Dr. Faisal Al-Khteeb Sheet Biochemistry 9 Tala Al-Hyasat 1 Electron transport chains in mitochondria Most eukaryotic cells have mitochondria, which produce ATP from products of the citric acid cycle, fatty acid oxidation, and amino acid oxidation. At the mitochondrial inner membrane, electrons from NADH and succinate pass through the electron transport chain to oxygen, which is reduced to water. * Electrons from NADH pass to complex 1 , Electrons from succinate pass to complex 2 The electron transport chain comprises an enzymatic series of electron donors and acceptors. Each electron donor passes electrons to a more electronegative acceptor, which in turn donates these electrons to another acceptor, a process that continues down the series until electrons are passed to oxygen, the most electronegative and terminal electron acceptor in the chain. *electrons flow from low to large reduction potential Passage of electrons between donor and acceptor releases energy, which is used to generate a proton gradient across the mitochondrial membrane by actively “pumping” protons into the intermembrane space, producing a thermodynamic state that has the potential to do work. The entire process is called oxidative phosphorylation, since ADP is phosphorylated to ATP using the energy of hydrogen oxidation in many steps. A small percentage of electrons do not complete the whole series and instead directly leak to oxygen, resulting in the formation of the free-radical superoxide, a highly reactive molecule that contributes to oxidative stress and has been implicated in a number of diseases and aging. BACKGROUND
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Dr. Faisal Al-Khteeb Sheet Biochemistry 9 Tala Al-Hyasat
1
Electron transport chains in mitochondria
Most eukaryotic cells have mitochondria, which produce ATP from products of the
citric acid cycle, fatty acid oxidation, and amino acid oxidation. At the mitochondrial
inner membrane, electrons from NADH and succinate pass through the electron
transport chain to oxygen, which is reduced to water.
* Electrons from NADH pass to complex 1 , Electrons from succinate pass to complex 2
The electron transport chain comprises an enzymatic series of electron donors and
acceptors. Each electron donor passes electrons to a more electronegative acceptor,
which in turn donates these electrons to another acceptor, a process that continues
down the series until electrons are passed to oxygen, the most electronegative and
terminal electron acceptor in the chain.
*electrons flow from low to large reduction potential
Passage of electrons between donor and acceptor releases energy, which is used to
generate a proton gradient across the mitochondrial membrane by actively “pumping”
protons into the intermembrane space, producing a thermodynamic state that has the
potential to do work. The entire process is called oxidative phosphorylation, since
ADP is phosphorylated to ATP using the energy of hydrogen oxidation in many steps.
A small percentage of electrons do not complete the whole series and instead directly
leak to oxygen, resulting in the formation of the free-radical superoxide, a highly
reactive molecule that contributes to oxidative stress and has been implicated in a number of diseases and aging.
Dr. Faisal Al-Khteeb Sheet Biochemistry 9 Tala Al-Hyasat
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Last lecture we talked about Flavin Mononucliotide , Iron Sulfur Centers and
Coenzyme Q.
Now ,
Protein contains a heme group (made of a porphyrin ring plus iron). Unlike the heme groups of hemoglobin, the cytochrome iron atom is reversibly converted from its ferric (Fe+3) to its ferrous (Fe+2) form as a normal part of its function as a reversible carrier of electrons. Electrons are passed along the chain from coenzyme Q to cytochromes bc1 (complex lll) ,c , and a + a3 ( complex lV )
Notes :
- Cytochrome c is associated with the outer face of the inner membrane.
- Cyto means cell ,, chrome means pigment ( they are colored
because of the presence of the heme group which: has iron , show
alternation of single – double bonds ).
- The basic unit of heme group is tetrapyrrole each one is formed by 5 members : 4C and 1N , each N atom is bound to the iron which locates at the centre .
ABSORPTION SPECTRA
- Absorption of light can be measured by spectrophotometer (in absorber unit).
- More concentrated molecule , more absorption of light .
- Every compound has characteristic absorption spectra .
- Compounds differ in : 1) their ability to absorb light 2) in the wave length at
which the light absorption occur (red, green , …)
- If you measure the light absorption at various wavelength at the visible region (
350 – 700 nm ) , you will find that it's differ according to the change of the
wave length.
- You can change the light absorption by changing the wave length.
- Cytochroms can absorb light because of the alternation of single – double
bonds .
cytochromes
(Heme- containing protein)
cytochrome
pyrrole
Dr. Faisal Al-Khteeb Sheet Biochemistry 9 Tala Al-Hyasat
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- Reduced and oxidized form of cyt c have different absorption spectra (curve
for each one ) .
Cytochrome c was isolated and purified.
Absorption spectra of both the oxidized and the
reduced cytochrome c are shown in the figure . (
blue- reduced , red- oxidized )
- Three classes of cytochromes a,b and c , they differ in the heme group . - different types of the heme group have different side chains attached to the tetrapyrrole ring ( minor differences ) -can be distinguished by differences in their light-absorption( different absorption spectra ) - Each one has different reduction potential - we can distinguish between the oxidized and reduced form . - Absorption spectra is affected by the protein environment ( isolated, pure , free form )
- At this wave length : absorption is
maximum for reduced form , while there
is no absorption for the oxidized form
(approximately )
- We can mesure the percentage of the
reduced form in the mixture after
measuring the absorption at this wave
length.
Dr. Faisal Al-Khteeb Sheet Biochemistry 9 Tala Al-Hyasat
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-NADH Dehydrogenase ( according to its function : remove
the hydrogen from the substrate ( NADH ))
-Membrane- Spanning. -binding sites to NADH . -More than 25 polypeptide chain - Tightly bound FMN group ( can't destroy the structure of complex l ) -Seven Fe-S centers of at least two different types ( involved in the electron transfer ) - Drop in energy≈ -13 to 14 kcal when electrons pass from NADH to Co Q , this
energy is more than enough to synthesis ATP .
NOTE : Co Q is lipid soluble
The free proton plus the hydride ion carried by NADH are transferred to NADH dehydrogenase, a protein complex ( complex l ) embedded in the inner mitochondrial membrane . complexl has a tightly bound molecule of flavin mononucleotide (FMN, a coenzyme structurally related to FAD) that accepts the two hydrogen atoms ( 2 H + and 2 e- ) becoming FMNH2. NADH dehydrogenase also contains several iron atoms paired with sulfur atoms to make iron-sulfur centers . These are necessary for the transfer of the hydrogen atoms to the next member of the chain, ubiquinone (known as coenzyme Q).
- The coenzyme Q - cytochrome c — oxidoreductase( the donner or the substrate is Co Q , the acceptor is cyt c )
- sometimes called the cytochrome bc1 complex ( the common name )
- or Cytochrome reductase and at other times complex III, is the third
complex in the electron transport chain • Catalyzes the transfer of electrons from QH2 to cytochrome c • 11 subunits including two cytochrome subunits • Contain iron sulfur center
Dr. Faisal Al-Khteeb Sheet Biochemistry 9 Tala Al-Hyasat
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• Contain three heme groups in two cytochrome subunits – bL and bH in cytochrome b ( L means low ability , H means high ability ) – c type in cytochrome c1 • Contain two CoQ binding sites ( 2 molecules bind at the same time )
THE Q CYCLE
The Q cycle describes a series of reactions that describe how the sequential oxidation
and reduction of the lipophilic electron carrier, ubiquinol-ubiquinone ( Coenzyme Q),
can result in the net pumping of protons across a lipid bilayer ( the inner
mitochondrial membrane).
Round 1:
1. Cytochrome b binds a ubiquinol and a ubiquinone. 2. The 2Fe/2S center and BL heme ( cyt bL in the slide ) each pull an electron off
the bound ubiquinol, releasing two hydrogens into the intermembrane space. 3. One electron is transferred to cytochrome c1 from the 2Fe/2S centre, while
another is transferred from the BL heme to the BH Heme( cyt bH in the slide ) . 4. Cytochrome c1 transfers its electron to cytochrome c (not to be confused with
cytochrome c1), and the BH Heme transfers its electron to a nearby ubiquinone, resulting in the formation of a ubisemiquinone.
5. Cytochrome c diffuses. The first ubiquinol (now oxidised to ubiquinone) is released, while the semiquinone remains bound.
Dr. Faisal Al-Khteeb Sheet Biochemistry 9 Tala Al-Hyasat
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Round 2:
1. A second ubiquinol is bound by cytochrome b. 2. The 2Fe/2S center and BL heme each pull an electron off the bound ubiquinol,
releasing two hydrogens into the intermembrane space. 3. One electron is transferred to cytochrome c1 from the 2Fe/2S centre, while
another is transferred from the BL heme to the BH Heme. 4. Cytocrome c1 then transfers its electron to cytochrome c, while the nearby
semiquinone picks up a second electron from the BH Heme, along with two protons from the matrix.
5. The second ubiquinol (now oxidised to ubiquinone), along with the newly formed ubiquinol ( fully reduced ) are released.
_ I copied everything related to ( the Q cycle ) from wiqipedia .
The enzyme cytochrome c oxidase or Complex IV, is a large transmembrane protein complex.
It receives an electron from each of four cytochrome c molecules, and transfers them to one oxygen molecule, converting molecular oxygen to two molecules of water. In the process, it binds four protons from the inner aqueous phase to make water, and in addition translocates four protons across the membrane, helping to establish a transmembrane difference of proton electrochemical potential that the ATP synthase then uses to synthesize ATP.
Remember that :
-the oxidized form of coenzyme Q ( Q ubiquinone )
- the reduced form of coenzyme Q ( QH2 ubiquinol )
Simple way to memorize it (OL = all electrons so it's the reduced form)
Dr. Faisal Al-Khteeb Sheet Biochemistry 9 Tala Al-Hyasat
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. Passes electrons from Cytocrome c to oxygen ( cyt c is a small protein found in the periphery of the membrane ) • Contains cytochrome a and a3 • Contains two copper ( e- carrier group ) • Contains oxygen binding sites • O2 must accept 4 electrons to be reduced to two H2O ( 4cyt c to reduce 1 O2 ) • Cytochrome c is one electron carrier 4Cyt cred + 4H+ + O2 → 4Cyt cox + 2H2O • Partial reduction of O2 is hazardous ( if the o2 take 1 e- , radicals will be formed,
they can oxidize cellular components like lipids … )
Parialy reduced O2 shouldn't be realesed
The dr said that we didn't have to memorize these steps.
Dr. Faisal Al-Khteeb Sheet Biochemistry 9 Tala Al-Hyasat