lecture 6 bchm2971
DESCRIPTION
Lecture 6 BCHM2971. Biochemical thermodynamics: ATP and redox reactions. Oxygen’s double edged sword. Thermodynamics and mechanisms of storing and spending energy. Proton gradient. fuel. ADP. spend. WORK. release. store. store. spend. NAD. NADH. C0 2. ATP. Glycolysis Krebs. - PowerPoint PPT PresentationTRANSCRIPT
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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!!