Electron transfer, coupled with oxidative phosphorylation

Coupling of electron transfer with oxidative phosphorylation... 

COUPLING OF ELECTRON TRANSFER WITH OXIDATIVE PHOSPHORYLATION... 

The spatial separation between the components of the electron transport chain and the site of ATP synthesis was incompatible with simple interpretations of the chemical coupling hypothesis. In 1964, Paul Boyer suggested that conformational changes in components in the electron transport system consequent to electron transfer might be coupled to ATP formation, the conformational coupling hypothesis. No evidence for direct association has been forthcoming but conformational changes in the subunits of the FI particle are now included in the current mechanism for oxidative phosphorylation. 

How is a concentration gradient of protons transformed into ATP We have seen that electron transfer releases, and the proton-motive force conserves, more than enough free energy (about 200 lcJ) per mole of electron pairs to drive the formation of a mole of ATP, which requires about 50 kJ (see Box 13-1). Mitochondrial oxidative phosphorylation therefore poses no thermodynamic problem. But what is the chemical mechanism that couples proton flux with phosphorylation ... 

All of these reactions release energy. In biological oxidations much of the energy is utilized to form ATP from ADP and inorganic phosphate (Section 15-5F). That is to say, electron-transfer reactions are coupled with ATP formation. The overall process is called oxidative phosphorylation. 

The oxygen formed clearly comes from H20 and not from C02, because photosynthesis in the presence of water labeled with lgO produces oxygen labeled with 180, whereas carbon dioxide labeled with 180 does not give oxygen labeled with 180. Notice that the oxidation of the water produces two electrons, and that the formation of NADPH from NADP requires two electrons. These reactions occur at different locations within the chloroplasts and in the process of transferring electrons from the water oxidation site to the NADP reduction site, adenosine diphosphate (ADP) is converted to adenosine triphosphate (ATP see Section 15-5F for discussion of the importance of such phosphorylations). Thus electron transport between the two photoprocesses is coupled to phosphorylation. This process is called photophosphorylation (Figure 20-7). 

In true fermentation, the free energy drop between substrate (say glucose) and anaerobic end products is always modest by comparison with respiration, because fermentation is never based on electron transfer chains coupled to phosphorylation. Rather, true fermentations depend upon a variety of oxidation-reduction reactions involving organic compounds, C02, molecular hydrogen, or sulfur compounds. All these reactions are inefficient in terms of energy yield (moles ATP per mole substrate fermented), and, therefore, the mass of cells obtainable per mole of substrate is much smaller than with respiratory-dependent species. 

The employment of cell organelles has been limited to mitochondrial and microsomal fractions (Table 15). As early as 1976, Guilbault et al. coupled pig heart mitochondrial electron transfer particles (ETP) with an oxygen electrode. This development has been further pursued by Aizawa et al. (1980c). Decoupling of the electron transfer from oxidative phosphorylation makes ETP catalyze the following reactions ... 

These conjectures were soon supported by experimental evidence obtained with mitochondrial preparations, first by Chance and Hollunger and later by many others, " who demonstrated that reversed electron transfer could involve not only flavin-coupled systems but also NAD-, NADP, and cytochrome-linked reactions. It was subsequently realized that ATP itself might not be directly involved in the energy transfer, and the notion of a high-energy intermediate of oxidative phosphorylation was invoked... 

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