Metabolism

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Transcript Metabolism

Electron Transport System
1
There are 2 Ways to Make ATP
1.
Substrate phosphorylation
2. Electron transfer-dependent
oxidative phosphorylation
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2 Glycolytic Reactions Make ATP by
Substrate-level Phosphorylation
--1,3-BPG is an energy –rich molecule with a greater phosphoryl-transfer
potential than that of ATP. Thus, it can be used to power the ATP
synthesis from ADP.
--This is called substrate-level phosphorylation because the phosphate
donor is a Substrate with high phosphoryl-transfer potential.
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2 Glycolytic Reactions Make ATP by
Substrate-level Phosphorylation
PEP has high phosphoryl-transfer potential, pyruvate
(ketone) is much more stable than enol form.
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There are 2 Ways to Make ATP
1.
Substrate phosphorylation
2. Electron transfer-dependent
oxidative phosphorylation
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How do we obtain lots of ATP?
Food (carbohydrates)
Glucose
Glycolysis
TCA
Glycolysis
ATP
Little
(~4 ATP)
After TCA cycle,
energy is extracted
In the form of reduced
Coenzymes, FADH2
and NADH
Reduced coenzymes
(NADH + H+, FADH2)
O2
ETC
H2O
Lots
(~28-30 ATP)
ATP
Electron transport and
Oxidative phosphorylation:
Involved many steps,
Sequestered in special
environment.
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Glucose
Minimal TCA Cycle
NADH + H+
Pyruvate
O
NADH + H+
CH3C-SCoA
(2C)
CoASH
4C
6C
NADH + H+
NADH + H+
CO2
FADH2
4C
GTP GDP
NADH +
CO2
H+
1 GTP
3 NADH
+1 FADH2
10 ATP/cycle
And releases
two CO2
NOTE: 1 NADH  2.5 ATP; 1 FADH2  1.5 ATP; 1 GTP  1 ATP so get 1 + 7.5 + 1.5 = 10 ATP/cycle
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Where in the cell does electron
transport and oxidative
phosphorylation occur?
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Mitochondria
TCA enzymes
b-oxidation
ATP synthase
Permeable Outer
Mtch Membrane
Intermembrane
Space
Inner Mtch
Membrane
e- transport
chain
M DNA
Matrix
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Mitochondria
--A mitochondrion is bounded by a double membrane, with
an intermembrane space.
--Outer M: permeable to most ions and small molecules
--The inner membrane: highly impermeable, Highly folded “cristae”.
most molecules require transporters (exceptions: O2, CO2).
provide large surface area for the transport proteins,
several FAD-dependent dehydrogenases and
all enzymes and proteins of oxidative phosphorylation
--The matrix is the fluid-filled interior of the mitochondrion.
oxidative enzymes like pyruvate dehydrogenase (acetyl Co A formation)
glutamate dehydrogenase, TCA cycle enzymes, fatty acid oxidation
enzymes
--Note that glycolysis occurs outside the mitochondrion in the cytosol,
whereas the citric acid cycle occurs in the matrix.
--The electron transport system is located on the cristae, both TCA cycle and11
oxidative phosphorylation occur within the mitochondrion.
Electron Transport System (ETS)
The electron transport system is located in the cristae of
mitochondria
It is a series of protein/prosthetic group carriers that pass
electrons from one to the other.
Electrons are donated to the ETS by NADH and FADH2
As a pair of electrons is passed from carrier to carrier,
energy is released and is used to form ATP
At the end of the electron transport chain, oxygen receives
the energy-spent electrons, resulting in the production of
water.
½ O2 + 2 e- + 2 H+ → H2O
(Oxygen is the final electron acceptor)
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Redox Reactions
reduction
e-
A
e+
B
A
+
B
oxidation
O oxidation
R
I
is
I
L loss of electrons G
reduction
is
gain of electrons
Reductant (A): is oxidized, electron donor
Oxidant (B): is reduced, electron acceptor
How are redox potentials
determined?
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Half cell reactions measure electromovtive force
Ethanol gives up e to H+ to form H2
H2 gives up e to Fe3+ to form H+
Oxidant
Reductant
Reductant
Oxidant
Standard:
1M H+
1atm H2 gas
E0’ of H+/H2 is
0 volts
Sample
Reference
Neg value = oxidized form has a lower affinity for electrons than does H2
(e.g., NADH a strong reducing agent has a negative reduction potential)
Pos value = oxidized form has a higher affinity for electrons than does H2
(e.g., Oxygen a strong oxidizing agent has a positive reduction potential)
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--Biochemists use E0’,
the value at pH 7.
--Chemists use E0, the
value in 1M H+.
--The prime denotes
that pH 7 is the
standard state.
--Thus, these values
are different in
chem textbooks.
A strong reducing agent, NADH is poised to donate electrons,
has a negative reduction potential, whereas a strong oxidizing agent O2 is ready
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to accept electrons and has a positive reduction potential.
Partial reactions
By convention, reduction potentials (as in Table 18.1)
refer to partial reactions are written as:
oxidant + e-
reductant
OVERALL REACTION
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Redox reactions
Redox pairs act as e- carriers
Reductant + oxidant  oxidized reductant + reduced oxidant
Free energy is released in the transfer of ereduction
(RIG)
e-
A
e+
B
A
+
B
oxidation
(OIL)
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Standard free-energy changes of an oxidationreduction reaction can be determined
DG0’: standard free energy change
– for a redox reaction
• is related to the difference in E0 between the e- acceptor and
donor
DG0’ = -nFDE’0
DG0’ = standard free-energy change
F= faraday constant = 23.06 kcal/mol/V
(required to remember!)
n = number of electrons
DE’0 = Change in reduction potential
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Determining: DG0’: standard free energy change
DE’0 = E’0 (acceptor) - E’0 (doner)
Pyruvate
NADH
DG0’ = -nFDE’0
F= faraday constant = 23.06 kcal/mol/V
n = number of electrons
DG0’ =
=
=
-2 x 23.06 kcal/mol/V x [-0.19 – (-0.32) V ]
-2 x 23.06 kcal/mol/V x 0.13V
-6.0 kcal/mol
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1.14 Volt potential favors formation of proton gradient
Acceptor donor
DG0’ = -nFDE’0 = -nF (E’0 acceptor – E’0 donor)
= -2 x 23.06 kcal/mol/V x [0.82V- (-0.32V)]
= -2 X 23.06 kcal/mol/V x 1.14V
= -52.6 kcal/mol
Note: DG0’ = -7.3 kcal/mol for the hydrolysis of ATP
The driving force of oxi phos is the elec-trans potential of NADH or FADH2 rel.
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to that of O2. The released energy is used to generate a proton gradient, then
for ATP synthesis
Driving e- Transport
Electron carriers at the beginning of the
chain are more - E0’ than those at the end
– so e- flow spontaneously from
NADH (E’0 = –0.32 v) or FADH2 (E’0 = –0.22V)
to O2 (E’0 = +0.82 volts)
Neg reduction potential = oxidized form has a lower affinity for
electrons and so transfers them most easily to an acceptor
Pos reduction potential = will be the strongest oxidizing substance and
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have a higher affinity for electrons
The electron transport system consists of four protein
complexes and two mobile carriers.
NADH-Q Oxidoreductase
Succinate-Q reductase
complexes
Q-cytochrome c Oxidoreductase
Cytochrome c Oxidase
Coenzyme Q
carrier
Cytochome c
The mobile carriers transport electrons between the
complexes, which also contain electron carriers.
The carriers use the energy released by electrons as
they move down the carriers to pump H+ from the matrix
into the intermembrane space of the mitochondrion.
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NAD+/NADH
Fumarate/
Succinate
Cytochrome C
(+3) / (+2)
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A very strong electrochemical gradient is
established with few H+ in the matrix and many in
the intermembrane space.
The cristae also contain an ATP synthase
complex through which hydrogen ions flow down
their gradient from the intermembrane space into
the matrix.
The flow of three H+ through an ATP synthase
complex causes a conformational change, which
causes the ATP synthase to synthesize ATP from
ADP + P.
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Mitochondria produce ATP by chemiosmosis,
so called because ATP production is tied to an
electrochemical gradient, namely an H+
gradient.
Once formed, ATP molecules are transported
out of the mitochondrial matrix.
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Mitchell’s Postulates for Oxidative Phosphorylation
1. The respiratory and photosynthetic electron transfer chains should be
able to establish a proton gradient
2. The ATP synthases should use the proton-motive force to drive the
phosphorylation of ADP
3. Energy-transducing membranes should be “impermeable” to protons. If
proton conductance is established (uncouplers), a proton-motive force
should not form and ATP synthesis should not occur.
4. Energy-transducing membranes should possess specific exchange
carriers to permit metabolites to permeate in the presence of high
membrane potential
Intermembrane
ADP
ATP-ADP
Antiporter
Mitochondrial
matrix
ATP
H+
H+ H+
H+
e27
ADP + Pi
ATP