Download Oxidative Phosphorylation and Photosynthesis

Oxidative Phosphorylation
Catabolism of proteins, fats,
and carbohydrates in the
three stages of cellular
respiration
• Stage 1: oxidation of fatty acids,
glucose, and some amino acids
yields acetyl-CoA.
• Stage 2: oxidation of acetyl groups
in the citric acid cycle to form NADH
and FADH2
• Stage 3: electrons are funneled into
a chain of electron carriers reducing
O2 to H2O. This electron flow drives
the production of ATP.
The products of oxidation of organic
compounds are CO2 a electrons
bound to “reduction equivalents”
NADH a FADH2
Oxidative phosphorylation is the final
step in energy producing metabolism
in aerobic organisms
Oxidative phosphorylation is a
process in which the energy of
electrons accumulated in reduction
equivalents is used to produce ATP
In the nature
How can be the energy stored and
later used to build up chemical
substances?
Pumped-Storage Power Plant
Chemiosmotic theory
• Unifying principles of ATP
production
• Chemiosmotic driving force of the
difference in proton concentration
How to achieve such difference in
proton concentration?
• Biological oxidation reactions give
reduced NADH and FADH2
• Energy released during their
oxidation is used to transfer
protons across the membrane
Energy from Reduced Fuels is Used
to Synthesize ATP
• Carbohydrates, lipids, and amino acids are
the main reduced fuels for the cell
• Electrons from reduced fuels are used to
reduce NAD+ to NADH or FAD to FADH2.
• In oxidative phosphorylation, energy from
NADH and FADH2 is used to make ATP
Oxidative Phosphorylation
• 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
to ATP
Oxidative Phosphorylation
Structure of a Mitochondrion
• The proton gradient is
established across the
inner membrane
• The cristae are
convolutions of the
inner membrane and
serve to increase the
surface area
Metabolic Sources of Reducing Power
Some Important Reactions Catalyzed by NAD(P)H-Linked Dehydrogenases
Structure of the Inner
Mitochondrial Membrane
- Electron Transport Chain -
(Citric acid cycle)
NADH:Ubiquinone Oxidoreductase
(Complex I)
NADH:Ubiquinone Oxidoreductase
(Complex I)
• 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
• Non-covalently bound flavin mononucleotide
(FMN) accepts two electrons from NADH
• Several iron-sulfur centers pass one electron at
the time toward the ubiquinone binding site
(Citric acid cycle)
Coenzyme Q or Ubiquinone
• Final akceptor of electrons from the complex I
• Central mobile electron carrier in respiratory chain
dissolved in mitochondrial membrane
• Upon accepting two electrons, it picks up two
protons to give an alcohol, ubiquinol
• Ubiquinol can freely diffuse in the membrane,
carrying electrons to a Komplex III
• Ultimately it also transports protons over the
mitochondrial membrane
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 inter-membrane space (P)
• Experiments suggest that about 4 protons are
transported per one NADH
NADH + Q + 5H+N = NAD+ + QH2 + 4 H+P
• Reduced coenzyme Q picks up two protons
• Despite 50 years of study, it is still unknown how the four
protons are transported across the membrane
(Citric acid cycle)
Succinate Dehydrogenase
(Complex II)
• Enzyme of Citric Acid Cycle
• FAD accepts two electrons from succinate
• Electrons are then passed via iron-sulfur centers
to ubiquinone that becomes reduced QH2
• Succinate Dehydrogenase
(Complex II)
Succinate Dehydrogenase - structure
Complex I and Complex II reduce ubichinon
to ubichinol QH2
Ubichinol transports electrons and protons
to Complex III
Complex I and II are the main donators of electrons to Q
However
There are more processes on the inner mitochondrial
membrane that can reduce ubichinon
• b oxidation of fatty acids
• Glycerol 3-phosphate dehydrogenase – forming
dihydroxyacetone phosphate – a way how to transport
reduced equivalents into mitochondria
• Alternative plant NAD(P)H dehydrogenases
(Citric acid cycle)
Cytochrome bc1 Complex
(Complex III)
Cytochrome bc1 Complex
(Complex III)
• Uses two electrons from QH2 to reduce
two molecules of cytochrome c
• Ubichinone:cytochrome c oxidoreductase
• Dimer of two cytochrome b subunits
• Cavity in the middle for ubichinone binding
The Q Cycle
• Two electron carrier - QH2 - must give electrons to
one electron carrier - heme of cytochrome
• Experimentally, four protons are transported
across the membrane per two electrons that reach
Cytochrome 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
Cytochrome c
• Cytochrome c is a soluble heme-containing
protein in the intermembrane space
• Heme iron can be either ferrous (Fe3+, oxidized)
or ferric(Fe2+, reduced)
• Cytochrome c carries a single electron from the
cytochrome bc1 complex to cytochrome oxidase
the next Complex IV
(Citric acid cycle)
Cytochrome Oxidase
(Complex IV)
Cytochrome Oxidase
(Complex IV)
• Mammalian cytochrome oxidase is a membrane
protein with 13 subunits
• Contains two heme groups
• Contains copper ions
– Two ions (CuA) form a binuclear center
– Another ion (CuB) bonded to heme forms FeCu center
Cytochrome Oxidase Passes Electrons to O2
Cytochrome Oxidase Passes
Electrons to O2
• Electron is transferred from Cyt c to CuA centre,
to heme a, to heme a3-CuB centre and finally to
oxygen
• Four electrons are used to reduce one oxygen
molecule 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 inter-membrane space by an
unknown mechanism
Summary of the Electron Flow in the
Respiratory Chain
Energy of the Electron Flow in the
Respiratory Chain
NADH + H+ + 1/2O2
NAD+ + H2O
DG’o = -nFDE’o
NADH/NAD+ = -0.32V
O2/H2O = 0.816
DE’o = 1.14V
DG’o = 220 kJ/mol (NADH)
Proton-motive Force
• The proteins in the electron transport chain use the
energy of NADH (FADH2) oxidation to:
• create the electrochemical proton gradient by one of
the three means:
– active transport of protons across the membrane
– reduced coenzyme Q pick up protons from the
matrix
– oxidation of QH2 releases protons at the intermembrane side
Proton-motive Force
• Chemical potential energy due to
difference in proton concentration
• Electrical potential energy due to the
separated charge over the membrane
Energy of the Proton-motive Force
NADH + 11H+ + 1/2O2
NAD+ + 10H+ + H2O
DG’o = 2.3RT DpH + FDy
DpH = 0.75
Dy = 0.15 V
DG’o = 19 kJ/mol (of protons)
10 protons / 1 NADH
190 (out of 220) kJ/mol is accumulated
Chemiosmotic Model for ATP Synthesis
• Electron transport sets up a proton-motive force
• Energy of proton-motive force drives synthesis of ATP
Mitochondrial ATP Synthase Complex
Mitochondrial ATP Synthase Complex
• The proton-motive force causes rotation of the
central shaft 
• This causes a conformational change within all the
three b pairs
• The conformational change in one of the three
pairs promotes condensation
of ADP and Pi into ATP
• Cylinder of c subunits, e and 
rotate relative to a, b2, d and , b
subunits
• For each 120o the  gets into
contact with b and changes its
conformation into b-empty state
• This forces the other b subunits into
b-ADP and b-ATP conformations
• In one rotation three ATP are
synthesized
• ATP hydrolysis makes reverse
rotation and proton transfer over the
membrane
Passage of protons
through Fo initiates
conformational
changes on F1
• Torque force of F1 is approximately 4000 Nm
• It corresponds to an example:
You are staying at the bottom of the swiming
pool full of water with a stick in a hand long
about 500 m moving the same speed as it is in
the picture
Nonintegral Stoichiometry of ATP
Synthesis
Before Chemiosmotic Coupling – assumption for integral
ratio of ATP per NADH (or oxygen)
Experimental P/O ratio (ATP to 1/2O2) – usually between 2
and 3 for NADH (and 1 to 2 for succinate)
4 protons are needed for 1 ATP – 3 for 120o F1 rotation and
1 for ADP transport into mitochondrial matrix
10 protons are produced from 1 NADH (gives 2 electrons to
reduce 1/2O2)
6 protons produced from 1 succinate
P/O is 2.5 for NADH and 1.5 for succinate
ATP Yield From Complete Oxidation of
Glucose
Shuttle system to transport NADH in
mitochondria
NADH cannot be transported into
mitochondria
Malate – Aspartate shuttle
NADH cannot be transported into
mitochondria
Malate – Aspartate shuttle
Energy Transformation
in Plants by
Photosynthesis
Light Energy is Converted to ATP in
Plant Chloroplasts
Energy of Light and Synthesis of ATP
• Localized on thylakoid membranes of chloroplasts
• Light induced extraction of electrons from a
chlorophyll – photo-oxidation of chlorophyll
• Electron is passed via a chain of membrane
transporters to the ultimate electron acceptor,
NADP+
• During the membrane transport, protons are
transported across the membrane making proton
gradient
• Energy of the proton gradient is used drive synthesis
of ATP
Light-Induced Redox Reactions
and Electron Transfer
Proton gradient built up during
photosynthesis drives synthesis of ATP
Energy of Light and Synthesis of ATP
• Water is the source of electrons that are
passed via a chain of transporters to the
ultimate electron acceptor, NADP+
• Oxygen is the byproduct of water oxidation
Z - scheme
Photosynthetic reactions
produce carbohydrates from
carbon dioxide using light
energy in two steps
• Energy of light is absorbed and used
to build up proton gradient on
membrane for ATP synthesis and to
produce NADPH.
• NADPH and ATP are used in the
carbon-assimilation reactions to
reduce CO2 into trioses and other
sugars.
ATP and NADPH are used to
convert CO2 to sugars
• Ribulose-1,5-bisphosphate-carboxylase/oxygenase
(Rubisco)
• Fixation of CO2 by Rubisco to ribulose-1,5bisphosphate (5 C) – production of two molecules of
3-phosphoglycerate (3 C)
• ATP required
• Reduction of 3-phosphoglycerate by NADPH and
regeneration of ribulose-1,5-bisphosphate
Calvin cycle
Flow of Protons: Mitochondria,
Chloroplasts, Bacteria
• According to endosymbiotic theory,
mitochondria and chloroplasts arose from
entrapped bacteria
• Bacterial cytosol became:
• mitochondrial matrix
• chloroplast stroma
Learning objectives
• Electron transport chain in mitochondria
• The reduced cofactors pass electrons into the mitochondrial electron
transport chain
• Stepwise electron transport is accompanied by the directional
transport of protons across the membrane against their concentration
gradient
• The energy in the electrochemical proton gradient drives synthesis of
ATP
• ATP is produced by coupling the proton flow via ATP synthase and
the conformational changes in the active site
• Photosynthesis and ATP production