Energy metabolism Part 7 - How does the electron
transport chain generate ATP?
In this episode, we’ll finally reveal how
ATP is synthesized from the reducing equivalents
NADH and H+ and FADH2, which are formed in
glycolysis, beta-oxidation, and the citric
acid cycle.
NAD+ and FAD are regenerated in the respiratory
chain.
The ATP produced in this process is a major
contributor to cellular energy production.
Let’s refresh our knowledge: The reduced
forms NADH and H+ and FADH2 are built when
NAD+ and FAD each pick up two electrons and
two protons.
While the reducing equivalents are reduced
in this way, the molecules from which the
electrons and protons originate are oxidized.
At first glance, it’s somewhat surprising
that the reducing equivalents can be used
to form ATP, as their oxidation doesn’t
cleave any energy-rich bonds.
So, where’s the energy for ATP synthesis
derived from?
Let’s find an answer to this question.
This time around, we don’t even need any
molecular structures for the answer.
The respiratory chain is a series of multienzyme
complexes that are embedded in the inner mitochondrial
membrane.
As its name suggests, this complex is involved
in cellular respiration.
The involved proteins are called complex I,
complex II, coenzyme Q - also termed ubiquinone,
complex III, cytochrome C, and complex IV.
By the way, we’ve already introduced complex
II in the citric acid cycle as succinate dehydrogenase.
There, the enzyme converts succinate to fumarate
and transfers two electrons and protons to
FAD and, therefore, to the respiratory chain.
FADH2 transfers its electrons and protons
to complex II, in contrast to NADH and H+,
which transfers its electrons and protons
to complex I.
To do so, they need to be present in the mitochondrial
matrix.
Since the citric acid cycle and beta-oxidation
take place in the mitochondrial matrix, there’s
no problem with the resulting reducing equivalents.
But that's not the case for NADH and H+ from
glycolysis, which is produced in the cytosol.
Its electrons and protons can’t easily pass
through the mitochondrial membrane to reach
the proteins of the respiratory chain.
The outer mitochondrial membrane possesses
many porins and, therefore, doesn’t represent
an obstacle.
However, NADH and H+ can’t permeate the
inner mitochondrial membrane.
Its protons and electrons need to be transported
through the membrane using special transport
systems, such as the malate-aspartate shuttle.
This transport operates as follows: Oxaloacetate
is initially reduced to malate in the cytosol,
regenerating cytosolic NAD+.
Malate is subsequently transported through
the inner membrane.
In the mitochondrial matrix, malate is oxidized
again to oxaloacetate, and its protons and
electrons are used to reduce mitochondrial
NAD+ to NADH and H+.
This way, electrons and protons of the reducing
equivalents have passed through the mitochondrial
membrane and into the matrix without the need
to translocate NADH and H+.
Proteins of the respiratory chain decouple
proton and electron transport.
In fact, this ability is so important that
the entire system is usually referred to as
the electron transport chain, highlighting
this part of the system's function.
Let’s take a detailed look at electron transport:
Whether electron transport uses or provides
energy depends on how easily the involved
molecules accept or release electrons.
Energy is emitted when electrons are transferred
from a molecule that isn’t very capable
of accepting electrons to one that easily
accepts them.
This electron transport occurs spontaneously.
A molecule's ability to accept electrons can
be quantified by its redox potential: The
more positive the redox potential, the easier
a molecule accepts electrons.
Consequently, the transfer of electrons from
a molecule with a low redox potential to one
with a higher potential occurs spontaneously.
The electrons pass from a high energy state
to a lower one.
The diagram shown here is often used to illustrate
this relationship between molecules.
Note that the redox potential on the y-axis
increases towards the bottom here.
In the electron transport chain, the electrons
gradually pass along the various proteins:
They’re shuttled from complex I or II to
coenzyme Q, and from there to complex III,
and further to cytochrome C. Cytochrome C
carries the electrons to the last protein
of the electron transport chain, complex IV.
Here the electrons need to leave the electron
transport chain to prevent the process from
stopping.
For this, the electrons are transferred to
oxygen.
In this reaction, one molecule of oxygen yields
two molecules of water.
For each water molecule, two electrons and
two protons leave the electron transport chain.
Because oxygen serves as the final electron
acceptor of the electron transport chain,
the entire system depends on aerobic conditions.
Without oxygen, the electrons can’t leave
the electron transport chain, and the proteins
remain in a reduced state.
Therefore, under anaerobic conditions, NAD+
and FAD aren’t regenerated and are depleted.
Consequently, the metabolic pathways depending
on these reducing equivalents are also unable
to run.
In the end, this final redox reaction of the
electron transport chain turns the citric
acid cycle, beta-oxidation, and partially
glycolysis into aerobic metabolic processes.
By the way, you can find out more about the
differences between aerobic and anaerobic
metabolism in the next episode of this course.
Now, on to proton transport: In the beginning,
protons are transported together with electrons
until they reach coenzyme Q.
There, proton and electron transport are decoupled.
Each time an electron is passed from a higher
to a lower energy state in the electron transport
chain, the difference in energy is released.
This energy is used by the protein complexes
to pump protons from the mitochondrial matrix
into the intermembrane space, creating a proton
gradient across the inner mitochondrial membrane.
Complex I pumps four protons into the intermembrane
space, while complex II doesn’t pump any
protons.
Coenzyme Q and complexes III and IV each release
two protons into the intermembrane space.
This results in an important difference in
proton numbers for the two reducing equivalents.
As NADH and H+ transfers its electrons to
complex I and FADH2 to complex II, NADH and
H+ increases the proton gradient by 10 protons
and FADH2 by only six.
The higher the proton gradient, the more energy
stored and ATP formed.
ATP synthesis is carried out by a protein
with an extraordinary mode of action, called
ATP synthase.
Here’s a short overview of ATP production:
ATP synthase transports protons along the
gradient, back into the mitochondrial matrix.
The energy set free by the transport of just
over three protons is utilized to synthesize
one molecule of ATP from ADP and phosphate.
This reaction is known as oxidative phosphorylation.
Through the interaction of the proteins in
the respiratory chain and ATP synthase, one
NADH and H+ provides around 2.5 molecules
of ATP, while one FADH2 yields around 1.5
molecules of ATP.
Therefore, the respiratory chain increases
the energy balance of the various metabolic
pathways: Glycolysis provides an additional
five molecules of ATP this way, the citric
acid cycle an additional nine ATP molecules
per acetyl-CoA unit, mitochondrial beta-oxidation
an additional four ATP molecules per C2 unit,
and peroxisomal beta-oxidation as well as
the oxidation of pyruvate to acetyl-CoA provide
an additional 2.5 molecules of ATP each.
So, you can see how large a role the electron
transport chain plays in energy production.
Now, what’s the clinical relevance of the
respiratory chain?
Undesirable outcomes of defects include mitochondrial
myopathies, in which there’s a deficiency
or malfunction in mitochondrial enzymes.
They can affect all protein complexes, thereby
interfering with energy metabolism.
The name "myopathies" indicates that these
diseases affect mainly skeletal muscle, an
organ with high energy consumption.
Myopathies become evident through, for example,
muscle weakness.
In addition, some toxic agents inhibit the
electron transport chain, therefore having
a lethal effect.
These include cyanides and carbon monoxide,
which both bind to complex IV and prevent
the reduction of oxygen to water.
On the other hand, some drugs’ mechanism
of action is based on a desired inhibition
of the electron transport chain.
One example are the biguanides, which include
metformin, the most widely used drug in treating
type II diabetes.
Metformin selectively inhibits complex I of
the respiratory chain.
The resulting inhibition of ATP synthesis
leads to a decrease in ATP levels, while ADP
and AMP concentrations increase.
In liver cells, AMP inhibits the activity
of the enzyme adenylyl cyclase.
As a result, cAMP production and, therefore,
hepatic gluconeogenesis is reduced, sinking
blood sugar levels.
Metformin acts as a glucagon antagonist since
glucagon stimulates cAMP synthesis via adenylyl
cyclase and thereby increases blood glucose
levels.
Let’s finish off by summarizing the most
important aspects of the respiratory chain:
The chain occurs in the inner mitochondrial
membrane and comprises complexes I to IV,
coenzyme Q, and cytochrome C.
In the respiratory chain, the electrons and
protons of the reducing equivalents NADH and
H+ and FADH2 are used to produce water.
The respiratory chain depends on oxygen because
it’s the acceptor of electrons and protons
in the final reaction catalyzed by complex
IV.
The energy released during electron transfer
is stored temporarily in a proton gradient,
which is used by ATP synthase to generate
ATP.
Through oxidative phosphorylation, around
2.5 molecules of ATP are formed per NADH and
H+ and approximately 1.5 molecules of ATP
per FADH2.
Before we look more closely at the differences
between aerobic and anaerobic metabolism in
the next episode, we’ve prepared a quiz
for you.
