Electron transport chain generation

When Mitchell first described his chemiosmotic hypothesis in 1961, little evidence existed to support it, and it was met with considerable skepticism by the scientific community. Eventually, however, considerable evidence accumulated to support this model. It is now clear that the electron transport chain generates a proton gradient, and careful measurements have shown that ATP is synthesized when a pH gradient is applied to mitochondria that cannot carry out electron transport. Even more relevant is a simple but crucial experiment reported in 1974 by Efraim Racker and Walther Stoeckenius, which provided specific confirmation of the Mitchell hypothesis. In this experiment, the bovine mitochondrial ATP synthasereconstituted in simple lipid vesicles with bac-teriorhodopsin, a light-driven proton pump from Halobaeterium halobium. As shown in Eigure 21.28, upon illumination, bacteriorhodopsin pumped protons... 

Note that, under aerobic conditions, the two NADH molecules that are synthesized are reoxidized via the electron transport chain generating ATP. Given the cytoplasmic location of these NADH molecules, each is reoxidized via the glycerol 3-phosphate shuttle (see Topic L2) and produces approximately two ATPs during oxidative phosphorylation or via the malate-aspartate shuttle (see Topic L2) and produces approximately three ATPs during oxidative phosphorylation. 

In the first step, FAD accepts hydrogens from a fatty acyl CoA. A double bond is produced between the a- and p-carbons, and an enoyl CoA is formed. The FADH2 that is produced interacts with the electron transport chain, generating ATP. 

Figure 18.34 Overview of oxidative phosphorylation. The electron-transport chain generates a proton gradient, which is used to synthesize ATP.

The light reactions of photosynthesis closely resemble the events tive phosphorylation, In both, the flow of high-energy electrons through an electron-transport chain generates a proton-motive force. This force A4T synthesis through the action of an ATP synthase. In photosynthesis the electrons are also used directly to reduce NADP to NAD PH. 

The oxidation of fatty acids to acetyl CoA in the p-oxidation spiral conserves energy as FAD(2H) and NADH. FAD(2H) and NADH are oxidized in the electron transport chain, generating ATP from oxidative phosphorylation. Acetyl CoA is oxidized in the TCA cycle or converted to ketone bodies. 

Figure 3.21 Hydrogen and electron carriers in the mitochondrial electron transport chain — generation of a transmembrane proton gradient.

Engelhardt s experiments in 1930 led to the notion that ATP is synthesized as the result of electron transport, and, by 1940, Severo Ochoa had carried out a measurement of the P/O ratio, the number of molecules of ATP generated per atom of oxygen consumed in the electron transport chain. Because two electrons are transferred down the chain per oxygen atom reduced, the P/O ratio also reflects the ratio of ATPs synthesized per pair of electrons consumed. After many tedious and careful measurements, scientists decided that the P/O ratio was 3 for NADH oxidation and 2 for succinate (that is, [FADHg]) oxidation. Electron flow and ATP synthesis are very tightly coupled in the sense that, in normal mitochondria, neither occurs without the other. 

The two processes are electron transport and oxidative phosphorylation. NADH is reoxidised by the process of electron transport using the electron transport chain and the energy released from this process is harnessed by oxidative phosphorylation to generate ATP. We noted earlier that the two processes are intimately linked or coupled. Normally one cannot proceed without the other. 

During dtric add production there is massive generation of NADH but little demand for ATP. Thus the situation could quickly arise where there is no further ADP available for oxidative phosphorylation within the cells. This means that the electron transport chain cannot operate and no further oxidation of NADH can occur. 

Fig. 10. Redox systems of the photosynthetic electron transport chain incorporated in the thylakoid membrane. Irradiation causes the generation of a proton gradient (after Trebst and Hauska135))...

The electron transport chain gets its substrates from the NADH and FADH2 supplied by the TCA cycle. Since the TCA cycle and electron transport are both mitochondrial, the NADH generated by the TCA cycle can feed directly into oxidative phosphorylation. NADH that is generated outside the mitochondria (for example, in aerobic glycolysis) is not transported directly into the mitochondria and oxidized—that would be too easy. 

Rotenone inhibits the transfer of electrons from NADH into the electron transport chain. The oxidation of substrates that generate NADH is, therefore, blocked. However, substrates that are oxidized to generate FADH2 (such as succinate or a-glycerol phosphate) can still be oxidized and still generate ATP. Because NADH oxidation is blocked, the NADH pool becomes more reduced in the presence of rotenone since there s nowhere to transfer the electrons. 

From the data of literature it is known that water-soluble derivatives of fullerenes are able to be localized in mitochondria and influence their state as well as enzyme system (Foley et al., 2002). Such intracellular localization of fullerenes C60 could explain biologic effects under irradiation, because generation of free oxygen radicals in the cells occurs during emission of electrons from electron-transport chain of mitochondria. 

In contrast to substrate level phosphorylation in glycolysis, mitochondrial oxidative phosphorylation is an efficient process in that it generates in excess of 30 ATP per mole of glucose. In essence, the movement of electrons along the respiratory chain or electron transport chain is coupled with phosphorylation of ADP. 

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