Oxidative Phosphorylation

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Oxidative Phosphorylation
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Learning Objectives
Describe the anatomy of a mitochondria.
Define free energy, and endergonic and exergonic
as these terms apply to chemical reactions and/or
electron transfers.
Describe oxidative phosphorylation with respect
to how the electron flow through the membrane
complexes generates energy, and then how this
energy is coupled to ATP synthesis. Describe
coupled and uncoupled oxidative
phosphorylation. Describe the function of brown
fat.
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Outer Membrane
Biochemical anatomy of a mitochondria
Freely permeable to small molecules and ions
Matrix
Inner Membrane
Contains
Impermeable to most small molecules and ions,
including H
Pyruvate dehydrogenase complex
Contains
Respiratory electron carriers (Complexes I-IV)
(oxidative phosphorylation)
TCA cycle enzymes
Amino acid oxidation enzymes
ATP synthase (FoF1)
Fatty acid b-oxidation enzymes
ADP-ATP translocases
Other membrane transporters
DNA, ribosomes
Mg2, Ca2, K
Porin channels
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free-energy, G expresses the amount of energy
available to do work in a chemical reaction at
constant temperature and pressure.
DG the change in free energy as the result of a
chemical reaction and/or the transfer of
electrons.
exergonic pertains to a reaction in which DG is
negative, i.e. the reaction proceeds with the
release of energy. endergonic pertains to a
reaction in which DG is positive, i.e. the system
gains free-energy.
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Oxidative Phosphorylation
Overview
(1) Electrons flow through a chain of
membrane-bound carriers. (2) The free energy made
available by this downhill (exergonic) electron
flow is coupled to the uphill transport of
protons across a proton-impermeable membrane,
conserving the free energy of fuel oxidation as a
transmembrane electrochemical gradient. (3) The
transmembrane flow of electrons down this
concentration gradient through specific protein
channels provides the energy for ATP synthesis,
catalyzed by ATP synthase. This membrane enzyme
complex couples proton flow to phosphorylation of
ADP.
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The electron carriers of the respiratory chain
are organized into membrane-embedded
supramolecular complexes that can be physically
separated. Each of these complexes can transfer
electron through a portion of the respiratory
chain.
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Protein components of the mitochondrial electron
transport chain
MasskDA
Number of Subunits
Prosthetic groups
Enzyme complex
Cytochrome c is not part of any complex it is
freely soluble and moves between Complexes III
and IV.
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Complex I NADHubiquinone oxidoreductase
Complex I catalyzes two simultaneous and
obligately coupled processes
(1) the exergonic transfer to Q of a hydride ion
from NADH and a H from the matrix.
(2) the endergonic transfer of 4 H from the
matrix to the intermembrane space.
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Complex I is therefore a proton pump driven by
the energy of electron transfer. The reaction it
catalyzes is vectorial it moves protons in a
specific direction from one location (the matrix,
which becomes negatively charged with the
departure of protons) to another (the
intermembrane space, which becomes positively
charges).
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Path of electrons from NADH, succinate, fatty
acyl-CoA, and glycerol 3-phosphate to ubiquinone
(Q).
ETF electron transferring flavoprotein
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(No Transcript)
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Path of electrons from NADH, succinate, fatty
acyl-CoA, and glycerol 3-phosphate to ubiquinone
(Q).
Electrons from NADH pass through a flavoprotein
to a series of iron-sulfur proteins (in Complex
I) and then to Q. Electrons from succinate pass
through a flavoprotein and several iron-sulfur
centers (in Complex II) on the way to
Q. Glycerol-3-phosphate donates electrons to a
flavoprotein (glycerol-3-phosphate dehydrogenase)
on the outer face of the inner mitochondrial
membrane, from which they pass to Q. Acyl-CoA
dehydrogenase (the first enzyme of b-oxidation)
transfers electrons to electron-transferring
flavoprotein (ETF), from which they pass via ETF
ubiquinone oxidoreductase to Q.
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Complex II Succinate dehydrogenase
This is the only TCA cycle enzyme that is
membrane-imbedded. Electrons pass from succinate
to FAD, then through the Fe-S centers to
ubiquinone.
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Cytochrome bc1 Complex
Complex III
Complex III couples the transfer of electrons
from ubiquinol (QH2) to cytochrome c with the
vectorial transport of protons from the matrix to
the intermembrane space.
Crystal structure of monomer
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Complex IV Cytochrome Oxidase
Cytochrome c is a soluble protein of the
intermembrane space. After its single heme
accepts an electron from Complex III, cytochrome
c moves to Complex IV to donate the electron to a
binuclear copper center in that enzyme.
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Path of electrons through Complex IV
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Summary of the flow of electrons and protons
through the four complexes of the respiratory
chain
4 H
4 H
4 H
ubiquinone (Q)
Cyt c
III
IV
Q
I
II
O2
2 H2O
NADH
NAD
succinate
fumarate
4H
H
Oxygen is the terminal electron acceptor in the
respiratory chain.
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The transfer of electrons from NADH through the
electron transport chain to molecular oxygen is
highly exergonic (energy releasing). Much of this
energy is used to pump protons out of the
mitochondrial matrix. This establishes a gradient
of both charge and proton concentration. The
energy stored in this gradient is called the
proton-motive force. When protons flow
spontaneously down this electrochemical gradient,
energy is made available to do work. This energy
drives the synthesis of ATP from ADP and Pi via
the enzyme ATP synthase.
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Mitochondrial intermembrane space
Mitochondrial matrix
The inner mitochondrial membrane separates two
compartments of different H concentration
resulting in differences in chemical
concentration (pH) and charge distribution across
the membrane. The net effect is the
proton-motive force.
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Chemiosmotic Model
ATP synthase
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ATP Synthase the structure of the FoF1 complex
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Inhibitors of electron transport
Blocks electron transfer from Fe-S center to
ubiquinone
Blocks electron transfer Cyt b to Cyt c1
Inhibits cytochrome oxidase
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Uncouplers of oxidative phosphorylation
DNP has a dissociable proton and is hydrophobic.
It carries protons across the inner mitochondrial
membrane, dissipating the proton gradient.
Without the gradient, ATP cannot be synthesized.
But the flow of electrons is not inhibited.
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Most newborn mammals, including humans, have a
type of adipose tissue called brown fat. This
tissue has a unique protein in the inner
mitochondrial membrane called thermogenin or
uncoupling protein. This is a short-circuit for
protons and the energy of oxidation is dissipated
as heat to help maintain the body temperature of
the newborn.