19.9
A cadeia de transporte de elétrons
A cadeia de transporte de elétrons, ou fosforilação oxidativa, é um processo exotérmico no qual a energia livre liberada durante as reações de transfe…
A cadeia de transporte de elétrons ou ETC é o estágio final da respiração celular, onde NADH e FADH2 iniciam uma série de reações redox.
No complexo I, o NADH doa dois elétrons em diferentes aceptores de elétrons, reduzindo Q para QH2.
No complexo II, o FADH2 transfere elétrons via Fe-S para uma molécula Q, formando outro QH2.
O QH2 gerado nessas reações então se difunde para o complexo III e transfere elétrons para o citocromo c por meio de uma série de reações chamadas de ciclo Q.
O citocromo c reduzido se move para o complexo IV, onde, após uma série de transferências de elétrons, o oxigênio aceita elétrons e se combina com prótons para produzir água.
À medida que os elétrons passam pelos complexos I, III e IV, a energia liberada é usada para bombear prótons para o espaço intermembranar.
Os prótons bombeados podem então fluir por seu gradiente de concentração e ativar a complexa V ou ATP sintase para produzir ATP a partir de ADP e fosfato inorgânico.
No geral, o ETC produz 32 moléculas de ATP a partir de uma molécula de glicose, tornando-se o principal estágio de contribuição de energia da respiração 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.