19.9
La cadena de transporte de electrones
La cadena de transporte de electrones o fosforilación oxidativa es un proceso exotérmico en el que la energía libre liberada durante las reacciones de…
La cadena de transporte de electrones o ETC es la etapa final de la respiración celular, donde NADH y FADH2 comienzan una serie de reacciones redox.
En el complejo I, el NADH dona dos electrones a través de diferentes aceptores de electrones, reduciendo Q a QH2.
En el complejo II, FADH2 transfiere electrones a través de Fe-S a una molécula Q, formando otra QH2.
La QH2 generada en estas reacciones luego se difunde al complejo III y transfiere electrones al citocromo c a través de una serie de reacciones llamadas ciclo Q.
El citocromo c reducido pasa al complejo IV, donde después de una serie de transferencias de electrones, el oxígeno acepta electrones y se combina con protones para producir agua.
A medida que los electrones pasan a través de los complejos I, III y IV, la energía liberada se utiliza para bombear protones al espacio intermembrana.
Los protones bombeados pueden entonces fluir por su gradiente de concentración y activar el complejo V o ATP sintasa para producir ATP a partir de ADP y fosfato inorgánico.
En general, el ETC produce 32 moléculas de ATP a partir de una molécula de glucosa, lo que lo convierte en la principal etapa contribuyente de energía de la respiración celular.
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Q1: What happens to electrons as they move through the electron transport chain?
Electrons from NADH and FADH2 pass through a series of protein complexes, losing energy at each step. At complex I, NADH donates electrons that reduce ubiquinone to QH2. These electrons continue through electron transport chain complex III and IV, where oxygen finally accepts them and combines with protons to produce water.
Q2: How does the electron transport chain generate ATP?
As electrons move through complexes I, III, and IV, energy released pumps protons into the intermembrane space, creating a concentration gradient. Protons flow back through ATP synthase down this gradient, activating the enzyme to convert ADP and inorganic phosphate into ATP, producing approximately 32 ATP molecules per glucose molecule.
Q3: What role does ubiquinone play in the electron transport chain?
Ubiquinone, or Q, acts as a mobile electron carrier between complexes. At complex I, NADH reduces Q to QH2. At complex II, FADH2 also transfers electrons to Q. The QH2 then diffuses to complex III, where it participates in the Q cycle to transfer electrons to cytochrome c.
Q4: Why are electron transport chain inhibitors dangerous to cells?
Inhibitors like rotenone block electron transfer and cause reactive oxygen species accumulation, damaging mitochondrial DNA and cellular components. Carbon monoxide inhibits complex IV by competing for oxygen-binding sites, causing electron accumulation and superoxide radical generation. These effects disrupt ATP production and can lead to cell death.
Q5: How does rotenone interfere with complex I function?
Rotenone, a pesticide, blocks the Q-binding site at complex I, preventing electron transfer from the Fe-S cluster to ubiquinone. This inhibition halts the electron transport chain and increases reactive oxygen species production, which damages mitochondrial components and can ultimately cause cell death.
Q6: What is the Q cycle and where does it occur?
The Q cycle is a series of reactions at complex III where reduced cytochrome c receives electrons from QH2. This process involves electron transfer between cytochrome b and cytochrome c subunits. Antimycin A, an antibiotic, blocks this cycle by interfering with the ubiquinone binding site, halting electron transport.
Q7: How does oligomycin inhibit ATP production?
Oligomycin, an antibiotic, binds to and blocks the proton channel of ATP synthase, preventing protons from flowing through the enzyme. Without proton flow, the rotary motion needed for ATP synthesis cannot occur, stopping the conversion of ADP to ATP despite an active electron transport chain.