chapter 19 Flashcards
Oxidative Phosphorylation
-Both involve the e- flow through membrane-bound carrier
-ΔG’° made available by this electron flow is coupled to an uphill transport of protons across a proton-impermeable membrane generating a transmembrane electro-chemical potential gradient (ΔE’°)
-The transmembrane flow of protons back down their concentration gradient through specific protein channels provides the free energy for the synthesis of ATP catalyzed by the ATP synthase complex that couples proton flow to ADP phosphorylation
-Components of e- flow chain and the large functional complexes
-Path of e- flow
-Proton movements coupling to e- flow
-“Rotational catalysis “ captures E of proton flow in ATP
-Regulatory mechanisms of coordinative OP
Energy from reduced fuels is used to synthesize ATP in animals
-Carbohydrates, lipids, and amino acids are the main reduced fuels for the cell
-Electrons from reduced fuels are transferred to reduced cofactors NADH or FADH2
-In oxidative phosphorylation, energy from NADH and FADH2 are used to make ATP
Energy Flow in Cellular Respiration
Oxidative Phosphorylation
-Electrons from the reduced cofactors NADH and FADH2 are passed to proteins in the
-Electrons from the reduced cofactors NADH and FADH2 are passed to proteins in the respiratory chain
-In eukaryotes, oxygen is the ultimate electron acceptor for these electrons
-Energy of oxidation is used to phosphorylate ADP
Photophosphorylation
-In photosynthetic organisms light causes charge separation between a pair chlorophyll molecules
-Energy of the oxidized and reduced chlorophyll molecules is used to drive synthesis of ATP
-Water is the source of electrons that are passed via a chain of protein transporters to the ultimate electron acceptor, NADP+
-Oxygen is the byproduct of water oxidation
Chemiosmotic Theory
ATP synthesis is highly thermodynamically unfavorable
-phosphorylation of ADP is not a result of a direct reaction between ADP and some high-energy phosphate carrier
-Energy needed to phosphorylate ADP is provided by the flow of protons down the electrochemical gradient
-the energy released by electron transport is used to transport portions against the electrochemical gradient
Chemiosmotic energy coupling requires membranes
-The proton gradient needed for ATP synthesis can be stably established across a membrane that is impermeable to ions
–Plasma membrane in bacteria
—Inner membrane in mitochondria
—Thylakoid membrane in chloroplasts
-Membrane must contain proteins that couple the “downhill” flow of electrons in the electron-transfer chain with the “uphill” flow of protons across the membrane
-Membrane must contain a protein that couples the “downhill” flow of protons to the phosphorylation of ADP
Structure of a Mitochondrion
Double membrane leads to four distinct compartments:
Outer Membrane:
–Relatively porous membrane allows passage of metabolites
Intermembrane Space (IMS):
–similar environment to cytosol
–higher proton concentration (lower pH)
Inner Membrane
–Relatively impermeable, with proton gradient across it
–Location of electron transport chain complexes
–Convolutions called Cristae serve to increase the surface area
Matrix
–Location of the citric acid cycle and parts of lipid and amino acid metabolism
–Lower proton concentration (higher pH)
Mitochondria: the site of oxidative phosphorylation
OM- Readily permeable to small <Mr 5,000 molecules and ions moving through integral membrane porins
IM- Impermeable to most small molecules and ions including H+. Specific transporters allow molecules/ions to pass
Electron Are Funneled to Universal Electron Acceptors
Universal electron acceptors:
Nicotinamide nucleotides (NAD+ or NADP+)
Electron Are Funneled to Universal Electron Acceptors that are used in ETC
Universal electron acceptors:
Flavin nucleotides (FMN or FAD)
-flavoproteins contain tightly bound FAD or FMN
-oxidized flavoprotein can accept one or two electrons
Mitochondrial Electron Transport
-Consists of a series of
-Most are integral
-Reducing equivalent is the term used to Chain
-Consists of a series of sequentially acting electron carriers
-Most are integral proteins with prosthetic groups that can accept or donate one or two electrons
-Reducing equivalent is the term used to designate a single electron equivalent transferred in an oxidation-reduction reaction and fall into three categories
Types of Electron Transfer and Electron-carrying Molecules
Three types of electron tranfers in OP:
1. direct transfer of e as in reduction of Fe3+ to Fe2+
2. transfer as a hydrogen atom (H + + e-)
3. transfer as a hydride ion (:H-) which bears two e
Four types of electron carrying molecules:
1. NAD and flavoproteins (NADH, FADH2, FMN)
2. Ubiquinone
3. Iron containing protein- cytochromes
4. Iron containing protein- iron-sulfur proteins
Coenzyme Q or Ubiquinone
-Ubiquinone is a lipid-soluble conjugated dicarbonyl compound that readily accepts electrons
-Upon accepting two electrons, it picks up two protons to give an alcohol, ubiquinol
-Ubiquinol can freely diffuse in the membrane, carrying electrons with protons from one side of the membrane to another side
-Coenzyme Q is a mobile electron carrier transporting electrons from Complexes I and II to Complex III
Coenzyme Q or Ubiquinone
-Small hydrophobic,
-Can freely diffuse in the
-Can carry
-Similar to
-Small hydrophobic, Lipid soluble benzoquinone with long isoprenoid side chain
-Can freely diffuse in the mitochondrial inner membrane
-Can carry both electrons and protons
-Similar to plant plastoquinone and bacterial menaquinone
Cytochromes
-one electron carriers
-Iron coordinating prophoryin ring derivatives
-a, b or c differ by ring additions
Iron-Sulfur Clusters
-one electron carriers
-coordinating by cysteines in the protein
-containing equal number of iron and sulfur atoms
Different types of Fe-S centers (Note: Fe is not in heme but with organic sulfur or cys. residues or both)
reduce potential
-looking at redox potentials, may give the order of carriers, not that the actual potential under cellular conditions depends on the concentration of the reduced and oxidized forms
-o2 has the highest reduction potential, that is why it is last electron acceptor
Free Energy of Electron Transport
Reduction Potential (E)
∆E′o = E′o (e- acceptor) – E′o (e- donor)
∆G′o = –nF∆E′o
For negative ΔG need positive ΔE
E(acceptor) > E(donor)
Electrons are transferred from lower (more negative) to higher (more positive) reduction potential.
Free Energy released is used to pump proton, storing this energy as the electrochemical gradient
Method For Determining Sequence of e Carriers
IF an electron donor and oxygen are added and specific blockers of the chain, a characteristic pattern of reduced and oxidized forms are seen to confirm the order of the carriers
e- Carriers Function in Multienzyme Complexes
1.2.3.4
I NADH dehydrogenase
II Succinate dehydrogenase
III Ubiquinone: cytochrome c oxidoreductase
IV cytochrom oxidase
Flow of Electrons from Biological Fuels into the Electron-Transport Chain
Complex I:
NADH to ubiquinone
Complex II:
succinate to ubiquinone
NADH:ubiquinone oxidoreductase,
a.k.a. Complex I
-One of the
-Over 40 different
-NADH binding site in the
-Noncovalently bound
-Several
-One of the largest macro-molecular assemblies in the mammalian cell
-Over 40 different polypeptide chains, encoded by both nuclear and mitochondrial genes
-NADH binding site in the matrix side
-Noncovalently bound flavin mononucleotide (FMN) accepts two electrons from NADH
-Several iron-sulfur centers pass one electron at a time toward the ubiquinone binding site
Complex I Overall:
Overall:
NADH + H+ + Q → NAD + +QH2
NADH:Ubiquinone oxidoreducase is a proton pump
-Transfer of two electrons from NADH to ubiquinone is accompanied by a transfer of protons from the matrix (N) to the intermembrane space (P)
-Experiments suggest that about four protons are transported per one NADH
NADH + Q + 5H+N = NAD+ + QH2 + 4 H+P
-Reduced coenzyme Q picks up two protons
-Protons are transported by proton wires
A series of amino acids that undergo protonation and deprotonation to get a net transfer of a proton from one side of a membrane to another
Succinate Dehydrogenase,a.k.a. Complex II
-FAD accepts two electrons from succinate
-Electrons are passed, one at a time, via iron-sulfur centers to ubiquinone, which becomes reduced QH2
-Does not transport protons
Complex II: Succinate to Ubiquinone
-Succinate dehydrogenase is the only
-Contains
-Contains a heme
-Succinate dehydrogenase is the only membrane bound enzyme of the TCA cycle.
-Contains 5 prosthetic groups, 4 different protein subunits with subunits C and D being integral membrane each with 4 transmembrane helices
-Contains a heme b binding site for ubiquinone binding
Ubiquinone:Cytochrome c Oxidoreductase, a.k.a. Complex III
-Uses two electrons from QH2 to reduce two molecules of cytochrome c
-Additionally contains iron-sulfur clusters, cytochrome bs, and cytochrome cs
-The Q cycle results in four additional protons being transported to the IMS
The Q Cycle
-Experimentally, four protons are transported across the membrane per two electrons that reach CytC
-Two of the four protons come from QH2
-The Q cycle provides a good model that explains how two additional protons are picked up from the matrix
-Two molecules of QH2 become oxidized, releasing protons into the IMS
-One molecule becomes rereduced, thus a Net transfer of four protons per reduced Coenzyme Q
Cytochrome c
-The second mobile electron carrier
-A soluble heme-containing protein in the intermembrane space
-Heme iron can be either ferric (Fe3+, oxidized) or ferrous (Fe2+, reduced)
-Cytochrome c carries a single electron from the cytochrome bc1 complex to cytochrome oxidase
Cytochrome c absorbs visible light
-Intense Soret band near 400 nm absorbs blue light and gives cytochrome c an intense red color
-Cytochromes are named by the position of their longest-wavelength (α) peak
Cytochrome Oxidase, a.k.a. Complex IV
-Mammalian cytochrome oxidase is a membrane protein with 13 subunits
-two heme groups: a and a3
-Contains copper ions
–CuA: two ions that accept electrons from Cyt c
–CuB: bonded to heme a3 forming a binuclear center that transfers four electrons to oxygen
Cytochrome oxidase passes electrons to O2
-how many electrons are used to reduce one oxygen molecule?
-Four electrons are used to reduce one oxygen molecule (O2) into two water molecules
-Four protons are picked up from the matrix in this process
-Four additional protons are passed from the matrix to the intermembrane space
Electron flow through Complex IV
Overall Reaction:
4 cyt c (reduced) + 8H+N + O2 → 4 cyt c (oxidized) + 4 H+P + 2H2O
Multiple complexes associate together
to form a
respirasome
A putative respirasome composed of Complexes III and IV. (a) Purified supercomplexes (Complexes III & IV), from yeast, visualized by EM. The electron densities of hundreds of images were averaged to yield this composite view. (b) The x-ray–derived structures of one molecule of Complex III (red; from yeast) and two of Complex IV (green; from bovine heart) could be fitted to the electron-density map to suggest one possible mode of interaction of these complexes in a respirasome.
Summary of the Electron Flow in the Respiratory Chain
-In transferring two electrons from NADH, the ΔG’º is -220 kJ/mol
-The vectorial equation: NADH + 11H+N + ½ O2 → NAD+ + 10 H+P + H2O
-ΔG’º for pumping one proton out is about 20kJ/mol, so to pump 10 out, is about 200kJ/mol
Summary of Electron Transport
Complex I Complex IV
1NADH + 11H+(N) + ½O2 ——> NAD+ + 10H+(P) + H2O
Complex II Complex IV
FADH2 + 6H+(N) + ½O2 ——> FAD + 6H+(P) + H2O
Difference in number of protons transported reflects differences in ATP synthesized.
Proton-Motive Force
The proteins in the electron-transport chain created the electrochemical proton gradient by one of three means:
actively transport protons across the membrane
—Complex I and Complex IV
Chemically remove protons from the matrix
—Reduction of CoQ and reduction of oxygen
Release protons into the intermembrane space
—Oxidation of QH2
The Chemiosmotic Model
-The chemical mechanism that couples proton flux with photophosphorylation
-Proposed by Peter Mitchell
-Chemiosmotic involves simultaneously a chemical reaction and a transport process.
-When isolated mitochondria are suspended in a buffer containing ADP, Pi and succinate (an oxidizable substrate) three things occur: 1. succinate is oxidized to fumarate; 2. molecular oxygen is consumed and 3. ATP is synthesized
-Inhibitors of either electron passage to oxygen (carbon monoxide or cyanide) or ATP synthesis (oligomycin) block the process- indicating that both processes are coupled
Chemiosmotic Model for ATP Synthesis
-Electron transport sets up a proton-motive force
-Energy of proton-motive force drives synthesis of ATP
-ATP synthesis requires electron transport
-Electron transport also requires ATP synthesis
Electron Transport Is Coupled to ATP Synthesis
Treatment of the mitochondria with a detergent or physical shearing- reduces the membrane to fragments that can still catalyze the electron transfer but no ATP is synthesized
Chemical uncouplers such as 2,4-dinitrophenol (DNP) and carbonylcyanide-p-trifluoromethoxyphenlhydrazone (FCCP)- both are weak hydrophobic acids that can cross the mitochondrial membrane that can become protonated, cross the membrane and release the proton to dissipate the proton gradient
Generation of ATP without an oxidizable substrate
Ionophores such as valinomycin allow inorganic ions to pass through membranes and dissipate the electrical contribution of the electrochemical gradient across the membrane
If an artificial proton gradient is made across the mitochondrial inner membrane, then the inner membrane should be able to synthesize ATP in the absence of an oxidizable substrate.
Mitochondrial ATP Synthase Complex
-An F-type ATPase, catalyzes the formation of ATP from ADP and Pi accompanied by the flow of protons from P to N side of the inner mitochondrial membrane
-Also called Complex V, contains two functional units:
–F1(a peripheral membrane protein)
-Soluble complex in the matrix
-Individually catalyzes the hydrolysis of ATP
—F0(o: oligomycin (inhibitor of OP)-sensitive)
-Integral membrane complex
-Transports protons from IMS to matrix, dissipating the proton gradient
-Energy transferred to F1 to catalyze phosphorylation of ADP
Mitochondrial ATP Synthase Complex
-In F1 removed vesicles, Fo
-When F1 is added back,
-In the absence of a proton gradient, the equilibrated ATP
For the continued synthesis of ATP, the synthase cycles between a form that binds ATP
-In F1 removed vesicles, Fo remains and is able to catalyze electron transfer from NADH to oxygen, but a proton gradient cannot be formed since protons can leak through Fo
-When F1 is added back, it plugs the proton pore in Fo and ATP can be made
-The free energy of synthesis of ATP on the enzyme surface is readily reversible. This is because ATP is stabilized relative to ADP and Pi and is bound more strongly than either ADP or Pi. This extra binding energy drives the equilibrium of the reaction towards ATP formation.
-In the absence of a proton gradient, the equilibrated ATP does not leave the enzyme surface.
-For the continued synthesis of ATP, the synthase cycles between a form that binds ATP very strongly and a conformation that releases ATP.
The F1
catalyzes
ADP + Pi ATP
-F1 has 9 subunits of 5 different types, α3β3γδε
-Hexamer arranged in three αβ dimers
Dimers can exist in three different conformations:
–Open: empty
–Loose: binding ADP and Pi
–Tight: catalyzes ATP formation and binds product
Coupling Proton Translocation to ATP Synthesis
-Proton translocation causes a rotation of the F0 subunit and the central shaft γ
-This causes a conformational change within all the three αβ pairs
-The conformational change in one of the three pairs promotes condensation of ADP and Pi into ATP
Other Uses of the Proton Motive Force
-rimary role is to furnish energy for ATP synthesis
-Also used to drive transport processes since the inner mitochondrial membrane is impermeable to charged species. For example, ATP must be moved out of the mitochondria and ADP and Pi must be moved in. Adenine nucleotide translocase which is an inner membrane protein binds ADP3- in the intermembrane space, transports it to the mitochondrial matrix in exchange for ATP4- that is simultaneously transported outward. Note that 4 negative charges are moved out. Due to proton pumping the matrix has a net negative charge and this proton motive force drives ATP-ADP exchange.
-The other translocase is the phosphate translocase which together with the other translocase and ATP synthase make up the ATP synthasome
Malate-Aspartate Shuttle
liver
kidney
heart (32 ATP per glucose)
Glycerol-3-Phosphate Shuttle
skeletal muscle and brain, 30 ATP
Stoichiometries of oxygen consumption and ATP synthesis
-IT is difficult to exactly measure this in intact mitochondria since other reactions occur in them apart from electron transport and ATP synthesis.
-Many experiments have yielded P/O ratio per pair of electrons being moved down the electron transport chain to be between 2 and 3 with NADH as the electron donor and between 1 and 2 with succinate as the donor.
-In looking at the numbers in term of protons being pumped, 10 protons are pumped out for every 2 electrons being moved from NADH and 6 protons for succinate. The number of protons to drive synthesis of an ATP molecule is 4, so the accepted P/O ratio is 2.5 for NADH and 1.5 for succinate.
Regulation of Oxidative Phosphorylation
Primarily regulated by substrate availability
–NADH and ADP/Pi
–Due to coupling both substrates required for electron transport and ATP synthesis
Inhibitor of F1 (IF1)
–Prevents hydrolysis of ATP during low oxygen
–Only active at lower pH, encountered when electron transport it stalled (i.e., low oxygen)
Inhibition of OxPhos leads to accumulation of NADH
–Causes feedback inhibition cascade up to PFK-1 in glycoysis
Inhibition of Oxidative Phosphorylation by IF1
-IF1 exists in a dimeric form when the pH of the cell is low.
-Under oxygen starvation conditions, most of the ATP produced is via glycolysis and the pyruvate produced is converted to lactate thereby dropping the pH of the cytosol and the mitochondrial matrix.
-The IF1 dimer binds to the ATP synthase to inhibit the ATPase activity. When aerobic metabolism resumes, pH levels rise again and the IF1 dimer is destabilized thereby leaving the ATP synthase.
Regulation of Oxidative Phosphorylation
Relative amounts of ATP/ADP control rates of electron transfer; oxidative phosphorylation; citric acid cycle; pyruvate oxidation; glycolysis. ATP is an allosteric regulator of PFK-1 and pyruvate kinase. Citrate also inhibits PFK-1 and produces concerted allosteric inhibition of the PFK-1
Mitochondria in Thermogenesis: Brown Fat
-Brown fat- a type of adipose tissue in newborns and hibernating animals, serves to generate heat to keep the newborn warm. The tissue is brown due to the large numbers of mitochondria ( cytochromes whose heme groups strongly absorb light)
-These mitochondria have a unique protein in their inner membrane called thermogenin or uncoupling protein that provides a path for protons to return to the mitochondrial matrix without going through the ATP synthase complex.
-As a result, energy is not conserved as ATP but dissipated as heat
Defective oxidative phosphorylation in pancreaticβcells blocks
insulin secretion
carry 1, 2 or 1 and 2
cytochrome, iron sulfur- carry 1
FAD or FMN- carry 1 or 2
NADH, ubiquinone-carry 2
promote
AMP, ADP, NAD+ promote
inhibit
ATP, NADH, Citrate
