Electron transport chain coupling

How does ATP production in chloroplasts resemble the process in mitochondria The two photosystems are linked by an electron transport chain coupled to the production of ATP. A proton gradient drives the production of ATP in photosynthesis, as it does in mitochondrial respiration. 

Glycolysis and the citric acid cycle (to be discussed in Chapter 20) are coupled via phosphofructokinase, because citrate, an intermediate in the citric acid cycle, is an allosteric inhibitor of phosphofructokinase. When the citric acid cycle reaches saturation, glycolysis (which feeds the citric acid cycle under aerobic conditions) slows down. The citric acid cycle directs electrons into the electron transport chain (for the purpose of ATP synthesis in oxidative phosphorylation) and also provides precursor molecules for biosynthetic pathways. Inhibition of glycolysis by citrate ensures that glucose will not be committed to these activities if the citric acid cycle is already saturated. 

This is a crucial point because (as we will see) proton transport is coupled with ATP synthesis. Oxidation of one FADHg in the electron transport chain results in synthesis of approximately two molecules of ATP, compared with the approximately three ATPs produced by the oxidation of one NADH. Other enzymes can also supply electrons to UQ, including mitochondrial 5w-glyc-erophosphate dehydrogenase, an inner membrane-bound shuttle enzyme, and the fatty acyl-CoA dehydrogenases, three soluble matrix enzymes involved in fatty acid oxidation (Figure 21.7 also see Chapter 24). The path of electrons from succinate to UQ is shown in Figure 21.8. 

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 energy released in catabolic pathways is used in the electron-transport chain to make molecules of adenosine triphosphate, ATP. ATP, the final result of food catabolism, couples to and drives many otherwise unfavorable reactions. 

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. 

The reason for this is that reoxidation of NADH via the alternative electron transport chain (not coupled to oxidative phosphorylation) liberates heat. 

By the mid-1950s, therefore, it had become clear that oxidation in the tricarboxylic acid cycle yielded ATP. The steps had also been identified in the electron transport chain where this apparently took place. Most biochemists expected oxidative phosphorylation would occur analogously to substrate level phosphorylation, a view that was tenaciously and acrimoniously defended. Most hypotheses entailed the formation of some high-energy intermediate X Y which, in the presence of ADP and P( would release X and Y and yield ATP. A formulation of the chemical coupling hypothesis was introduced by Slater in 1953,... 

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. 

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. 

Reaction 15.12 would be catalyzed by the electron-transport chain, with coupled phosphorylation, and all the oxygen in the sulfite product would be derived from water (in contrast to the oxygenase, in which two thirds of the oxygen atoms in sulfite come from dioxygen). Overall, Equations 15.11 and 15.12 produce the same result as Equation 15.10. 

Figure 1 7.3 The electron transport chain of mitochondria and the coupling of electron transfer reactions to the creation of a proton concentration gradient across the inner mitochondrial membrane. This proton concentration gradient is ultimately employed to drive the synthesis of ATP by ATP synthase, noted here as complex V. (Reproduced from D. Voet and J. G Voet, Biochemistry, 3rd edn, 2004 2004, Donald and Judith G. Voet. Reprinted with permission of John Wiley and Sons, Inc.)...

To begin with, let us return to the aerobic catabolism of simple sugars such as glucose to yield two molecules of pyruvate -I- two molecules of ATP - - two molecules of NADH. We noted just above that coupling the oxidation of the two molecules of NADH to the electron transport chain yields an additional six molecules of ATP, three for each molecule of NADH, for a total of eight. Now let s ask what happens when we further metabolize the two molecules of pyruvate via the pyruvate dehydrogenase complex and the citric acid cycle. 

First, the conversion of pyruvate to acetyl-SCoA produces one molecule of NADH. Coupling the oxidation of the NADH to the electron transport chain will generate... 

Electron transport chain a series of multienzyme complexes organized in the inner layer of the mitochondrial membrane that catalyze the transport of electrons from reduced coenzymes to molecular oxygen coupled to the synthesis of ATP. 

Most compounds oxidized by the electron transport chain donate hydrogen to NAD+, and then NADH is reoxidized in a reaction coupled to reduction of a flavoprotein. During this transformation, sufficient energy is released to enable synthesis of ATP from ADP. The reduced flavoprotein is reoxidized via reduction of coenzyme Q subsequent redox reactions then involve cytochromes and electron transfer processes rather than hydrogen transfer. In two of these cytochrome redox reactions, there is sufficient energy release to allow ATP synthesis. In... 

Oxidizible substrates from glycolysis, fatty acid or protein catabolism enter the mitochondrion in the form of acetyl-CoA, or as other intermediaries of the Krebs cycle, which resides within the mitochondrial matrix. Reducing equivalents in the form of NADH and FADH pass electrons to complex I (NADH-ubiquinone oxidore-ductase) or complex II (succinate dehydrogenase) of the electron transport chain, respectively. Electrons pass from complex I and II to complex III (ubiquinol-cyto-chrome c oxidoreductase) and then to complex IV (cytochrome c oxidase) which accumulates four electrons and then tetravalently reduces O2 to water. Protons are pumped into the inner membrane space at complexes I, II and IV and then diffuse down their concentration gradient through complex V (FoFi-ATPase), where their potential energy is captured in the form of ATP. In this way, ATP formation is coupled to electron transport and the formation of water, a process termed oxidative phosphorylation (OXPHOS). 

Blocking electron transfer by any one of these inhibitors stops electron flow from substrate to oxygen because the reactions of the electron transport chain are tightly coupled like meshed gears. 

Site-specific inhibitors Site-specific inhibitors of electron transport have been identified and are illustrated in Figure 6.10. These compounds prevent the passage of electrons by binding to a component of the chain, blocking the oxidation/reduction reaction. Therefore, all electron carriers before the block are fully reduced, whereas those located after the block are oxidized. [Note Because electron transport and oxidative phosphorylation are tightly coupled, site-specific inhibition of the electron transport chain also inhibits ATP synthesis.]... 

Electron transport chain shown coupled to the transport of protons. [Note Complex II is not shown.]... 

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