3.2 Electron transport and oxidative phosphorylation

Electron transport and oxidative phosphorylation are key processes in cellular energy production. They involve the transfer of electrons through protein complexes in the mitochondrial membrane, creating a proton gradient that drives ATP synthesis.

These processes are the final stages of cellular respiration, following glycolysis and the citric acid cycle. They're crucial for generating the majority of ATP in eukaryotic cells, especially in high-energy-demanding tissues like the brain and heart.

Electron transport chain components

Structure and organization of the ETC

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• The electron transport chain (ETC) is a series of protein complexes embedded in the inner mitochondrial membrane that facilitate the transfer of electrons from reducing equivalents (NADH and FADH2) to molecular oxygen
• The main components of the ETC are Complex I (NADH dehydrogenase), Complex II (succinate dehydrogenase), Complex III (cytochrome bc1 complex), and Complex IV (cytochrome c oxidase)
• Electron carriers, such as ubiquinone (coenzyme Q10) and cytochrome c, shuttle electrons between the complexes
• The flow of electrons through the ETC is coupled with the pumping of protons (H+) from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient

Function and importance of the ETC

• The ETC plays a crucial role in cellular respiration by harnessing the energy released from the oxidation of reducing equivalents (NADH and FADH2) to generate an electrochemical proton gradient
• The proton gradient generated by the ETC is utilized by ATP synthase to drive the synthesis of ATP through oxidative phosphorylation
• Disruptions in the ETC can lead to mitochondrial disorders characterized by defects in energy production and a wide range of clinical manifestations (Leigh syndrome, MELAS syndrome)
• The efficiency of the ETC can be affected by factors such as the availability of substrates, the integrity of the mitochondrial membrane, and the presence of uncoupling agents that dissipate the proton gradient without ATP synthesis (2,4-dinitrophenol)

Oxidative phosphorylation and ATP synthesis

Coupling of electron transport and ATP synthesis

• Oxidative phosphorylation is the process by which the energy released from the transfer of electrons through the ETC is used to generate ATP
• The electrochemical proton gradient generated by the ETC drives the synthesis of ATP through the enzyme ATP synthase (Complex V)
• As protons flow down their concentration gradient through ATP synthase, the enzyme undergoes conformational changes that catalyze the phosphorylation of ADP to form ATP
• The coupling of electron transport and ATP synthesis is known as chemiosmotic coupling, as it involves the movement of ions (protons) across a membrane

Efficiency and regulation of oxidative phosphorylation

• Oxidative phosphorylation is a highly efficient process, producing approximately 26-32 molecules of ATP per glucose molecule oxidized, compared to only 2 ATP molecules generated during glycolysis
• The efficiency of oxidative phosphorylation can be affected by factors such as the availability of substrates, the integrity of the mitochondrial membrane, and the presence of uncoupling agents (thermogenin in brown adipose tissue)
• The regulation of oxidative phosphorylation involves the control of electron flow through the ETC and the modulation of ATP synthase activity
• Mitochondrial disorders that disrupt oxidative phosphorylation can lead to defects in energy production and a wide range of clinical manifestations (Leigh syndrome, MELAS syndrome)

Proton gradients for ATP synthesis

Generation and maintenance of the proton gradient

• The proton gradient, also known as the proton motive force (PMF), consists of two components: a pH gradient (ΔpH) and an electrical potential difference (Δψ) across the inner mitochondrial membrane
• The proton gradient is generated by the pumping of protons from the mitochondrial matrix into the intermembrane space by the ETC complexes (Complex I, III, and IV)
• The maintenance of the proton gradient is essential for the continuous synthesis of ATP through oxidative phosphorylation
• Factors that disrupt the proton gradient, such as uncoupling agents or mitochondrial membrane damage, can impair ATP synthesis and lead to cellular energy deficits

Harnessing the proton gradient for ATP synthesis

• The energy stored in the proton gradient is harnessed by ATP synthase to drive the synthesis of ATP from ADP and inorganic phosphate (Pi)
• Protons flow down their electrochemical gradient through the F0 subunit of ATP synthase, causing the rotation of the c-ring
• The rotation of the c-ring is coupled to conformational changes in the F1 subunit of ATP synthase, which catalyzes the formation of ATP
• The number of protons required for the synthesis of one ATP molecule varies among species, typically ranging from 2.7 to 3.3 protons per ATP

Cellular respiration: Electron transport and oxidative phosphorylation

Integration with other stages of cellular respiration

• Electron transport and oxidative phosphorylation represent the final stages of cellular respiration, following glycolysis and the citric acid cycle
• The reducing equivalents (NADH and FADH2) generated during glycolysis and the citric acid cycle are oxidized by the ETC, providing the energy for ATP synthesis through oxidative phosphorylation
• The ATP generated through oxidative phosphorylation is essential for various cellular processes, including biosynthesis, transport, and mechanical work
• Disruptions in electron transport or oxidative phosphorylation can have far-reaching effects on cellular metabolism and energy homeostasis

Importance of oxidative phosphorylation in cellular energy production

• Oxidative phosphorylation is the primary source of ATP in most eukaryotic cells, accounting for the majority of cellular energy production
• The high efficiency of oxidative phosphorylation allows cells to generate large amounts of ATP from the complete oxidation of glucose and other energy-rich molecules (fatty acids, amino acids)
• Tissues with high energy demands, such as the brain, heart, and skeletal muscle, rely heavily on oxidative phosphorylation for their energy needs
• Impairments in oxidative phosphorylation can lead to cellular energy deficits, mitochondrial disorders, and a wide range of pathological conditions (neurodegenerative diseases, cardiovascular disorders)