Chapter 18: Electron Transport and Oxidative Phosphorylation Flashcards
What is Electron Transport and Oxidative Phosphorylations function?
The citric acid cycle thus produces the reduced coenzymes NADH and FADH2, which then pass their electrons to O 2 to produce H2 O in the processes of electron transport and oxidative phosphorylation.
The process of electron transport results in a transmembrane proton concentration gradient that drives ATP synthesis
the electrons from reduced fuel molecules are transferred to molecular oxygen in eukaryotes. We also examine how the energy of fuel oxidation is conserved and used to synthesize ATP.
The 12 electron pairs released during glucose oxidation are not transferred directly to O2 . Rather, they are transferred to the coenzymes NAD +and FAD to form 10 NADH and 2 FADH 2
The mitochondrion is the site of ?
the site of eukaryotic oxidative metabolism.
What all does Mitochondria contain?
pyruvate dehydrogenase, the citric acid cycle enzymes, the enzymes catalyzing fatty acid oxidation, and the enzymes and redox proteins involved in electron transport and oxidative phosphorylation.
the cell’s “power plant.”
A mitochondrion is bounded by a smooth outer membrane and contains an extensively invaginated inner membrane known as?
cristae
reflects the type of cell and its respiratory activity.
The large protein complexes mediating electron transport and oxidative phosphorylation are embedded in the inner mitochondrial membrane, so the respiration rate varies with membrane surface area.
The inner membrane divides the mitochondrion into what two compartments?
the intermembrane space and the internal matrix.
The matrix contains the soluble enzymes of oxidative metabolism as well as substrates, nucleotide cofactors, and inorganic ions. It also contains DNA, RNA, and ribosomes—that generates only 13 of the more than 1500 mitochondrial proteins. The remainder are encoded by nuclear genes and hence must be imported into the mitochondrion.
What is important for compartimentalization between mitochondria and cytosol/What mechanism results in the compartmentalization of metabolic functions between cytosol and mitochondria?
What does it generate?
The controlled impermeability of the inner mitochondrial membrane to most ions and metabolites permits the generation of ion gradients across this barrier and results in the compartmentalization of metabolic functions between cytosol and mitochondria.
components and permeability for both the inner and outer membrane?
the outer mitochondrial membrane contains porins, proteins that permit the free diffusion of molecules of up to 10 kD
The inner membrane, which is ~75% protein by mass, is considerably richer in proteins than is the outer membrane. It is permeable only to O2 , CO2 , and H2 O and contains, in addition to respiratory chain proteins, numerous transport proteins that control the passage of metabolites such as ATP, ADP, pyruvate, Ca2+ , and phosphate
The intermembrane space is therefore equivalent to the cytosol in its concentrations of metabolites and ions.
?????
What is produced in the cytosol by glycolysis that must gain access to the mitochondrial electron-transport chain for aerobic oxidation? The inner mitochondrial membrane lacks a transport protein specifically for it. How are “reduction equivalents” transported into mitochondria?
NADH
However, the inner mitochondrial membrane lacks an NADH transport protein.
the malate–aspartate shuttle (Fig. 16-20), in which, when run in reverse, cytosolic oxaloacetate is reduced to malate for transport into the mitochondrion. When malate is reoxidized in the matrix, it gives up the reducing equivalents that originated in the cytosol.
The glycerophosphate shuttle is expressed at variable levels in different animal tissues and is especially active in insect flight muscle (the tissue with the largest known sustained power output).
What is the ADP–ATP translocator?
Most of the ATP generated in the mitochondrial matrix through oxidative phosphorylation is used in the cytosol.
The inner mitochondrial membrane contains an ADP–ATP translocator (also called the adenine nucleotide translocase) that transports ATP out of the matrix in exchange for ADP produced in the cytosol by ATP-consuming reactions.
ATP out, ADP in
antiport electrogenic
ATP is synthesized from ADP + Pi in the mitochondrion but is utilized in the cytosol. How is Phosphate imported into the mitochondrion? What drives this process?
The Pi is returned to the mitochondrion by the phosphate carrier, an electroneutral Pi-H+ symport that is driven by ΔpH.
- cotransporter for Pi and H+
- phosphate import into mitochondrion driven by transmembrane proton gradient
- proton gradient not only driving force for ATP synthesis, also for transport of substrates ADP, Pi
Whatt drives the transport of ATP, ADP, and Pi.
the free energy of the proton gradient drives the transport of ATP, ADP, and Pi.
protons flow down their concentration gradient and bind to ATP synthase which turns and makes ATP from ADP
The proton gradient generated by proton pumping during the electron transport chain is a stored form of energy. When protons flow back down their concentration gradient (from the intermembrane space to the matrix), their only route is through ATP synthase, an enzyme embedded in the inner mitochondrial membrane.
In the electron transport chain, electrons are passed from one molecule to another, and energy released in these electron transfers is used to form an electrochemical gradient. In chemiosmosis, the energy stored in the gradient is used to make ATP.
So, where does oxygen fit into this picture? Oxygen sits at the end of the electron transport chain, where it accepts electrons and picks up protons to form water. If oxygen isn’t there to accept electrons (for instance, because a person is not breathing in enough oxygen), the electron transport chain will stop running, and ATP will no longer be produced by chemiosmosis.
The electron transport chain is a series of proteins and organic molecules found in the inner membrane of the mitochondria. Electrons are passed from one member of the transport chain to another in a series of redox reactions. Energy released in these reactions is captured as a proton gradient, which is then used to make ATP in a process called chemiosmosis. Together, the electron transport chain and chemiosmosis make up oxidative phosphorylation
How do NADH and FADH2 participate in additional substrate oxidation reactions. after transferring their electrons to other substances?
they are reoxidized to NAD +and FAD
electrons from the reduces coenzymes NADH and FADH2 go through what before reducing O2 to H2O
they pass through a series of redox centers in the electron transport chain
during electron transfer what happens to protons?
protons are translocated out of the mitochondrion to form an electrochemical gradient/proton gradient across the inner mitochondrial membrane whose free energy drives ATP synthesis from ADP and P ithrough oxidative phosphorylation.
what are electron carriers?
The electron carriers that ferry electrons from NADH and FADH 2 to O2 are located in the inner mitochondrial membrane. They contain redox centers are highly mobile, and others are less mobile components of large protein complexes.
- carry electrons from NADH and FADH 2 to O2
describe the thermodynamic efficiency of electron transport
an exergonic process
by inspecting the standard reduction potentials of the redox centers
efficiency, under standard conditions: 35 %
efficiency, under cellular conditions: ~ 70 %
Oxidation of NADH and FADH 2 is carried out by?
the electron-transport chain, a series of four protein complexes containing redox centers with progressively greater affinities for electrons (increasing standard reduction potentials). Electrons travel through the chain from lower to higher standard reduction potentials
What complexes/carriers are involved in the electron transport chain?
Electrons are carried from Complexes I and II to Complex III by the lipid coenzyme Q (CoQ or ubiquinone; so named because of its ubiquity in respiring organisms), and from Complex III to Complex IV by the small soluble protein cytochrome c.
ATP is not synthesized by complexes I, III, or IV.
purpose of inhibitors?
The sequence of events in electron transport was elucidated largely through the use of specific inhibitors and later corroborated by measurements of the standard reduction potentials of the redox components.
What is Complex 1 Function?
Complex I (NADH–coenzyme Q oxidoreductase), passes electrons from NADH to CoQ, is the largest protein complex in the inner mitochondrial membrane.
Complex I Accepts Electrons from NADH
Complex I catalyzes oxidation of NADH by CoQ:
In mammals, it consists of 44 different subunits with a total mass of ∼980 kD. Eukaryotes and many prokaryotes share 14 “core” subunits.
L shaped protein
contains multiple coenzymes
electron transport coupled to proton translocation across the membrane not yet completely resolved in this step
What is coenzyme Q
aka ubiquinone
so named because of its ubiquity in respiring organisms)
highly unpolar, i.e. soluble in membrane environment
Complex I Contains Multiple Coenzymes. What are they?
Complex I contains one molecule of flavin mononucleotide (FMN, a redox-active prosthetic group that differs from FAD only by the absence of the AMP group) and eight (in mammals), or nine or ten (in prokaryotes), iron–sulfur clusters. (FES)
1 FMN and 8-10 FES
What are the Oxidation States of FMN & CoQ?
FMN and CoQ can each adopt three oxidation states (Fig. 18-10). They are capable of accepting and donating either one or two electrons because their semiquinone forms are stable (these semiquinones are stable free radicals, molecules with an unpaired electron). FMN is tightly bound to proteins; however, CoQ has a hydrophobic tail that makes it soluble in the inner mitochondrial membrane’s lipid bilayer.
Which complex contains all of the exnyme’s prosthetic centers?
The 140-Å-high peripheral arm of the T. thermophilus Complex I (Fig. 18-9a) contains all of the enzyme’s prosthetic redox centers: an FMN, seven [4Fe–4S] clusters, and two [2Fe–2S] clusters
Thus, NADH reduces FMN in a two-electron reaction (formally, a hydride transfer), which in turn passes these electrons, one by one, through the “wire” of Fe–S clusters, to the CoQ.
Complex I Translocates How Many Protons in Each Reaction Cycle?
How are they translocated?
four protons are translocated from the matrix to the intermembrane space.
Bacteriorhodopsin
What is a Bacteriorhodopsin?
What drives its function?
Bacteriorhodopsin is a light-driven proton pump: It obtains the free energy required for pumping protons through the absorbance of light by its retinal prosthetic group.
a proton-translocating complex
Light-Induced Isomerization
CoQ Reduction Mechanically Drives Proton Translocation.
Complex 2 other name and function?
What are its redox groups? What are its subunits? Why is it important?
Complex II (succinate–coenzyme Q oxidoreductase), which contains the citric acid cycle enzyme succinate dehydrogenase (Section 17-3F), passes electrons from FADH of succinate to CoQ.
Its redox groups include succinate dehydrogenase’s covalently bound FAD to which electrons are initially passed, one [4Fe–4S] cluster, a [3Fe–4S] cluster (essentially a [4Fe–4S] complex that lacks one Fe atom), one [2Fe–2S] cluster, and one cytochrome b 560 (cytochromes are discussed in Box 18-1). All of its four subunits are encoded by nuclear genes.
FAD, FES, and Cytochrome b560
The free energy for electron transfer from succinate to CoQ (Fig. 18-7) is insufficient to drive ATP synthesis
Compare complex 1 and 2?
Complex 1 pumps 4 H+
Complex II does not pump H+
Complex II not really an electron carrier
complex 1 is for transfer of electrons from nadH
complex 2 is for FADH
What are cytochromes?
- electron-transport heme proteins
- heme groups contain iron that alternates between Fe 2+ and Fe3+
- iron ion is surrounded by porphyrin system
- have characteristic absorbance spectra
- can transfer electrons over 10-20 Å
are redox-active proteins that occur in all organisms except a few types of obligate anaerobes. These proteins contain heme groups that alternate between their Fe(II) and Fe(III) oxidation states during electron transport.
The heme groups of the reduced Fe(II) cytochromes have prominent visible absorption spectra consisting of three peaks: the α, β , and γ (Soret) bands.
complex 3 functions and other name? How many subunits in animals and bacterium?
Complex III (also known as coenzyme Q–cytochrome c oxidoreductase or cytochrome bc1) passes electrons from reduced CoQ to cytochrome c and pumps 4 protons out via Q cycle
It contains two b-type cytochromes, one cytochrome c1 , and one [2Fe–2S] cluster ((“Rieske center”) in which one of the Fe atoms is coordinated by two His residues rather than two Cys residues
animals: 11 subunits bacterium: 4 subunits
How does Complex III pump protons from the matrix to the intermembrane space?
The Q cycle
The essence of the Q cycle is that CoQH 2 undergoes a two-cycle reoxidation in which the semiquinone, CoQ−∙, is a stable intermediate.
Is cytochrome c soluble or insoluble electron carrier?
soluble in the intermembrane space.
unlike other cytochromes
Complex IV other name and function?
Cytochrome c oxidase (Complex IV) catalyzes the one-electron oxidations of four consecutive reduced cytochrome c molecules and the concomitant fourelectron reduction of one O 2molecule./Complex IV Reduces Oxygen to Water
mammals: 13 subunits bacterium: 4 subunits
both contain:
heme a
heme a3
CuB
Cu A center
What is Oxidative Phosphorylation?
electron transport causes proton transfer across the inner mitochondrial membrane from the matrix, a region of low [H+ ], to the intermembrane space (which is in contact with the cytosol), a region of high [H+ ] to form an electrochemical gradient across the inner mitochondrial membrane. whose energy is used to endergonically synthesize ATP from ADP+Pi
via ATP synthase
What is ATP Synthase function?
The endergonic synthesis of ATP from ADP and P i in mitochondria is catalyzed by an ATP synthase (also known as Complex V) that is driven by the electrontransport process.
Where does ATP Synthase get its energy from?/ What links ATP synthase to electrons transport?
The free energy released by electron transport through Complexes I–IV must be conserved in a form that the ATP synthase can use. Such energy conservation is referred to as energy coupling.
ATP synthesis is coupled to electron transport through the production of a transmembrane proton gradient during electron transport by Complexes I, III, and IV.
What is the Chemiosmotic Theory?
Summary: The Chemiosmotic Theory Links Electron Transport to ATP Synthesis/Electron transport and ATP synthesis are coupled via a transmembrane proton gradient
Explanation: Mitchell’s theory states that the free energy of electron transport is conserved by pumping H+ from the mitochondrial matrix to the intermembrane space to create an electrochemical H + gradient across the inner mitochondrial membrane. The electrochemical potential of this gradient is harnessed to synthesize ATP
proposed in 1961 by Peter Mitchell,
Describe the permeability of the inner mitochondrial membrane and how this affects oxidative phosphorylation.
The inner mitochondrial membrane (or the plasmamembrane of bacteria) has to be impermeable for H+ , OH-, and other ions so it can pump H+ out.
Compounds that make the inner membrane permeable for protons dissipate the gradient; no ATP synthesis occurs. Electron transport is “uncoupled” from ATP synthesis./allow electron transport (from NADH and succinate oxidation) to continue but inhibit ATP synthesis
Where does electron-transporting happen in bacteria?
An entirely analogous process occurs in bacteria, whose electron-transporting machinery is located in their plasma membranes
What generates a proton gradient? What complexes?
Electron transport, as we have seen, causes Complexes I, III, and IV to transport protons across the inner mitochondrial membrane from the matrix, a region of low [H+ ], to the intermembrane space (which is in contact with the cytosol), a region of high [H+ ].
What is the point of the free energy sequestered by the resulting electrochemical gradient?
It powers ATP synthesis.
The free energy change of transporting a proton from one side of the membrane to the other has what 2 components?
a chemical (based on concentration difference) as well as an electrical (based on charge) component, since H + is an ion
what is electrochemical gradient also known as?
“electrochemical gradient”, also called “protonmotive force”, “pmf”
Why is the export of protons from the mitochondrial matrix (against the proton gradient) is an endergonic process?
Since the pH outside the mitochondrion (side 2) is less than the pH of the matrix (side 1),
(positive DG)
Because formation of the proton gradient is an endergonic process, discharge of the gradient is exergonic. (negative DG)
An ATP molecule’s estimated physiological free energy of synthesis? How many protons does It require for synthesis?
around +40 to +50 kJ ∙ mol−1 , is too large for ATP synthesis to be driven by the passage of a single proton back into the mitochondrial matrix; at least two protons are required.
sometimes 3 per atp synthesized
What mechanism does the flow of protons drive?
ATP Synthase also known as proton-pumping ATP synthase and F1F0-ATPas
Describe ATP Synthase structure, 2 components, and function?
What mechanism is crucial?
a multisubunit transmembrane protein composed of two functional units, F 0 and F1 .
The influx of protons through the F 0 component of ATP synthase (F1 F0 -ATPase) drives its F 1 component to synthesize ATP from ADP + P i via the binding change mechanism, a process that is mechanically driven by the F 0 -mediated rotation of F1 ’s γ subunit with respect to its catalytic α 3 β 3 assembly.
- The F 0 component of ATP synthase includes a c-ring whose rotation is driven by the dissipation of the proton gradient and drives conformational changes in the F 1 component.
Subunit rotation Is crucial
Describe the Binding change mechanism.
The sites, rotation, how affinity is determined, where does atp synthesis occur
What mechanism is crucial?
Subunit rotation Is crucial
F 1has three chemically and genetically similar and interacting catalytic protomers/sites known as (α β units),but conformationally distinct with different affinities for ATP and ADP(high, medium, low)
O, the open conformation, has very low affinity for ligands and is catalytically inactive; L binds ligands loosely and is catalytically inactive; T binds ligands tightly and is catalytically active.
1) ADP and P ibind to site L.
(2) An energy-dependent conformational change converts binding site L to T, T to O, and O to L. (3) ATP is synthesized at site T and ATP is released from site O.
- affinity determined by position of central stalk
- rotation → sites switch their affinities
- catalysis (ATP synthesis/hydrolysis) occurs in high-affinity (tight) site
- ATP synthesis from ADP and P i in the high-affinity site occurs spontaneous. It is the release of ATP from this site that requires (most of the) energy
What is the brownian ratchet?
represents proton flow through F0
- H + channel located between a and c subunits
- Translocation of 1 H + rotates c-ring of rotor by 360°/n
(n = 8-15)
- Rotation of g subunit of rotor by 120 o results in synthesis of 1 molecule of ATP
- Rotation of g subunit of rotor by 360 o results in synthesis of 3 molecules of ATP
H+
- Synthesis of 1 ATP requires translocation of 8/3 to 15/3 H+
The P/O ratio relates?
the moles of ATP produced to the moles of O reduced
the number of ATPs synthesized per oxygen reduced
What are uncouples?
Uncouplers - make the membrane permeable for protons, thereby dissipating the gradient.
Agents that discharge the proton gradient can uncouple oxidative phosphorylation from electron transport.
Uncouplers uncouple electron transport from oxidative phosphorylation. Instead, what is generated? Give an example
What are uncouplers blocked by?
Heat. (thermogenesis)
Thermogenesis in Brown Adipose Tissue
- Brown adipose tissue occurs in newborn and in coldadapted animals. (brown – rich in mitochondria)
- BAT mitochondria contain uncoupling protein, which contains a proton channel and allows flux of protons back into the mitochondrial matrix.
Uncoupling protein is blocked by ATP, ADP, GTP, GDP.
Oxidative phosphorylation is controlled by?
the ratio [NADH]/[NAD+] and by the ATP mass action ratio.
Aerobic metabolism is more efficient than anaerobic metabolism. However, aerobic organisms must guard against damage caused by reactive oxygen species.
- If transport of 1 electron pair through the electron transport chain results in transport of 10 H+ , and flux of 10 H + is necessary to produce 3 ATP, the P/O ratio is 3.
- If flux of 8 H + is necessary (8 c subunits) to produce 3 ATP, the P/O ratio should be 3.75.
- If flux of 15 H + is necessary (15 c subunits) to produce 3 ATP, the P/O ratio should be 2.
- The empirically determined P/O ratio for NADH as donor is often ~ 2.5; with FADH 2 as donor, it is ~ 1.5.
????
Oxidative phosphorylation is controlled by?
-the ratio [NADH]/[NAD+] and by the ATP mass action ratio/ the ATP and NADH concentrations
-The concentrations of ATP, ADP, and P i in the mitochondrial matrix depend on the activities of the transport proteins that import these substances from the cytosol. Thus, the ADP–ATP translocator and the P i transporter may play a part in regulating oxidative phosphorylation.
-The rate of oxidative phosphorylation is coordinated with the cell’s other oxidative pathways.
-pathway from NADH to cyt c close to equilibrium-appears to be controlled by availability of substrate, cyt c(Fe2+ )
- constant recycling required
- ATP is not produced more rapidly than necessary
- efficient control necessary
control site of oxidation phosphorylation?
irreversible steps such as the cytochrome c oxidase reaction (the terminal step of the electrontransport chain)
When an individual is at rest what happens to the ATP mass action ratio, cytochrome c activity and oxidative phosphorylation?
At rest: ATP mass action ratio ↑
[cyt c(Fe2+ )]/[cyt c (Fe3+ )] ↓
cytochrome c oxidase activity ↓
oxidative phosphorylation ↓
advantages and disadvantages of aerobic metabolism?
Advantages:
* much higher energy yield from given amount of fuel
* oxidative detoxification possible (cytochrome P450)
Cells Are Equipped with Antioxidant Mechanisms. Antioxidants destroy oxidative free radicals such as O 2−· and ∙OH.
Disadvantages:
* absolute O 2 dependence in most organisms (O 2 deprivation → heart attack, stroke)
* reactive oxygen species as byproducts/it produces by leaking electrons
-Aerobic metabolism is more efficient than anaerobic metabolism. However, aerobic organisms must guard against damage caused by reactive oxygen species.
What are the reactive oxygen species radicals?
reactive species is
* partially reduced oxygen
* highly reactive
* harmful to cell/organism lipids, DNA, and enzymes
-mostly harms mitochondria
involved in Parkinson’s, Alzheimer’s, Huntington’s diseases; also in “normal” aging process
