matrix
•2 NADH / glucose – Krebs
•In mitochondrial matrix
•6 NADH and 2 FADH2/ glucose
-The transfer of electrons is not directly to oxygen but through coenzymes. -There are 2 sites of entry for electrons into the electron transport chain:
NAD+ or FAD
Both are coenzymes for dehydrogenase enzymes. Nicotinamide coenzymes: NAD+
Always a 2-electron reaction transferring 2 e- and 2 H+ The flavin coenzymes / flavoproteins :
-flavin adenine dinucleotide (FAD) always a 2-electron reaction transferring 2 e- and 2 H+
-it can accept/donate 1 or 2 .FMN has an important role in mediating e-transfer between carriers that e-transfer 2 e-(e.g., NADH) and those that transfer 1 e-(e.g., Fe+++).
Iron-sulfur Centers (clusters)
-Iron-sulfur centers (Fe-S) are prosthetic groups containing 1-4 iron atoms
Iron-sulfur centers transfer only one electron, even if they contain two or more iron atoms.
Ubiquinone
Coenzyme Q(CoQ, Q or ubiquinone) is lipid-soluble. It dissolves in the hydrocarbon core of a membrane.
the only electron carrier not bound to a protein.
it can accept/donate 1 or 2 e-.Q can mediate e-transfer between 2 e-that transfer and 1 e-carriers
Cytochromes
-proteins that accept electrons from QH2 or FeS -Ultimately transfers the electrons to oxygen.
-Cytochromesare electron carriers containing hemes . Hemes in the 3 classes of cytochrome (a,b,c) differ in substituents on the porphyrin ring.
-Some cytochromes(b,c1,a,a3) are part of large integral membrane protein complexes.
-Cytochrome c is a small, water-soluble protein. THE ELECTRON TRANSPORT CHAIN:
Support for this order of events:
1. Energetically favorable. electrons pass from lower to higher standard reduction potentials
2. Spectra: the absorption spectrum for the reduced carrier differs from that of its oxidized form.
carriers closer to oxygen are more oxidized.
3. Specific inhibitors. Those before the blocked step should be reduced and those after be oxidized.4. Assay of individual complexes.NADH can reduce complex I but not the other complexes
.
COMPLEX 1:
•Has NADH binding site – NADH reductase activity
•NADH -→NAD+
– NADH ---> FMN--->FeS---> ubiquinone – ubiquinone ---> ubiquinone H2
– 4 H+pumped/NADH
COMPLEX 2:
succinate ---FAD — ubiquinone – Contains coenzyme Q
– FADH2binding site •FAD reductase activity •FADH2--→FAD
Reason for no proton pump by complex 2:
1)It is not a transmembrane protein and hence cannot pump proton from matrix to intermembrane space.
2)E produced by electron transfer in complex 2 is so small that it cannot pump
proton from matrix to intermembrane space,because it produces very small G.
COMPLEX 3:
ubiquinone -→ubiquinone ox
•while cyt C gets reduced •Also contains cytochromes b – proton pump 4H+ •Adds to gradient 8 H+/ NADH 4 H+/ FADH2 COMPLEX 4: -reduction of oxygen -cytochrome oxidase
-cyt a+a3 red---> oxidized state -oxygen ---> water
– 2 H++ 2 e-+ ½
O2-- – transfers e-one at a time to oxygen -Pumps 2H+out
– Total of 10 H+/ NADH – Total of 6 H+/ FADH2
INHIBITORS OF ETS:
Complex 1= Rotenone,Amytol Complex 2= Malonate,Carboxyn
Complex 3= Antimycin,Mixothiozole,Dimercaparol
Complex 4= Cyanite, Azide, Carbon monoxide.(These three inhibitor are known as warburg inhibitor.
UNCOUPLER:
A) Synthetic uncoupler: 2,4-dinitrophenol,dicumarol Uncoupler has two properties:
-It should be hydrophobic which help its movement in mitochondrial membrane -It should have ionizable group which can carry a proton from intermembrane space to matrix.
Uncoupler has 2 stages, one is protonated and other is deprotonated.Proton is taken from intermembrane space and release into matrix by these uncoupler which finally function just reverse to the pump which decrease the
electrochemical gradient and ATP synthesis.
B) Natural uncoupler: Ionophores are antibiotics that generate pore in membrane to kill the bacteria.
Valonomycin decrease membrane potential component of proton motive force without direct effect on pH gradient .
Nigericin disrupt the chemical gradient without changing membrane potential.
SHUTTLE SYSTEM:
•NADH made in cytosol
•Can’t get into matrix of mitochondrion •2 mechanisms
– In muscle and brain
•Glycerol phosphate shuttle – In liver and heart
•Malate / aspartate shuttle
•In muscle and brain
•Each NADH converted to FADH2inside mitochondrion – FADH2enters later in the electron transport chain
– Produces 1.5 ATP
Total ATP per glucose in muscle and brain •Gycerol phosphate shuttle
– 2 NADH per glucose -→2 FADH2
– 2 FADH2X 1.5 ATP / FADH2……….3.0 ATP –2 ATP in glycoysis………2.0 ATP – From pyruvate and Krebs
•12.5 ATP X 2 per glucose ………..25.0 ATP Total = 30.0 ATP/ glucose
MALATE ASPARTATE SHUTTLE:
