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.