Respiratory chain electron transport

NADH and reduced substrate dehydrogenase-flavoproteins (FPH2) must be continually reoxidized for mitochondrial oxidations to proceed. This is achieved by the electron transport chain (respiratory chain) which is a series of redox carriers of graded redox potential in the inner mitochondrial membrane (Appendix 1) that catalyzes the net reactions ... 

Fig. 1. Overview of the electron transport chain (respiratory chain).

In contrast to common usage, the distinction between photosynthetic and respiratory Rieske proteins does not seem to make sense. The mitochondrial Rieske protein is closely related to that of photosynthetic purple bacteria, which represent the endosymbiotic ancestors of mitochondria (for a review, see also (99)). Moreover, during its evolution Rieske s protein appears to have existed prior to photosynthesis (100, 101), and the photosynthetic chain was probably built around a preexisting cytochrome be complex (99). The evolution of Rieske proteins from photosynthetic electron transport chains is therefore intricately intertwined with that of respiration, and a discussion of the photosynthetic representatives necessarily has to include excursions into nonphotosynthetic systems. 

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

The electron transport chain system responsible for the respiratory burst (named NADPH oxidase) is composed of several components. One is cytochrome 6558, located in the plasma membrane it is a heterodimer, containing two polypeptides of 91 kDa and... 

Polymorphonuclear leucocytes (PMNs) employ a system comprising myeloperoxidase, hydrogen peroxide, and a halide factor to kill microorganisms and tumour cells. This process is sometimes loosely called the respiratory burst , which refers to the sudden rise in oxygen consumption by the phagocytosing neutrophils that is independent of the mitochondrial electron transport chain. 

Flutolanil is an inhibitor of succinate dehydrogenase complex (Complex II), in the mitochondrial respiratory electron transport chain. ... 

Ubiquinones (coenzymes Q) Q9 and Qi0 are essential cofactors (electron carriers) in the mitochondrial electron transport chain. They play a key role shuttling electrons from NADH and succinate dehydrogenases to the cytochrome b-c1 complex in the inner mitochondrial membrane. Ubiquinones are lipid-soluble compounds containing a redox active quinoid ring and a tail of 50 (Qio) or 45 (Q9) carbon atoms (Figure 29.10). The predominant ubiquinone in humans is Qio while in rodents it is Q9. Ubiquinones are especially abundant in the mitochondrial respiratory chain where their concentration is about 100 times higher than that of other electron carriers. Ubihydroquinone Q10 is also found in LDL where it supposedly exhibits the antioxidant activity (see Chapter 23). 

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. 

All tissues except mature red blood cells are able to manufacture haem for use in the respiratory cytochrome proteins of the electron transport chain. However, the liver is an especially important site of haem synthesis because it (a) is a major organ of erythropoiesis in utero and (b) haem-containing cytochrome-P450 (CYP-450) enzymes play significant roles in hepatic detoxification of drugs, toxins and endogenous waste products (Section 6.4). 

As in the respiratory chain (see p. 140), the light reactions cause electrons to pass from one redox system to the next in an electron transport chain. However, the direction of transport is opposite to that found in the respiratory chain. In the respiratory chain, electrons flow from NADH+H to O2, with the production of water and energy. 

---