Electron Transport Chain Definition

The Electron Transport System also referred to as the Electron Transport Chain is a collection of chemical reactions which transform the redox energy created by the oxidation of NADH and FADH2 into proton-motive force. This process is then utilized to synthesize ATP through an enzyme reaction called oxidative phosphorylation, which necessitates conformational modifications in the ATP synthase complex.

• Cellular respiration ends with oxidative phosphorylation.
• The process of transferring electrons from organic molecules to oxygen at this step concurrently releases energy.
• Molecular oxygen serves as the ultimate electron acceptor in aerobic respiration, whereas alternative acceptors, such as sulfate, are present in anaerobic respiration.
• This series of events is crucial because it converts ATP into ADP and then back into ATP, making use of the body’s finite supply of ATPs roughly 300 times every day.
• The respiratory chain, also called the electron-transport chain, is a group of four protein complexes located in the inner mitochondrial membrane and carrying out electron flow.
• During this final step of cellular respiration, ATP generation is powered by the energy of oxidation. All oxidative stages in the breakdown of carbohydrates, lipids, and amino acids converge.

Electron Transport Chain Location

• The high-energy electrons are present in the mitochondria where the citric acid cycle is occurring. Mitochondria are the place where the electron transport chain happens in eukaryotes.
• The mitochondrion is a double-membraned organelle made up of an inner membrane that is folded into cristae, or ridges, and an exterior membrane.
• The matrix and the intermembrane space are the two compartments in the mitochondria.
• The outer membrane has high ion permeability. The outer membrane has the enzymes needed for citric acid cycles, but the inner membrane is impermeable to various ions and comprises uncharged molecules, an electron transport chain, and enzymes that synthesize ATP.
• The volume of electron transport chains in the mitochondria depends on the place and role of the cell. The liver mitochondria contain 10,000 sets of electron transport chains, but the heart mitochondria possess three times as many electron transport chains.
• Enzymes like adenylate kinase are found in the intermembrane space, whereas ATP, ADP, AMP, NAD, NADP, and different ions like Ca2+, Mg2+, etc. are found in the matrix.

Electron Transport Chain Components/ Electron carriers

• From the substrate to the oxygen, a variety of electron carriers transfer electrons along the chain.
• The electron chain is made up of around 15 distinct chemical groups that may accept or transport electrons.

1. FMN (Flavin Mononucleotide)

• The electrons from NADH are transported to the flavin mononucleotide (FMN), reducing it to FMNH2 at the start of the electron transfer chain.

NAD + H+ + FMN → NAD + FMNH2

• NADH dehydrogenase is a catalyst for the transport of electrons.
• A number of iron-sulfur complexes (Fe-S) with a greater relative affinity for the electrons receive the electrons before being transferred to them in a subsequent step.

1. Ubiquinone (Co-enzyme-Q)

• Other electron carriers known as ubiquinone are located between the flavoproteins and cytochromes (UQ).
• Ubiquinone serves as the sole electron transporter in the respiratory chain that is not connected to a protein. As a result, the molecule can move between the flavoproteins and cytochromes.
• Following the electrons being transferred from FMNH2 via the Fe-S centers to the ubiquinone, transforming into UQH2, the oxidized form of flavoprotein (FMN) is liberated.

FMNH2 + UQ → FMN + UQH2

1. Cytochromes

• The following electron transporters are cytochromes, which are heme-containing red or brown proteins which transfer the electrons from ubiquinone to molecular oxygen.
• Every cytochrome, such as Fe-S centers, only transmits one electron, in contrast to other electron carriers like FMN and ubiquinone which transport two electrons.
• Between ubiquinone and molecular oxygen, there are five different cytochrome types with the letters a, b, c, and so on.
• These are so named because they can absorb light of various wavelengths. The longest wavelength is absorbed by cytochrome A, followed by the next longest by cytochrome B, and so on.

Electron Transport Chain Equation

• A succession of oxidation-reduction events that release energy makes up the electron transport chain. The following is a list of all the reactions in the electron transport chain:

NADH + 1/2O2 + H+ + ADP + Pi → NAD+ + ATP + H2O

Electron Transport Chain Complexes

• The electron transport chain that facilitates the movement of electrons from different electron carriers to molecular oxygen consists of a chain of four enzyme complexes.

A. Complex I (Mitochondrial complex I)

• NADH dehydrogenases and the Fe-S centers that catalyze the transfer of two electrons from NADH to ubiquinone make up Complex I in the electron transport chain (UQ).
• A proton gradient is produced as the complex simultaneously translocates four H+ ions through the membrane.

NADH + H+ + CoQ  →  NAD+ + CoQH2

• By reducing FMN to FMNH2 in a two-step electron transfer, NADH is first oxidized to nAD+.
• The two electrons are then transported to Fe-S centers and then ubiquinone during the oxidation of FMNH2 to FMN.

B. Complex II (Mitochondrial complex II)

• Succinic dehydrogenase, FAD, and Fe-S centers make up Complex II.
• Through FAD and Fe-S centers, the enzyme complex catalyzes the transfer of electrons from other donors such as fatty acids and glycerol-3 phosphate to ubiquinone.
• This complex coexists with Complex II, but unlike Complex I, Complex II does not translocate H+ across the membrane.

Succinate + FADH2 + CoQ → Fumarate + FAD+ + CoQH2

C. Complex III (Mitochondrial complex III)

• Cytochromes b, c, and a particular Fe-S center make up Complex III.
• Two electrons from reduced CoQH2 are transferred to two molecules of cytochrome c via the enzyme complex known as cytochrome reductase.
• The discharge of protons (H+) from the ubiquinone throughout the membrane, in the meantime, aids the proton gradient.
• The iron core (Fe3+) in cytochrome c is reduced to Fe2+ while the CoQH2 is oxidized back to CoQ.

CoQH2 + 2 cytc c (Fe3+)  →  CoQ + 2 cytc c (Fe2+) + 4H+

D. Complex IV (Mitochondrial complex IV)

• Complex IV is made up of cytochrome a and a3, also referred to as cytochrome oxidase.
• This complex, which is the final link in the chain, is in charge of transferring two electrons from cytochrome c to molecular oxygen (O2), which then reacts to generate water.
• Four protons are moved across the membrane in the interim, assisting the proton gradient.

4 cytc c (Fe 2+) + O2   →  4cytc c (Fe3+) + H2O

Electron Transport Chain Steps

In electron transfer chains, which entail the transfer of electrons from NADH to molecular oxygen, the following stages are involved:

1. Transfer of electrons from NADH to Ubiquinone (UQ)

• The TCA cycle’s -ketoglutarate dehydrogenase, isocitrate dehydrogenase, and malate dehydrogenase reactions yield NADH. NADH is produced by a variety of oxidation mechanisms, including the pyruvate dehydrogenase reaction that converts pyruvate into acetyl-CoA, the -oxidation of fatty acids, and others.
• The intermembrane gap receives the NADH generated in the mitochondrial matrix.
• The complex I is used by the NADH to then transmit the electrons to FMN that exists in the intermembrane space (NADH dehydrogenase).
• The electrons are subsequently transferred from the FMN to the Fe-S center (one electron to one Fe-S center), which in turn transfers them one at a time to CoQ to create semiquinone and eventually ubiquinol.
• Two protons are pumped across the membrane by the energy produced by the electron transfer, forming a potential gradient.
• The protons return to the matrix via the pore of the ATP synthase complex and produce ATP, which is a form of energy.

2. Transfer of electrons from FADH2 to CoQ

• FAD is reduced to FADH2 as a result of the oxidation of succinate to fumarate.
• Succinic dehydrogenase, a component of complex II, catalyzes the electron transport chain when it accepts electrons from FADH2.
• The electrons go via a string of Fe-S centers to reach CoQ, similar to complex I.
• But protons are not pumped across the membrane by complex II.

3. Transfer of electrons from CoQH2 to cytochrome c

• The reduced CoQH2 transfers electrons to cytochrome c after passing through cytochrome b and c1.
• The reduction of the Fe3+ in the cytochrome to Fe2+ is catalyzed by Complex II (cytochrome reductase).
• Two cytochrome molecules are reduced for every NADH oxidation since each cytochrome transfers one electron.
• Protons must be pumped across the membrane to support the potential gradient. Energy is created during the transport of electrons.
• The protons revert to the matrix via the pore of the ATP synthase complex and produce ATP as they did in the initial phase.

4. Transfer of electrons from cytochrome c to molecular oxygen

• Complex IV (cytochrome oxidase), which catalyzes the last step in the electron transfer chain, is responsible for moving electrons from cytochrome c to molecular oxygen.
• Because it requires two electrons to convert one molecule of oxygen to water, with every NADH oxidation, half of the oxygen is reduced to water.
• Similar to this, cytochrome c’s Fe2+ is converted to Fe3+. The energy generated during this process is used to push protons across the membrane.
• ATP is created when protons are sent back to the matrix.

Electron Transport Chain Products

The end products of the electron transport chain are:

30-32 ATPs and 44 moles of H2O

Stage	Direct products (net)	Ultimate ATP yield (net)
Glycolysis	2 ATP	2 ATP
	2 NADH	3-5 ATP
Pyruvate oxidation	2 NADH	5 ATP
Citric acid cycle	2 ATP/GTP	2 ATP
	6 NADH	15 ATP
	2 FADH2	3 ATP
Total		30-32 ATP

Frequently Asked Questions (FAQs) (Module Revision questions and answers)

Where is the electron transport chain located?

The electron transport chain is located in the mitochondria of a cell.

What is the purpose of the electron transport chain?

The purpose of electron transfer chains is the production of ATPs.

What does the electron transport chain do?

The electron transport chain produces ATPs from previous cycles’ precursors (NADH and FADH).

What are the three main steps in the electron transport chain?

The three main steps of the electron transfer chain are:

1. Transfer of electrons from NADH and FADH2 to CoQ
2. Transfer of electrons from CoQ to cytochrome c
3. Transfer of electrons from cytochrome c to molecular oxygen.

Where are the proteins of the electron transport chain located?

The proteins of the electron transport chain are located in the inner mitochondrial membrane of the mitochondria.

What are the products of the electron transport chain?

The products of the electron transport chains are ATPs and water.

What is the final electron acceptor of the electron transport chain?

The final electron acceptor in aerobic respiration is molecular oxygen while in anaerobic respiration, it can be sulfate or other molecules.

How many ATPs are formed in the electron transport chain?

A total of 30-32 ATPs are formed in the electron transport chain. But it depends upon the ATP per glucose in cellular respiration. In some cases, we can see the production of 38 ATPs also.

How many ATPs are utilized in the electron transport chain?

No ATPs are utilized in the electron transport chain.

What is the main function of the electron transport chain?

The main function of the electron transport chain is the production of ATPs from NADH and FADH.

What is the role of oxygen in the electron transport chain?

Oxygen in the electron transport chain is the final electron acceptor.

How does the electron transport chain work in cellular respiration?

The electron transport chain is the final stage of cellular respiration where most of the ATPs or energy is produced from glucose.

References

• Jain JL, Jain S, and Jain N (2005). Fundamentals of Biochemistry. S. Chand and Company.
• Nelson DL and Cox MM. Lehninger Principles of Biochemistry. Fourth Edition.
• Berg JM et al. (2012) Biochemistry. Seventh Edition. W. H Freeman and Company.

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