Electron Transport in the Energy Cycle of the Cell

The eukaryotic cell's most efficient path for production of vital ATP is the aerobic respiration that takes place in the mitochondria. After glycolysis, the pyruvate product is taken into the mitochondia and is further oxidized in the TCA cycle. This cycle deposits energy in the reduced coenzymes which transfer that energy through what is called the electron transport chain. 真核细胞产生重要ATP最有效的途径是线粒体进行的有氧呼吸。糖酵解后，丙酮酸产物被输送到线粒体并在TCA循环中进一步氧化。这个循环将能量储存在被还原的辅酶中，这些辅酶通过所谓的电子传递链将能量传递出去。

The energy given to the electrons of the reduced coenzyme NADH and to succinate by the TCA cycle is transferred in small steps in the inner membrane of the mitochondrion through a chain of five protein complexes. These small oxidation steps accomplish the conversion of ADP to the energy currency molecule ATP. This series of coupled reactions is often referred to as oxidative phosphorylation.

The energy used in the electron transport chain pumps protons across the inner mitochondrial membrane from the inner matrix to the intermembrane space, producing a strong hydrogen concentration gradient. This process was called chemiosmosis by its discover, Peter Mitchell. This difference in proton concentration produces both an electrical potential and a pH potential across the membranes. The protein complex ATP synthase then makes use of this membrane potential to accomplish the phosphorylation of ADP to ATP.

The Protein Complexes of the Electron Transport Chain

Many years of effort have been devoted to the study of the remarkable processes in the mitochondria. The electron transport chain is the final stage of aerobic respiration leading to the forming of ATP in the inner membrane of the mitochondrion. The emergent picture is that of coupled reactions through five protein structures associated with that inner membrane.

Complex I (NADH-coenzyme Q oxidoreductase). The reduced coenzyme NADH binds to Complex I and accomplishes the reduction of Coenzyme Q10. Electrons are transferred through Complex I using FMN (flavin mononucleotide) and a series of Fe-S clusters. The process accomplishes the pumping of four protons across the inner mitochondrial membrane to the intermembrane space. More detail 更多内容 复合体I（NADH-辅酶Q氧化还原酶）。还原态的辅酶NADH与复合体I结合，并完成辅酶Q10的还原。电子通过复合体I中的FMN（黄素单核苷酸）和一系列铁硫簇进行转移。这一过程使四个质子被泵送至线粒体内膜的内侧空间。

Complex II (Succinate-Q oxidoreductase). This complex forms a second entry point into the electron transport chain using the succinate product of the TCA cycle. More detail 更多内容 复合物II（琥珀酸-辅酶Q氧化还原酶）。该复合物通过三羧酸循环产生的琥珀酸，作为电子传递链的第二个入口点。

Complex III (Q-cytochrome c oxidoreductase). This complex accomplishes the oxidation of ubiquinol and the reduction of two molecules of cytochrome-c. Four hydrogens are pumped across the membrane to the intermembrane space. More detail 更多内容 复合物III（泛醌-细胞色素c氧化还原酶）。该复合物完成泛醌醇的氧化和两个细胞色素c分子的还原。四个氢离子被泵入膜间隙。

Complex IV (Cytochrome c oxidase). This final complex in the electron transport chain accomplishes the final transfer of the electrons to oxygen and pumps two protons across the membrane. This makes a total of 10 protons across the membrane for one NADH into the electron transfer chain. This protein complex makes use of the metal ions iron and copper in the operation of this electron transfer. More detail 更多内容 复杂IV（细胞色素c氧化酶）。这是电子传递链中的最后一步复合物，完成将电子最终传递给氧气，并将两个质子泵过膜。这使得每分子NADH进入电子传递链时，总共有10个质子穿过膜。该蛋白质复合物利用铁和铜这两种金属离子进行电子传递操作。

ATP Synthase. This complex makes use of the proton potential created by the action of the electron transport chain. It transports a proton down the gradient and uses the energy to complete the phosphorylation of ADP to ATP. The current model of its action is called the binding charge mechanism, and it appears that part of this large protein complex accomplishes a mechanical rotation in the process of phosphorylation and release of the ATP molecule. So part of its action is like a molecular motor. More on ATP synthase 关于ATP合成酶的更多内容 ATP合酶。这种复合物利用电子传递链作用所产生的质子势。它将质子从高浓度区域运输到低浓度区域，并利用这种能量将ADP磷酸化为ATP。目前对其作用机制的模型称为结合电荷机制，似乎在磷酸化和ATP分子释放的过程中，该大蛋白质复合物的一部分完成了机械旋转。因此，其部分作用类似于分子马达。