Lecture 6 BCHM2971
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Transcript of Lecture 6 BCHM2971
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Lect
ure
6 B
CH
M29
71
Biochemical thermodynamics: ATP and redox reactions.
Oxygen’s double edged sword
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Thermodynamics and mechanisms of storing and spending energy
fuel
release
C02
stor
e
Proton gradient
spend
NA
DN
AD
H
ADP
ATP
store
spen
d
WO
RK
e- transport chain
Redox and E
Glycolysis
Krebs
Oxidative
phosphorylation
Free energy G
coupling
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Plan for today’s lecture1. Free-energy currency is "spent" to drive
nonspontaneous reactions• G and coupling
2. Why is ATP the currency of free-energy?
3. Redox cycles of e- and H+ transfer:• redox potentials (E )
4. Mechanism of e- and H+ transfer:• Complex 4 of the electron transfer chain
5. Oxygen as the final acceptor of electrons
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Why eat?
• most metabolic reactions are not spontaneous
• require a source of free energy = G
• Energy released from food is eventually ‘saved’ in ATP
‘spent’ to drive energetically unfavourable reactions
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Free energy change (G)• Free energy change (G) of a reaction
determines its spontaneity
• negative G spontaneous ( products)ie: G products < G
reactants
For a reaction A + B C + D
G = Go' + RT ln[C] [D][A] [B]
R = gas constant; T = temp.
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For a reaction A + B C + D
G = Go' + RT ln[C] [D][A] [B]
standard free energy change
pH 7 ([H+] = 10-7M)
reactants & products = 1Mfree energy change of reaction under ‘other’ conditions (eg in the cell)
Value depends on actual [products] and [reactants]
G
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Hydrolysis of ATP • useful free-energy ‘currency’ • dephosphorylation reaction is very
spontaneousATP ADP + Pi
(Go' = -31 kJ/mol) G<0
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Spontaneous?
• Spontaneous does not indicate how quickly a reaction occurs
• ATP (and pals) are kinetically stable
(usually have free energies of activation)
• Rate low without enzyme
reaction
energy-ve G
Activation energy
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Spontaneous? Why doesn’t ATP explode??
• Spontaneous does not indicate how quickly a reaction occurs
• ATP (and pals) are kinetically stable
(usually have free energies of activation)
• Rate low without enzyme
reaction
energy-ve G
Activation energy
(lowered by enzyme)
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Spontaneous?
• Kinetic stability essential:
• reaction energy is then Controllable by catalysis
Can be coupled to useful reactions
reaction
energy-ve G
Activation energy
(lowered by enzyme)
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Adenine
P P P
Ribose
What makes the bonds in ATP‘high-energy”?
Phosphoanhydride bonds
O O CH2
• Phosphoanhydride bonds tend to have a large negative G (-30.5 kJ.mol-1)
• NB: bond energy is not necessarily high, just the free energy of hydrolysis.
ATP
Phosphoester
bond
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1. PhAnH bond has less stable resonance than its product
• Two strongly e- withdrawing groups compete for e- of the bridging oxygen
• No such competition in the hydrolysis product more stable
hydrolysis
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2. PhAnH bond has greater electrostatic repulsion than its product
• At pH 7, ATP has 3 –ve charges
• Repulsion is relieved by hydrolysis
more stable
hydrolysis
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3. Solvation energy
• Phosphoanhydride bond has smaller solvation energy than product
favours hydrolysis
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Phosphoryl group-transfer potential
• Measure of tendency of compound to transfer ~P to H20
• ATP is intermediate!• Can accept ~P from
compounds above• Or donate ~P to
compounds below
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•Other phosphorylated compounds–Phosphocreatine
•Thioesters–CoenzymeA (you will meet this in other lectures)
Other high energy compounds
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Phosphocreatine
phosphocreatine creatine
ADP ATP
When ATP P
• Higher P-group transfer potential than ATP• ‘reservoir’ of ~P for rapid ATP regeneration
Maintains constant level of ATP by swapping ~P=reversible ‘substrate-level phosphorylation’ in tissues with
high need (muscle, nerve)
When ATP
P
When ATP is low, phosphocreatine can lend a P to ADP to make ATP.
When ATP is replenished by catabolism, P is ‘paid back”.
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Why create high energy compounds?
• spontaneous reactions G<0 are often coupled with non-spontaneous reactions (G>0) to drive them forward.
• The free-energy change (G) for coupled reactions is the sum of the free-energy changes for the individual reactions.
Gcoupled = G reaction 1 + G reaction 2
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hexokinase
• Thus, ATP ADP +Pi (G<0) is coupled with non-spontaneous reactions (G>0) to drive them forward.
Glucose glucose-6-P + H20
G = 13.8 kJ.mol-1
ATP +H20 ADP +Pi
G = -30.5 kJ.mol-1
Glucose + ATP glucose-6-P + ADPOverall: spontaneous!
G = -16.3 kJ.mol-1
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Energy coupling with ion gradientEnergy can also be stored as an ion gradient
• eg oxidative phosphorylation
• Spontaneous H+
movement against gradient coupled to ATP synthesis
Proton gradient
-ve G
ATP
+ve
GADP
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How does energy from food get transferred to ATP for storage?
Controlled cycles of
oxidation and reduction
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Electron transport chain (ETC)
OXIDATION
REDUCTION
NAD+ NADHe-
OXIDATION
glucose CO2
e-H
IQ III IV
H2OO2
e-
REDUCTION
e-
H
e-
e-e-
Cyt C
Sequential transfer of H: (2e- and H) from fuels indirectly provides free energy for production of ATP. What causes transfer of e- and H+? How does this release energy to create an ion gradient?? Remember redox potentials?
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REDUCTION
B reduced
e-
OXIDATION
A reducedAoxidised
B oxidised
The tendency of a substance to undergo reduction
= E°’ (reduction potential)
E°’ = Affinity for electrons
E °' = E °‘ (acceptor) – E °‘ (donor)
gain electrons, gain Hlose O
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Reduction Potential and Relationship to Free Energy
E °' = E °'(acceptor) – E °'(donor)
Go' = – nFE °'
Faraday constant
# electrons transferred
**Don’t learn these equations! Just understand the implications of +ve or –ve values
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Go' = – nFE °'
• An electron transfer reaction is spontaneous (-ve G) if E°‘ is +ve
ie: when E °' of the acceptor > E °' of the donor
Electrons spontaneously flow from low high reduction potentials
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REDUCTION
B reduced
e-
OXIDATION
A reducedAoxidised
B oxidised
acceptor has higher E
Spontaneous if...
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Oxidised reduced
Hydride ion = 2e + H+
Accepts e- from fuel
thermodynamics of the ETChain
In ETC
• NAD accepts e- and H+ from fuel NADH• NADH donates e- and H+ to ETC
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NADH oxidation is spontaneous and releases free energy
E °' = E °'(acceptor) – E °'(donor)
E °‘ = 0.8 – (-0.3) = 1.13V
NAD+ + H+ + 2e- NADH
H2O½ O2 + 2H+ + 2e-
E°’ = -0.3 V
E°’ = +0.8 V
reduced
oxid
ised
O2 has greatest affinity for e-NADH becomes the e- donor
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NADH oxidation is spontaneous and releases free energy
NAD+ + H+ + 2e- NADH
H2O½ O2 + 2H+ + 2e-
reduced
oxid
ised REDUCTION
OXIDATION
E °‘= 1.13V
Go' = – nFE °‘
- ve +ve
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electrons are not transferred directly from NADH to O2
• rather pass through a series of intermediate electron carriers
• Why? This allows energy released to be coupled to protons pump.
• ultimately responsible for coupling the energy of redox to ATP synthesis.
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Electrons spontaneously flow from low to high reduction potentials
Increasing E
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One example in more detail: Complex IV (cytochrome c oxidase)
Transmembrane spanning -helices
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Complex IV (cytochrome c oxidase)
• Catalyses final reduction in the ETC
• O2 + 4 H+ + 4 e- 2 H2O (irreversible)
• The four electrons are transferred into the complex one at a time from cytochrome c.
• Results in pumping of 4 H+ across the membrane.
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Has 4 metal ‘redox centers’
• haem a3, (Fe)
• CuB
• CuA (=2 Cu atoms)
• haem a (Fe)
Ions in close proximity
= binuclear complex
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FIRST: 2e- passed from cytC by haem a-CuA to binuclear center
Cyt C
e-
• e- are passed one at a time
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Fe3+ Cu2+ Fe2+ Cu+
Fully oxidised Fully reduced
• H+ from matrix and hydroxyl from binuclear center H2O
• 2e- were passed from cytC by haem a-CuA to fully reduce Fe and Cu in the binuclear center
e- e-
Tyr
H+e-e-
O-H
OH TyrOHOHH
So far…
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Fe3+ Cu2+ Fe2+ Cu+
Fully reduced
Then, O2 binds
e- e-
Tyr
H+e-e-
O-H
OH TyrOHOHH
Fe2+ Cu+e- e-
TyrOH
OO
OO
This O2 is going to become O22-
It’s going to need 4 e-
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e-
Fe3+ Cu2+ Fe2+ Cu+
Fully oxidised
e- e-
Tyr
H+e-e-
O-H
OH TyrOHOHH
Fe2+ Cu+e- e-
TyrOH
OO
OO
Fe4+ Cu2+e-
e-
TyrO
O2- O-e-
The tricky bit!!
• 4e- are rearranged• Only 3e- can be donated by the
metal ions (see why?)• So 1e- ALSO must be donated
temporarily from tyrosine OXYFERRYL complex
H
Fe2+ - 2e- Fe4+ Cu + - 1e- Cu2+
Tyr-OH - 1e- -H+ Tyr-O.
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O22- shared between Cu and Fe
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e-
Fe3+ Cu2+ Fe2+ Cu+
Fully oxidised
e- e-
Tyr
H+e-e-
O-H
OH TyrOHOHH
Fe2+ Cu+e- e-
TyrOH
OO
OO
Fe4+ Cu2+e-
e-
TyrO
O2- O-e- H
e-H
H
e- Fe4+ Cu2+e-
Tyr
O2-e-
OHH
OH
1 more e- passed in via haem3-CuA to binuclear complex Reconverts tyrosine
And more H+ H2O
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e-
Fe3+ Cu2+ Fe2+ Cu+
Fully oxidised
e- e-
Tyr
H+e-e-
O-H
OH TyrOHOHH
Fe2+ Cu+e- e-
TyrOH
OO
OO
Fe4+ Cu2+e-
e-
TyrO
O2- O-e-
4th e- passed via h3CuA Regenerates Fe3+: Completed cycle!
HAnd one more H+
e-
H H
e- Fe4+ Cu2+e-
Tyr
O2-e-
OHH
OH
e-
H
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OO
H+
H+H+
H+
H+
H+
H+
H+
Meanwhile pumps 4 H+ were pumped
to proton gradient
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O2 as final e- acceptor
• Strong e- acceptor (high E)Provides thermodynamic force
• Also, controllable: reacts slowly unless catalysed by enzyme
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Disadvantages
• O2 + 4 e- safe 2H20
• BUT partial reduction DANGER!!!
• O2 + e- O2 – (superoxide)
• Can extract e- from other molecules ‘free radicals’
• Oxidisation of membranes, DNA, enzymes
• Implicated in Alzheimers, Parkinsons, aging
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Summary• Hydrolysis of ATP is spontaneous (–ve G)• Free energy of ATP coupled to non-
spontaneous reactions• Phospho-anhydride bond is ‘high energy’• Electrons spontaneously flow from low to
high EFood NAD e- transfer chain O2
• Free energy used to create proton gradient that is then ‘spent’ to make ATP
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The individual reactions are:• oxidation NADH NAD+ + H+ + 2e- Go= -158.2 kJ
spontaneous
• reduction ½ O2 + 2H+ + 2e- H2O Go= -61.9 kJ
spontaneous
• phosphorylation ADP ATP Go= +30.5 kJ
nonspontaneous
• The net reaction is obtained by summing the coupled reactions,
ADP + NADH + ½ O2 + 2H+ ATP + NAD+ + 2 H2OGo= -189.6 kJspontaneous
Coupled non-spontaneous work
Do NOT learn these values! Just know which are +ve or –ve/ spontaneous or not…understand concept of coupling!!