Respiratory chain-linked phosphorylation

Only 1 ATP is gained from the citrate cycle, namely by the oxidative decarboxylation of ketoglutarate. But almost all the carbon dioxide is produced in the cycle. Most of the energy expected from the decomposition of foodstuffs is produced in the respiratory chain. The reduced coenzyme (NADH2) is oxidized by the enzymes of the respiratory chain and at the end of the chain by atmospheric oxygen. The formation of water thus occurs in a stepwise fashion which at three places is coupled with the phosphorylation of ADP. Respiratory-chain-linked phosphorylation (or oxidative phosphorylation) is the most important source of chemical free energy. 

The reduced coenzymes are oxidized by the respiratory chain linked to formation of ATP. Thus, the cycle is the major route for the generation of ATP and is located in the matrix of mitochondria adjacent to the enzymes of the respiratory chain and oxidative phosphorylation. 

A slowly progressive congenital neuromuscular disorder was reported in which the respiratory chain-linked energy transfer at a level common to all three energy coupling sites of respiratory chain was defective.52 Uncouplers of mitochondrial oxidative phosphorylation (2,4-dinitrophenol and carbonylcyanide-m-chlorophenylhydrazone) (5) produced mitochondrial myopathy in rats.53... 

Thus, in one cycle, eight hydrogen atoms (H+ + e ) are transferred to hydrogen-transmitting coenzymes and later oxidized to water in the respiratory chain. This process is linked to oxidative phosphorylation, i.e., the synthesis of ATP from ADP and inorganic phosphate. 

This hypothesis presumes that early free-living prokaryotes had the enzymatic machinery for oxidative phosphorylation and predicts that their modern prokaryotic descendants must have respiratory chains closely similar to those of modern eukaryotes. They do. Aerobic bacteria carry out NAD-linked electron transfer from substrates to 02, coupled to the phosphorylation of cytosolic ADP. The dehydrogenases are located in the bacterial cytosol and the respiratory chain in the plasma membrane. The electron carriers are similar to some mitochondrial electron carriers (Fig. 19-33). They translocate protons outward across the plasma membrane as electrons are transferred to 02. Bacteria such as Escherichia coli have F0Fi complexes in their plasma membranes the F portion protrudes into the cytosol and catalyzes ATP synthesis from ADP and P, as protons flow back into the cell through the proton channel of F0. 

Oxidative phosphorylation is ATP synthesis linked to the oxidation of NADH and FADH2 by electron transport through the respiratory chain. This occurs via a mechanism originally proposed as the chemiosmotic hypothesis. Energy liberated by electron transport is used to pump H+ ions out of the mitochondrion to create an electrochemical proton (H+) gradient. The protons flow back into the mitochondrion through the ATP synthase located in the inner mitochondrial membrane, and this drives ATP synthesis. Approximately three ATP molecules are synthesized per NADH oxidized and approximately two ATPs are synthesized per FADH2 oxidized. 

The role of L-lactate dehydrogenase in the physiology of aerobic yeast is not clear. It has been shown that its presence in yeast depends on the availability of oxygen (306), and that in the presence of antimycin A, which inhibits electron transfer to cytochrome c from NADH-linked substrates, L-or D-lactate can partially support the growth of Saccharo-myces cerevisiae (307). Under these conditions, cyanide inhibited the growth. Therefore, it has been concluded that l- and D-lactate-cyto-chrome c reductases can feed electrons to the respiratory chain at the level of cytochrome c and provide energy through the third site of oxidative phosphorylation (307). 

Describe the chemiosmotic model of oxidative phosphorylation and relate experimental evidence that only the proton-motive force links the respiratory chain and ATP synthesis. 

Oxidative phosphorylation is an aerobic process. The production of ATP is linked to the transport of electrons to an oxygen molecule by the cytochromic respiratory chain. This oxygen molecule is the final acceptor of the electrons. These reactions occur in the mitochondria. 

Another important function of the kinases is the synthesis of ATP from ADP and energy-rich bound phosphate in substrate-linked phosphorylation (cf. Chapt. XV-7) and in respiratory-chain phosphorylation (Chapt. X-6). ATP hence represents a pool for energy-rich phosphates—and, in a sense, for chemical energy in general. 

The preceding paragraphs undoubtedly have revealed the complicated and diverse nature of carbohydrate metabolism, both on the level of interconversions among the carbohydrates and on that of degradative reactions for the production of energy. Part of the energy is derived anaerobically by substrate-linked phosphorylation the major part, however, is liberated in the respiratory chain. The situation is further complicated by the obvious fact that carbohydrate metabolism is not an isolated system of reactions, but is closely tied to other pathways and reaction cycles through common intermediates. A separate chapter (Chapt. XVIII) is devoted to such interrelationships. 

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