Biochemistry, Electron Transport Chain

The electron transport chain is a series of four protein complexes that couple redox reactions, creating an electrochemical gradient that leads to the creation of ATP in a complete system named oxidative phosphorylation. It occurs in mitochondria in both cellular respiration and photosynthesis. In the former, the electrons come from breaking down organic molecules, and energy is released. In the latter, the electrons enter the chain after being excited by light, and the energy released is used to build carbohydrates.

Aerobic cellular respiration is made up of three parts: glycolysis, the citric acid (Krebs) cycle, and oxidative phosphorylation. In glycolysis, glucose metabolizes into two molecules of pyruvate, with an output of ATP and nicotinamide adenine dinucleotide (NADH). Each pyruvate oxidizes into acetyl CoA and an additional molecule of NADH and carbon dioxide (CO2). The acetyl CoA is then used in the citric acid cycle, which is a chain of chemical reactions that produce CO2, NADH, flavin adenine dinucleotide (FADH2), and ATP. In the final step, the three NADH and one FADH2 amassed from the previous steps are used in oxidative phosphorylation, to make water and ATP.

Oxidative phosphorylation has two parts: the electron transport chain (ETC) and chemiosmosis. The ETC is a collection of proteins bound to the inner mitochondrial membrane and organic molecules, which electrons pass through in a series of redox reactions, and release energy. The energy released forms a proton gradient, which is used in chemiosmosis to make a large amount of ATP by the protein ATP-synthase.

Photosynthesis is a metabolic process that converts light energy into chemical energy to build sugars. In the light-dependent reactions, light energy and water are used to make ATP, NADPH, and oxygen (O2). The proton gradient used to make the ATP forms via an electron transport chain. In the light-independent reactions, sugar is made from the ATP and NADPH from the previous reactions.

In the electron transport chain (ETC), the electrons go through a chain of proteins that increases its reduction potential and causes a release in energy. Most of this energy is dissipated as heat or utilized to pump hydrogen ions (H+) from the mitochondrial matrix to the intermembrane space and create a proton gradient. This gradient increases the acidity in the intermembrane space and creates an electrical difference with a positive charge outside and a negative charge inside. The ETC proteins in a general order are complex I, complex II, coenzyme Q, complex III, cytochrome C, and complex IV.

• Complex I, also known as ubiquinone oxidoreductase, is made up of NADH dehydrogenase, flavin mononucleotide (FMN), and eight iron-sulfur (Fe-S) clusters. The NADH donated from glycolysis, and the citric acid cycle is oxidized here, transferring 2 electrons from NADH to FMN. Then they are transferred to the Fe-S clusters and finally from Fe-S to coenzyme Q. During this process, 4 hydrogen ions pass from the mitochondrial matrix to the intermembrane space, contributing to the electrochemical gradient. Complex I may also play an important role in causing apoptosis in programmed cell death.[1][2][3][4][1]
  • (NADH + H+) + CoQ + 4 H+(matrix) -> NAD+ + CoQH2 + 4 H+(intermembrane)

• Complex II, also known as succinate dehydrogenase, accepts electrons from succinate (an intermediate in the citric acid cycle) and acts as a second entry point to the ETC. When succinate oxidizes to fumarate, 2 electrons are accepted by FAD within complex II. FAD passes them to Fe-S clusters and then to coenzyme Q, similar to complex I. However; no protons are translocated across the membrane by complex II, therefore less ATP is produced with this pathway.[5][6]
  • Succinate + FAD -> Fumarate + 2 H+(matrix) + FADH2
  • FADH2 + CoQ -> FAD + CoQH2
    • Glycerol-3-Phosphate dehydrogenase and Acyl-CoA dehydrogenase also accept electrons from glycerol-3-P and fatty acyl-CoA, respectively. Inclusion of these protein complexes allows for the donation to the ETC by cytosolic NADH (glycerol-3-P acts as a shuttle to regenerate cytosolic NAD from NADH) and fatty acids undergoing beta-oxidation within the mitochondria (acyl-CoA is oxidized to enoyl-CoA in the first step, producing FADH2).[7][8]

• Coenzyme Q, also known as ubiquinone (CoQ), is made up of quinone and a hydrophobic tail. Its purpose is to function as an electron carrier and transfer electrons to complex III. Coenzyme Q undergoes reduction to semiquinone (partially reduced, radical form CoQH-) and ubiquinol (fully reduced CoQH2) through the Q cycle. This process receives further elaboration under Complex III.

• Complex III, also known as cytochrome c reductase, is made up of cytochrome b, Rieske subunits (containing two Fe-S clusters), and cytochrome c proteins. A cytochrome is a protein involved in electron transfer that contains a heme group. The heme groups alternate between ferrous (Fe2+) and ferric (Fe3+) states during the electron transfer. Because cytochrome c can only accept a single electron at a time, this process occurs in two steps (the Q cycle), in contrast to the single-step complex I and II pathways. Complex III also releases 4 protons into the intermembrane space at the end of a full Q cycle, contributing to the gradient. Cytochrome c then transfers the electrons one at a time to complex IV.[9][10][11]
  • Q Cycle:  
    • Step 1 in the Q cycle involves ubiquinol (CoQH2) and ubiquinone (CoQ) binding to two separate sites on complex III. CoQH2 transfers each electron to a different path. One electron goes to Fe-S and then cytochrome c, while the second electron is transferred to cytochrome b and then to CoQ bound at the other site. While this occurs, 2 H+ ions are released into the intermembrane space, contributing to the proton gradient. CoQH2 is now oxidized to ubiquinone and dissociates from the complex. The CoQ bound at the second site enters a transitional CoQH- radical state from accepting one of the electrons.
    • The second step of the cycle involves a repeat of the first: a new CoQH2 binds to the first site and transfers two electrons like before (and 2 more H+ ions released). Again, one electron passes to cytochrome c and one to cytochrome b, which this time works to reduce CoQH- to CoQH2 before it dissociates from complex III and can be recycled. In this way, one full cycle appears as follows:[12]
  • 2 CoQH2(site 1) + CoQ(site 2) + 2 Cyt c(ox) + 2 H+(matrix) -> 2 CoQ(site 1) + CoQH2(site 2) + 2 Cyt c(red) + 4 H+(intermembrane)
• Complex IV, also known as cytochrome c oxidase, oxidizes cytochrome c and transfers the electrons to oxygen, the final electron carrier in aerobic cellular respiration. The cytochrome proteins a and a3, in addition to heme and copper groups in complex IV transfer the donated electrons to the bound dioxygen species, converting it into molecules of water. The free energy from the electron transfer causes 4 protons to move into the intermembrane space contributing to the proton gradient. Oxygen reduces via the following reaction:[13][14]
  • 2 cytochrome c(red) + ½O2 + 4 H+(matrix) -> 2 cytochrome c(ox) + 1 H2O + 2 H+(intermembrane)

ATP synthase, also called complex V, uses the ETC generated proton gradient across the inner mitochondrial membrane to form ATP. ATP-synthase contains up of F0 and F1 subunits, which act as a rotational motor system. F0 is hydrophobic and embedded in the inner mitochondrial membrane. It contains a proton corridor that is protonated and deprotonated repeatedly as H+ ions flow down the gradient from intermembrane space to matrix. The alternating ionization of F0 causes rotation, which alters the orientation of the F1 subunits. F1 is hydrophilic and faces the mitochondrial matrix. Conformational changes in F1 subunits catalyze the formation of ATP from ADP and Pi. For every 4 H+ ions, 1 ATP is produced. ATP-synthase can also be forced to run in reverse, consuming ATP to produce a hydrogen gradient, as is seen in some bacteria.[15][16][17]

Nicotinamide adenine dinucleotide has two forms: NAD+ (oxidized) and NADH (reduced). It is a dinucleotide connected by phosphate groups. One nucleoside has an adenine base and the other nicotinamide. When involved in metabolic redox reactions, the mechanism is as shown in Reaction 1.

• Reaction 1: RH2 + NAD+ -> R + H+ + NADH

R is the reactant, for example, sugar.

NADH enters the ETC at complex I and produces a total of 10 H+ ions through the ETC (4 from complex I, 4 from complex III, and 2 from complex IV). ATP-synthase synthesizes 1 ATP for 4 H+ ions. Therefore, 1 NADH = 10 H+, and 10/4 H+ per ATP = 2.5 ATP per NADH (**some sources round up**). When NADH is oxidized, it breaks into NAD+, H+, and 2 e- as shown in Reaction 2.

• Reaction 2: NADH -> H+ +  NAD+ + 2 e-

Flavin adenine dinucleotide has 4 redox states, 3 of them being FAD (quinone, fully oxidized form), FADH- (semiquinone, partially oxidized), and FADH2 (hydroquinone, fully reduced). FAD is made up of an adenine nucleotide and a flavin mononucleotide (FMN), connected by phosphate groups. FMN is synthesized in part from vitamin B2 (riboflavin). FAD contains a highly stable aromatic ring, and FADH2 does not. When FADH2 oxidizes, it becomes aromatic and releases energy, as seen in Reaction 3. This state makes FAD a potent oxidizing agent, with an even more positive reduction potential than NAD. FADH2 enters the ETC at complex II and creates a total of 1.5 ATP (4 H+ from complex III, and 2 H+ from complex IV; 6/4 H+ per ATP = 1.5 ATP per FADH2 **some sources round up**).[18]

• Reaction 3: FADH2 -> FAD +  2 H+ + 2 e-

FAD also functions in several metabolic pathways outside of the ETC, including DNA repair (MTHF repair of UV damage), fatty acid beta-oxidation (acyl-CoA dehydrogenase), and synthesis of coenzymes (CoA, CoQ, heme).