Respiratory chain diagram

Diagram of the functional complexes of the electron transport system within the respiratory chain. Fnad = NADH dehydrogenase flavoprotein Fs = succinate dehydrogenase flavoprotein Fefn.h.) = nonheme iron. 

Figure 11.2 Electron transport in the respiratory chain. The diagram details the flow of electrons from the Krebs cycle intermediates malate and succinate via the electron transport chain (complexes 1, II, III and IV) to oxygen. 

Figure 13.1 Oxidative phosphorylation. A cartoon representation of electron and proton transport via the respiratory chain which produces ATP by oxidative phosphorylation. A concise version of this diagram, which is more appropriate for examination purposes, is shown in Chapter 11, Fig. 11.2.

From the previous section it is evident that our knowledge about the respiratory chain is still quite incomplete. We know which prosthetic groups participate (cf. diagram in Section 4). It remains to be clarified, however, to what proteins they are bound and what role the metals and any new cofactors might play. The reason for this unsatisfactory state of knowledge is that the enzymes under consideration are bound very firmly to the mitochondrial structure (cf. Chapt. XIX-3). Only very recently have techniques been developed to subdivide the mitochondria in such a manner that most of their activity is retained. The subunits thus obtained have been called electron-transport particles (Green and co-workers). Some of the catalytic capabilities have been sacrificed (e.g. the enzymes of the citric acid cycle). But they are still able to oxidize NADHs or succinate with consumption of Oj and formation of ATP (see below). With the further destruction of these subunits, the capacity for oxidative phosphorylation disappears. 

It should be noted that this constitutes a more specific version of the large general diagram in Section 4. Ubiquinone and cytochrome c are represented as auxiliary substrates the complexes within the frames are the true enzymes of the respiratory chain. Their composition, particularly with regard to prosthetic groups, is not yet fully understood. It is even possible that additional complexes may be involved. Many current controversies will hopefully be decided experimentally in the near future. 

From ferredoxin the electron pair returns to chlorophyll over a chain of redox catalysts (see left half of the diagram. Fig. 41). One of these redox catalysts— perhaps the first one—is the system plastoquinone/plastohydroquinone with a redox potential of 0.00 volt furthermore, cytochrome f is interposed here. The transport of electrons from plastoquinone to chlorophyll a E = H-0.45 volt) is coupled to a phosphorylation step just as in the respiratory chain, one inorganic phosphate is taken up and stored as ATP. 

Whereas the respiratory chain requires a continuous supply of substrate and oxygen for its operation, the phosphorylation just discussed does not. In the latter, the electrons are driven around their circuit by the light reaction. In addition to this cyclic photophosphorylation there is also a noncyclic photophosphorylation (the right half of the diagram), which includes the production of the reduction equivalent and of oxygen. This, however, is part of the second photosynthetic reaction. 

In this chapter we intend to describe briefly the structure and the main biochemical functions of the individual cellular components and to point out some principles of regulation. Knowledge of the individual reaction sequences and cycles (respiratory chain, glycolysis, citrate cycle, etc.) is a prerequisite. When in doubt, refer to the fold-out chart at the back of the book. A schematic diagram of cellular organization is presented in Fig. 47. 

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