Electron Transport Chain For Peaceful Sleep Learning

Tonight, we quietly explore the elegant molecular machinery of the electron transport chain—a vital cellular process that powers life itself through the gentle flow of electrons and energy. Benjamin Boster softly guides us through this fascinating biochemical pathway, where tiny particles dance through protein complexes to create the energy our cells need to thrive. Happy sleeping!

Transcript

Welcome to the I Can't Sleep podcast,

Where I help you drift off one fact at a time.

I'm your host,

Benjamin Boster,

And today let's learn about electron transport chain.

An electron transport chain,

ETC,

Is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions,

Both reduction and oxidation occurring simultaneously,

And couples this electron transfer with the transfer of protons,

H plus ions,

Across a membrane.

Many of the enzymes in the electron transport chain are embedded within the membrane,

And the flow of electrons through the electron transport chain is an exergonic process.

The energy from the redox reactions creates an electrochemical proton gradient that drives the synthesis of adenosine triphosphate,

ATP.

In aerobic respiration,

The flow of electrons terminates with molecular oxygen as the final electron acceptor.

In anaerobic respiration,

Other electron acceptors are used,

Such as sulfate.

In an electric transport chain,

The redox reactions are driven by the difference in the Gibbs free energy of reactants and products.

The free energy released when a higher-energy electron donor and acceptor convert to lower energy products,

While electrons are transferred from a lower to a higher redox potential,

Is used by the complexes in the electron transport chain to create an electrochemical gradient of ions.

It is this electrochemical gradient that drives the synthesis of ATP via coupling,

With oxidative phosphorylation with ATP synthase.

In eukaryotic organisms,

The electron transport chain and site of oxidative phosphorylation is found on the inner mitochondrial membrane.

The energy released by reactions of oxygen and reduced compounds,

Such as cytochrome c,

And,

Indirectly,

NADH and FADH2,

Is used by the electron transport chain to pump protons into the intermembrane space,

Generating the electrochemical gradient over the inner mitochondrial membrane.

In photosynthetic eukaryotes,

The electron transport chain is found on the thylakoid membrane.

Here,

Light energy drives electron transport through a proton pump,

And the resulting proton gradient causes subsequent synthesis of ATP.

In bacteria,

The electron transport chain can vary between species,

But it always constitutes a set of redox reactions that are coupled to the synthesis of ATP,

Through the generation of an electrochemical gradient and oxidative phosphorylation through ATP synthase.

Most eukaryotic cells have mitochondria.

Which produce ATP from reactions of oxygen with products of the citric acid cycle,

Fatty acid metabolism,

And amino acid metabolism.

At the inner mitochondrial membrane,

Electrons from NADH and FADH2 pass through the electron transport chain to oxygen,

Which provides the energy driving the process as it is reduced to water.

The electron transport chain comprises an enzymatic series of electron donors and acceptors.

Each electron donor will pass electrons to an acceptor of higher redox potential,

Which in turn donates these electrons to another acceptor,

A process that continues down the series until electrons are passed to oxygen,

The terminal electron acceptor in the chain.

Each reaction releases energy because a higher energy donor and acceptor convert to lower energy products.

Via the transferred electrons,

This energy is used to generate a proton gradient across the mitochondrial membrane by pumping protons into the inner membrane space,

Producing a state of higher free energy that has the potential to do work.

This entire process is called oxidative phosphorylation,

Since ADP is phosphorylated to ATP by using the electrochemical gradient that the redox reactions of the electron transport chain have established driven by energy-releasing reactions of oxygen.

Energy associated with the transfer of electrons down the electron transport chain is used to pump protons from the mitochondrial matrix into the inner membrane space,

Creating an electrochemical proton gradient delta pH across the inner mitochondrial membrane.

This proton gradient is largely but not exclusively responsible for the mitochondrial membrane potential,

Delta psi sub m.

It allows ATP synthase to use the flow of H+,

Through the enzyme,

Back into the matrix to generate ATP from adenosine diphosphate,

ATP,

And inorganic phosphate.

Complex I,

NADH coenzyme Q reductase,

Labeled 1,

Accepts electrons from the Krebs cycle electron carrier,

Nicotinamide adenine dinucleotide,

NADH,

And passes them to the coenzyme Q,

Ubiquinone,

Labeled Q,

Which also receives electrons from complex II,

Succinate dehydronase,

Labeled II.

Q passes electrons to complex III,

Cytochrome bc sub I complex,

Labeled III,

Which passes them to cytochrome c,

Or site c.

Site c passes electrons to complex IV,

Cytochrome c oxidase,

Labeled IV.

Four membrane-bound complexes have been identified in mitochondria.

Each is an extremely complex transmembrane structure that is embedded in the inner membrane.

Three of them are proton pumps.

The structures are electrically connected by lipid-soluble electron carriers.

The structures are electrically connected by lipid-soluble electron carriers and water-soluble electron carriers.

The overall electron transport chain can be summarized as follows.

Electrons from NADH and a proton,

H+,

Enter the electron transport chain at complex I.

From there they move to ubiquinone,

Or Q,

Then to complex III,

Then cytochrome c,

And finally to complex IV,

Where they help reduce oxygen to form water.

Separately,

Succinate feeds into complex II,

Which also passes electrons to Q,

Bypassing complex I.

In complex I,

NADH ubiquinone oxidoreductase,

Type I NADH dehydrogenase,

Or mitochondrial complex I,

EC 1.

6.

5.

3.

Two electrons are removed from NADH and transferred to a lipid-soluble carrier,

Ubiquinone,

Q.

The reduced product,

Ubiquinol QH2,

Freely diffuses within the membrane,

And complex I translocates four protons,

H+,

Across the membrane,

Thus producing a proton gradient.

Complex I is one of the main sites at which premature electron leakage to oxygen occurs,

Thus being one of the main sites of production of superoxide.

The pathway of electrons is as follows.

NADH is oxidized to NAD+,

By reducing flavin mononucleotide to FMNH2 in one 2-electron step.

FMNH2 is then oxidized in two 1-electron steps,

Through a semiquinone intermediate.

Each electron thus transfers from the FMNH2 to an iron-sulfur cluster,

From the iron-sulfur cluster to ubiquinone,

Q.

Transfers of the first electron results in the free radical semiquinone form of Q,

And transfer of the second electron reduces the semiquinone form to the ubiquinol form.

QH2.

During this process,

Four protons are translocated from the mitochondrial matrix to the intermembrane space.

As the electrons move through the complex,

An electron current is produced along the 180 angstrom widths of the complex within the membrane.

This current powers the active transport of four protons to the intermembrane space,

Per two electrons,

From NADH.

In Complex II,

Succinate dehydrogenase or succinate COQ reductase,

EC 1.

3.

5.

1,

Additional electrons are delivered into the quinone pool.

Q.

Originating from succinate and transferred via flavin adenine duonucleotide FAD to Q.

Complex II consists of four protein subunits.

Succinate dehydrogenase,

SDHA,

Succinate dehydrogenase ubiquinone iron-sulfur subunit mitochondrial,

SDHB,

Succinate dehydrogenase complex subunit C,

SDHC,

And succinate dehydrogenase complex subunit D,

SDHD.

Other electron donors,

E.

G.

Fatty acids and glycerol-3-phosphate,

Also direct electrons into Q via FAD.

Complex II is a parallel electron transport pathway to Complex I.

But,

Unlike Complex I,

No protons are transported to the intermembrane space in this pathway.

Therefore,

The pathway through Complex II contributes less energy to the overall electron transport chain process.

In Complex III,

Cytochrome bc-sub-1 complex,

Or CO-QH-sub-2 cytochrome c reductase,

EC 1.

10.

2.

2,

The Q cycle contributes to the proton gradient by an asymmetric absorption release of protons.

Two electrons are removed from QH-sub-2 at the Q-sub-0 site and sequentially transferred to two molecules of cytochrome c,

A water-soluble electron carrier located within the intermembrane space.

The two other electrons sequentially pass across the protein to the Q-sub-I site,

Where the quinone part of ubiquinone is reduced to quinol.

A proton gradient is formed by one quinol,

Two H-plus and two E-minus oxidations at the Q-0 site to form one quinone,

Two H-plus and two E-minus at the Q-I site.

In total,

Four protons are translocated.

Two protons reduce quinone to quinol,

And two protons are released from two ubiquinol molecules.

When electron transfer is reduced by a high membrane potential or respiratory inhibitors,

Such as antamycin A,

Complex III may leak electrons to molecular oxygen,

Resulting in superoxide formation.

This complex is inhibited by dimercapryl naphthoquinone and antamycin.

In Complex IV,

Cytochrome c oxidase,

EC 1.

9.

3.

1,

Sometimes called cytochrome AA3,

Four electrons are removed from four molecules of cytochrome c,

And transferred to molecular oxygen,

O2,

And four protons,

Producing two molecules of water.

The complex contains coordinated copper ions and several heme groups.

At the same time,

Eight protons are removed from the mitochondrial matrix,

Although only four are translocated across the membrane.

Contributing to the proton gradient,

The exact details of proton pumping in Complex IV are still under study.

Cyanide is an inhibitor of Complex IV.

According to the chemiosmotic coupling hypothesis,

Proposed by Nobel Prize in chemistry winner Peter D.

Mitchell,

The electron transport chain and oxidative phosphorylation are coupled by a proton gradient across the inner mitochondrial membrane.

The efflux of protons from the mitochondrial matrix creates an electrochemical gradient,

Proton gradient.

This gradient is used by the F0F1 ATP synthase complex to make ATP via oxidative phosphorylation.

ATP synthase is sometimes described as Complex V of the electron transport chain.

The F0 component of ATP synthase acts as an ion channel.

It provides for a proton flux back into the mitochondrial matrix.

It is composed of A,

B,

And C subunits.

Protons in the intermembrane space of mitochondria first enter the ATP synthase complex through an A subunit channel.

Then,

Protons move to the C subunits.

The number of C subunits determines how many protons are required to make the F0 turn one full revolution.

For example,

In humans,

There are 8 C subunits.

Thus,

8 protons are required.

After C subunits,

Protons finally enter the matrix through an A subunit channel that opens into the mitochondrial matrix.

This reflux releases free energy produced during the generation of the oxidized forms of the electron carriers,

NAD+,

And Q,

With energy provided by O2.

The free energy is used to drive ATP synthesis,

Catalyzed by the F1 component of the complex.

Coupling with oxidative phosphorylation is a key step for ATP production.

However,

In specific cases,

Uncoupling the two processes may be biologically useful.

The uncoupling protein thermogenin,

Present in the inner mitochondrial membrane of brown adipose tissue,

Provides for an alternative flow of protons back to the inner mitochondrial matrix.

Thyroxine is also a natural uncoupler.

This alternative flow results in thermogenesis rather than ATP production.

Reverse electron flow is the transfer of electrons through the electron transport chain through the reverse redox reactions.

Usually requiring a significant amount of energy to be used,

This can reduce the oxidized forms of electron donors.

For example,

NAD+,

Can be reduced to NADH by Complex I.

There are several factors that have been shown to induce reverse electron flow.

However,

More work needs to be done to confirm this.

One example is blockage of ATP synthase,

Resulting in a buildup of protons and therefore a higher proton motive force,

Inducing reverse electron flow.

In eukaryotes,

NADH is the most important electron donor.

The associated electron transport chain is NADH passes electrons to Complex I,

Which transfers them to Q,

Ubiquinone,

Then to Complex III,

Cytochrome C,

Complex IV,

And ultimately to molecular oxygen,

Where Complexes I,

III,

And IV are proton pumps,

While Q and Cytochrome C are mobile electron carriers.

The electron acceptor for this process is molecular oxygen.

In prokaryotes,

Bacteria,

And archaea,

The situation is more complicated because there are several different electron donors and several different electron acceptors.

The generalized electron transport chain in bacteria is electrons from a donor enter the chain and reduce quinone.

From there,

Electrons pass to the BC complex,

Then to cytochrome,

And finally to an oxidase of reductase,

Which transfers them to the terminal electron acceptor.

Another donor can feed directly into a dihydrogenase,

Which also passes electrons to cytochrome,

Then to an oxidase or reductase,

And finally to an acceptor.

Electrons can enter the chain at three levels,

At the level of dehydrogenase,

At the level of quinone pool,

Or at the level of a mobile cytochrome electron acceptor.

These levels correspond to successively more positive redox potentials,

Or to successively decreased potential differences relative to the terminal electron acceptor.

In other words,

They correspond to successively smaller Gibbs free energy changes for the overall redox reaction.

Individual bacteria use multiple electron transport chains,

Often simultaneously.

Bacteria can use a number of different electron donors,

A number of different dehydrogenases,

A number of different oxidases and reductases,

And a number of different electron acceptors.

For example,

E.

Coli,

When growing aerobically,

Uses glucose and oxalate,

As an energy source,

Uses two different NADH dehydrogenases and two different quinole oxidases,

For a total of four different electron transport chains operating simultaneously.

A common feature of all electron transport chains is the presence of a proton pump to create an electrochemical gradient over a membrane.

Bacterial electron transport chains may contain as many as three proton pumps,

Like mitochondria,

Or they may contain two or at least one.

In the current biosphere,

The most common electron donors are organic molecules.

Organisms that use organic molecules as an electron source are called organotrophs.

Chemoorganotrophs,

Animals,

Fungi,

Protists,

And photolithotrophs,

Plants and algae,

Constitute the vast majority of all familiar life forms.

Some prokaryotes can use inorganic matter as an electron source.

Such an organism is called a chemolithotroph,

Rock eater.

Inorganic electron donors include hydrogen,

Carbon monoxide,

Ammonia,

Nitrite,

Sulfur,

Sulfide,

Manganese oxide,

And ferrous iron.

Lithotrophs have been found growing in rock formations thousands of meters below the surface of the earth.

Because of their volume of distribution,

Lithotrophs may actually outnumber organotrophs and phototrophs in our biosphere.

The use of inorganic electron donors,

Such as hydrogen as an energy source,

Is of particular interest in the study of evolution.

This type of metabolism must logically have preceded the use of organic molecules and oxygen as an energy source.

Bacteria can use several different electron donors.

When organic matter is the electron source,

The donor may be NADH or succinate,

In which case electrons enter the transport chain via NADH dehydrogenase,

Similar to complex I in mitochondria,

Or succinate dehydrogenase similar to complex II.

Other dehydrogenases may be used to process different energy sources,

Formate dehydrogenase,

Lactate dehydrogenase,

Glyceraldehyde 3-phosphate dehydrogenase,

H2 dehydrogenase,

Electron transport chain.

Some dehydrogenases are also proton pumps,

While others funnel electrons into the quinone pool.

Most dehydrogenases show induced expression in the bacterial cell in response to metabolic needs,

Triggered by the environment in which the cells grow.

In the case of lactate dehydrogenase in E.

Coli,

The enzyme is used aerobically and in combination with other dehydrogenases.

It is inducible and is expressed when the concentration of DL-lactate in the cell is high.

Quinones are mobile,

Lipid-soluble carriers that shuttle electrons and protons between large,

Relatively immobile macromolecular complexes embedded in the membrane.

Bacteria use ubiquinone,

Coenzyme Q,

The same quinone that mitochondria use,

And related quinones such as menaquinone,

Vitamin K2.

Archaea in the genus Sulfolobus use calderiella quinone.

The use of different quinones is due to slight changes in redox potentials,

Caused by changes in structure.

The change in redox potentials of these quinones may be suited to change in the electron acceptors or variations of redox potentials in bacteria complexes.

A proton pump is any process that creates a proton gradient across a membrane.

Protons can be physically moved across a membrane,

As seen in mitochondrial complexes I and IV.

The same effect can be produced by moving electrons in the opposite direction.

The result is the disappearance of a proton from the cytoplasm and the appearance of a proton in the periplasm.

Mitochondrial complex III is a second type of proton pump,

Which is mediated by a quinone,

The Q cycle.

Some dehydrogenases are proton pumps,

While others are not.

Most oxidases and reductases are proton pumps,

But some are not.

Cytochrome bc-sub-1 is a proton pump found in many,

But not all,

Bacteria,

Like not in E.

Coli.

As the name implies,

Bacterial bc-sub-1 is similar to mitochondrial bc-sub-1,

Complex III.

Cytochromes are proteins that contain iron.

They are found in two very different environments.

Some cytochromes are water-soluble carriers that shuttle electrons to and from large,

Immobile macromolecular structures embedded in the membrane.

The mobile cytochrome electron carrier in mitochondria is cytochrome c.

Bacteria use a number of different mobile cytochrome electron carriers.

Other cytochromes are found within macromolecules such as complex III and complex IV.

They also function as electron carriers,

But in a very different intermolecular solid-state environment.

Electrons may enter an electron transport chain at the level of a mobile cytochrome or guanone carrier.

For example,

Electrons from inorganic electron donors,

Nitrite,

Ferrous iron,

Electron transport chain,

Enter the electron transport chain at the cytochrome level.

When the electrons enter a redox level greater than NADH,

The electron transport chain must operate in reverse to produce this necessary higher energy molecule.

As there are a number of different electron donors,

Organic matter in organotrophs,

Inorganic matter in lithotrophs,

There are a number of different electron acceptors,

Both organic and inorganic.

As with other steps of the ETC,

An enzyme is required to help with the process.

It is most often used as the terminal electron acceptor in aerobic bacteria and facultative anaerobes,

And oxidase reduces the O2 to water while oxidizing something else.

In mitochondria,

The terminal membrane complex,

Complex IV,

Is cytochrome oxidase,

Which oxidizes the cytochrome.

Aerobic bacteria use a number of different terminal oxidases.

For example,

E.

Coli,

A facultative anaerobe,

Does not have a cytochrome oxidase or a BC-sub-1 complex.

Under aerobic conditions,

It uses two different terminal quinol oxidases,

Both proton pumps,

To reduce oxygen to water.

Reviews

This sent me off into peaceful slumber. I don’t think your voice is boring, though. I find it soothing, which helps me drift off to sleep. Thanks, Benjamin.