An electron transport chain is a group of proteins that transfer electrons through a membrane into the mitochondria to form a proton gradient that results in the creation of adenosine triphosphate (ATP). ATP is used by the cell as energy for the metabolic processes of cellular functions.
Electron Transport Chain
Electron transport systems, also called electron transport chains, are a series of reactions that convert the available redox energy from oxidation of NADH and FADH2 to a proton motive force that is used to synthesize. ATP occurs through a change in the ATP synthase complex through a process called ATP oxidative phosphorylation.
- Oxidative phosphorylation is the last step in cellular respiration.
- This step involves a range of oxygen from organic compounds to oxygen, while simultaneously releasing energy during the process.
- In aerobic respiration, the final electron acceptor is molecular oxygen while anaerobic respiration contains other acceptors such as sulfate.
- This chain of reactions is important because it involves the breakdown of ATP into ATP and its re-synthesis in the ATP process, leading to the use of limited ATP in the body approximately 300 times a day.
- Electrons flow into four large protein complexes that integrate into the inner mitochondrial membrane, called the respiratory chain or electron transport chain.
- This phase is important in the synthesis of energy because all the oxidative phases of a breakdown of carbohydrates, fats, and amino acids are converted to this last phase of cellular respiration, in which the synthesis of oxidative energy causes ATP.
The Electron Transport Chain Location
- As the citric acid cycle takes place in the mitochondria, high energy electrons are also present in the mitochondria. As a result, the electron transport chain in eukaryotes also takes place in the mitochondria.
- The mitochondrion is a double-membrane organelle that consists of an outer membrane and an inner membrane that is folded into a series of ridges called cristae.
- There are two compartments in the mitochondria; the matrix and the intermembrane space.
- The outer membrane is highly permeable to ions. It contains enzymes necessary for citric acid cycles while the inner membrane is impermeable to various ions and contains uncharged molecules, an electron transport chain, and enzymes synthesizing ATP.
- The number of electron transport chains in the mitochondria depends on the location and function of the cell. In liver mitochondria, there are 10,000 sets of electron transport chains, while cardiac mitochondria have three times more electron transport chains than in liver mitochondria.
- The intermembrane space contains enzymes like adenylate kinase, and the matrix contains ATP, ADP, AMP, NAD, NADP, and various ions like Ca2 +, Mg2 +, etc.
Electron Transport Chain Components/ Electron carriers
- The electrons in the chain are transferred from the substrate to oxygen via a series of electron carriers.
- There are around 15 different chemical groups that accept or transfer electrons through the electronic chain.
FMN (Flavin Mononucleotide)
At the start of the electron transfer chain, the electrons from NADH are transferred to the flavin mononucleotide (FMN) reducing it to FMNH2.
NAD + H + + FMN → NAD + FMNH2
The electron transfer is catalyzed by the action of NADH dehydrogenase.
The electrons are then transferred to a series of iron-sulfur complexes (Fe-S) which have a higher relative affinity towards the electrons.
- Between the flavoproteins and the cytochromes are other electron carriers called ubiquinone (UQ).
- Ubiquinone is the only electron in the respiratory chain that is not linked to a protein. This allows the molecule to move between flavoproteins and cytochromes.
- Once the electrons are transferred from FMNH2 via the Fe-S centers to ubiquinone, it becomes UQH2 and the oxidized form of flavoprotein (FMN) is released.
FMNH2 + UQ → FMN + UQH2
- The next electron carriers are cytochromes which are proteins of red or brown color containing a heme group that transports the electrons in a sequence going from ubiquinone to molecular oxygen.
- However, each cytochrome, like the Fe-S centers, transfers only one electron while other electron carriers like FMN and ubiquinone transfer two electrons.
- There are five types of cytochromes between ubiquinone and molecular oxygen, each designated by a, b, c, etc.
- These are named according to their ability to absorb light of different wavelengths (the cytochrome a absorbs the longest wavelength, b absorbs the next longest wavelength, etc.).
The Electron Transport Chain Equation
The electron transport chain consists of a series of oxidation-reduction reactions that lead to the release of energy. A summary of the reactions in the electron transport chain is:
NADH + 1/2O2 + H+ + ADP + Pi → NAD+ + ATP + H2O
Electron Transport Chain Complexes
A chain of four enzyme complexes is present in the electron transport chain which catalyzes the transfer of electrons through different electron transporters to molecular oxygen.
Complex I (Mitochondrial complex I)
- Complex I of the electron transport chain is formed by NADH dehydrogenases and Fe-S centers which catalyze the transfer of two electrons from NADH to ubiquinone (UQ).
- At the same time, the complex translocates four H + ions across the membrane, creating a gradient of protons.
- NADH + H + + CoQ → NAD + + CoQH2
- NADH is first oxidized to nAD + by reducing the FMN to FMNH2 in a two-step electron transfer.
- FMNH2 is then oxidized to FMN where the two electrons are first transferred to the Fe-S centers and then to ubiquinone.
The Complex II (Mitochondrial complex II)
Complex II consists of centers of succinic dehydrogenase, FAD, and Fe-S.
The enzyme complex catalyzes the transfer of electrons from other donors such as fatty acids and glycerol-3 phosphate to ubiquinone via the FAD and Fe-S centers.
This complex is parallel to Complex II, but Complex II does not translocate H + across the membrane, as in Complex I.
Succinate + FADH2 + CoQ → Fumarate + FAD+ + CoQH2
Complex III (Mitochondrial complex III)
- Complex III includes cytochrome b, c, and a specific Fe-S center.
- The enzyme complex, cytochrome reductase, catalyzes the transfer of two electrons from reduced CoQH2 to two molecules of cytochrome c.
- Meanwhile, the protons (H +) of ubiquinone are released through the membrane helping the proton gradient.
- CoQH2 has oxidized again to CoQ while the center of iron (Fe3 +) in cytochrome c is reduced to Fe2 +
CoQH2 + 2 cytc c (Fe3+) → CoQ + 2 cytc c (Fe2+) + 4H+
Electron Transport Chain Steps
The following steps are involved in the electron transfer chains that involve the movement of electrons from NADH to molecular oxygen:
Transfer of electrons from NADH to Ubiquinone (UQ)
- NADH is produced in various other cycles by the α-ketoglutarate dehydrogenase, isocitrate dehydrogenase and malate dehydrogenase reactions of the TCA cycle, by the pyruvate dehydrogenase reaction which converts pyruvate to acetyl-CoA, by β-oxidation of fatty acids, and by d ‘other oxidation reactions.
- The NADH produced in the mitochondrial matrix is transferred into the intermembrane space.
- The NADH then transfers the electrons to the FMN present in the intermembrane space via the complex I (NADH dehydrogenase).
- The FMN then passes the electrons to the Fe-S center (an electron at a Fe-S center) which then transfers the electrons, one at a time to CoQ, forming semiquinone and then ubiquinol.
- The electron transfer creates energy which is used to pump two protons through the membrane creating a potential gradient.
- Protons return to the matrix through the pores of the ATP synthase complex, forming energy in the form of ATP.
The Transfers of electrons from FADH2 to CoQ
- Oxidation of succinate to fumarate results in the reduction of FAD to FADH2.
- The electrons of FADH2 enter the electron transport chain catalyzed by complex II, succinic dehydrogenase.
- As in complex I, the electrons reach CoQ through a series of Fe-S centers.
- However, complex II does not pump protons through the membrane.
The Transfers of electrons from CoQH2 to cytochrome c
- Reduced CoQH2 transfers electrons through cytochrome b and c1 which ultimately reaches cytochrome c.
- Complex II (cytochrome reductase) catalyzes this process where the Fe3 + present in the cytochrome is reduced to Fe2 +.
- Each cytochrome transfers one electron each and thus two cytochrome molecules are reduced for the transfer of electrons for each oxidized NADH.
- Energy is produced during the transfer of electrons which is used to pump the protons through the membrane helping the potential gradient.
- The protons return to the matrix through the pores of the ATP synthase complex, forming energy in the form of ATP as in the first step.
Transfer of electrons from cytochrome c to molecular oxygen
- The last step in the electron transfer chain is catalyzed by the IV complex (cytochrome oxidase) where electrons are transferred from cytochrome c to molecular oxygen.
- Since two electrons are needed to reduce an oxygen molecule to water, for each NADH, half of the oxidized oxygen is reduced to water.
- Likewise, the Fe2 + of cytochrome c is oxidized to Fe3 +. The energy released during this process is used to pump protons through the membrane.
- The transfer of protons to the matrix leads to the formation of ATP.
Electron Transport Chain (ETC), Steps, and, Diagram