CHAPTER 19
Oxidative Phosphorylation
基本概念
Electron Transport
酵素
Chemiosmotic Theory
Structure of a Mitochondrion
Energy Flow in Cellular Respiration
Structure of a Mitochondrion
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 FADH 2.
In oxidative phosphorylation, energy from NADH and FADH2 is used to make ATP.
Double membrane leads to four distinct compartments:
- Outer membrane:
– relatively porous membrane; allows passage of metabolites
- Intermembrane space (IMS):
- Inner membrane
- Matrix
similar environment to cytosol
higher proton concentration (lower pH)
relatively impermeable, with proton gradient across it
location of electron transport chain complexes
Convolutions called cristae serve to increase the surface area.
location of the citric acid cycle and parts of lipid and amino acid metabolism
lower proton concentration (higher pH)
Iron-Sulfur Clusters
Coenzyme Q or Ubiquinone
Cytochromes
One-electron carriers
Iron coordinating porphoryin ring derivatives
a, b, or c differ by ring additions
One-electron carriers
Coordinating by cysteines in the protein
Containing equal number of iron and sulfur atoms
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.
Ubiquinone:Cytochrome c Oxidoreductase
a.k.a. Complex III
The Q Cycl
（Complex III）
Succinate Dehydrogenase
a.k.a. Complex II
Cytochrome Oxidase
a.k.a. Complex IV
NADH:Ubiquinone Oxidoreductase
a.k.a. Complex I
Summary of Electron Transport
Reactive Oxygen Species Can Damage Biological Macromolecules
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.
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
Succinate dehydrogenase is a single enzyme with dual roles:
convert succinate to fumarate in the citric acid cycle
capture and donate electrons in the electron transport chain
Uses two electrons from QH2 to reduce two molecules of cytochrome c
Additionally contains iron-sulfur clusters, cytochromeb, and cytochrome c
Clearance of electrons from the reduced quinones via the Q-cycle results in translocation of four additional protons to the intermembrane space.
Experimentally, four protons are transported across the membrane per two electrons that reach cyt c.
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.
Mammalian cytochrome oxidase is a membrane protein with 13 subunits.
Contains 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
Ubiquinone is naturally “leaky” and facilitates partial reduction of non-Complex III targets.
Single electron tranfers result in free radicals.
One method by which the cell can correct free-radical production of reduced glutathione, which fuels the glutathione shuttle
ATP Synthesis
Mitochondrial ATP Synthase Complex
Relationship of ETC and ATP Synthesis
Binding-Change Model
Inhibitors of the Electron Transport Chain Disrupt Oxidative Phosphorylation
Coupling Proton Translocation to ATP Synthesis
Chemiosmotic Model for ATP Synthesis
Evidence of Rotation
Electron transport sets up a proton-motive force.
Energy of proton-motive force drives synthesis of ATP.
As described, ATP synthesis requires electron transport.
But electron transport does not requires ATP synthesis.
F1
F2
soluble complex in the matrix
individually catalyzes the hydrolysis of ATP
integral membrane complex
transports protons from IMS to matrix, dissipating the proton gradient
energy transferred to F1 to catalyze phosphorylation of ADP
The F1 catalyzes ADP + Pi <=> ATP
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
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.
其他
Net Production of ATP via Catabolic Pathways
Glycerol-3-Phosphate Shuttle
Regulation of Oxidative Phosphorylation
Malate-Aspartate Shuttle
The Mitochondria Play an Initiating Role in Apoptosis
Net Production of ATP by Oxidation of Glucose (and Other Fuels) Varies
In prokaryotic systems, organelles do not segregate machinery, so all electron carriers can easily feed directly into the electron-transport chain.
In eukaryotic systems, organellar segregation prevents NADH from the cytosol from directly entering the electron-transport chain at Complex I.
NAD+ pools are kept segregated and cannot directly cross the mitochondrial inner membrane.
Two methods are used to feed the electrons from NADH from the cytosol into the mitochondria:
malate-aspartate shuttle
glycerol-3-phosphate shuttle
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
Primarily regulated by substrate availability
– NADH and ADP/Pi
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