Mitochondria reactions

Fig. 6. SCPj translocates cholesterol from the outer to the inner membrane in adrenal mitochondria. Reaction 1, transmembrane translocation of cholesterol reaction 2, interaction of cholesterol with cytochrome reaction 3, removal of pregnenolone. C, cholesterol AG, aminoglutethimide.

Fig. 39.10. Synthesis and degradation of proline. Reactions 1, 3, and 4 occur in mitochondria. Reaction 2 occurs in the cytosol. Synthesis and degradation involve different enzymes. The cyclization reaction (formation of a Schiff base) is nonenzymatic, i.e., spontaneous.

The biosynthesis of CoA in animals requires five enzymes. The first three are foimd only in the cytosol, and the other two are also found in mitochondria. Reaction IV is reversible. Four moles of ATP are required for the biosynthesis of one mole CoA from one mole of pantothenic acid. Two additional enzymes... 

The link with the final electron acceptor, O2, is the enzyme cytochrome c oxidase which spans the inner membrane of the mitochondrion. It consists of cytochromes a and a3 along with two, or possibly three, Cu atoms. The details of its action are not fully established but the overall reaction catalysed by the enzyme is ... 

Pymvate dehydrogenase is a mitochondrial enzyme, and fatty acid synthesis is a cytosohc pathway, but the mitochondrial membrane is impermeable to acetyl-CoA. Acetyl-CoA is made available in the cytosol from citrate synthesized in the mitochondrion, transported into the cytosol and cleaved in a reaction catalyzed by ATP-citrate lyase. 

ALASl. This repression-derepression mechanism is depicted diagrammatically in Figure 32-9. Thus, the rate of synthesis of ALASl increases greatly in the absence of heme and is diminished in its ptesence. The turnover rate of ALASl in rat liver is normally rapid (half-life about 1 hour), a common feature of an enzyme catalyzing a rate-limiting reaction. Heme also affects translation of the enzyme and its transfer from the cytosol to the mitochondrion. 

Poly(3HB) is synthesized in bacteria from acetyl-CoA by a three-step reaction (Fig. 1). The first enzyme of the pathway, 3-ketothiolase, catalyzes the condensation of two molecules of acetyl-CoA to form acetoacetyl-CoA. Aceto-acetyl-CoA reductase subsequently reduces acetoacetyl-CoA to R-3-hydroxy-butyryl-CoA, which is then polymerized by the PHA synthase to produce poly(3HB). Since acetyl-CoA is present in plant cells in the cytosol, plastid, mitochondrion, and peroxisome, the synthesis of poly(3HB) in plants could, in... 

We directed our initial attention to the mitochondrion-specific phospholipid, cardiolipin because it is particularly rich in polyunsaturated fatty acids which are vulnerable to oxidative attack. It seemed reasonable to speculate that mitochondrial cardiolipin may degrade by the enhanced peroxidation reactions during apoptotic cell death,... 

Under aerobic conditions, the hydrogen atoms of NtUDH are oxidised within the mitochondrion pyruvate is also oxidised in the mitochondrion (Figure 9.15). However, NADH cannot be transported across the inner mitochondrial membrane, and neither can the hydrogen atoms themselves. This problem is overcome by means of a substrate shuttle. In principle, this involves a reaction between NADH and an oxidised substrate to produce a reduced product in the cytosol, followed by transport of the reduced product into the mitochondrion, where it is oxidised to produce hydrogen atoms or electrons, for entry into the electron transfer chain. Finally, the oxidised compound is transported back into the cytosol. The principle of the shuttle is shown in Figure 9.16. 

Figure 9.16 The principle of the transfer shuttle of hydrogen atoms into the mitochondrion. A dehydrogenase in the cytosol generates XH from NADH. XH is transported into the mitochondrion where a second dehydrogenase catalyses a reaction in which the XH reduces NAD to NADH. X then returns to the cytosol. The nature of XH is considered in Figures 9.17 and 9.18.

In addition to the processes described above, there still remains one further process which, at least in some cells or tissues, is required prior to the utilisation of ATP in the cytosol that is, the transport of energy within the cytosol, via a shuttle. The transport of ATP out and ADP into the mitochondrion, via the translocase, results in a high ATP/ ADP concentration ratio in the cytosol. However, a high ratio means that the actual concentration of ADP in the cytosol is low, which could result in slow diffusion of ADP from a site of ATP utilisation back to the inner mitochondrial membrane. If sufficiently slow, it could limit the rate of ATP generation. To overcome this, a process exists that transports energy within the cytosol, not by diffusion of ATP and ADP, but by the diffusion of phosphocreatine and creatine, a process known as the phosphocreatine/creatine shuttle. The reactions involved in the shuttle in muscle help to explain the significance of the process. They are ... 

There is a logic to the route of these reactions through the mitochondrion. The [NADH]/[NAD+] ratio in the cytosol is 8 X 10 4, about 105 times lower than in mitochondria. Because cytosolic NADH is consumed in gluconeogenesis (in the conversion of 1,3-bisphos-... 

FIGURE 14-19 Alternative paths from pyruvate to phospho-enolpyruvate. The path that predominates depends on the glucogenic precursor (lactate or pyruvate). The path on the right predominates when lactate is the precursor, because cytosolic NADH is generated in the lactate dehydrogenase reaction and does not have to be shuttled out of the mitochondrion (see text). The relative importance of the two pathways depends on the availability of lactate and the cytosolic requirements for NADH by gluconeogenesis. 

Eugene Kennedy and Albert Lehninger showed in 1948 that, in eulcaiyotes, the entire set of reactions of the citric acid cycle takes place in mitochondria. Isolated mitochondria were found to contain not only all the enzymes and coenzymes required for the citric acid cycle, but also all the enzymes and proteins necessaiy for the last stage of respiration—electron transfer and ATP synthesis by oxidative phosphoiylation. As we shall see in later chapters, mitochondria also contain the enzymes for the oxidation of fatty acids and some amino acids to acetyl-CoA, and the oxidative degradation of other amino acids to a-ketoglutarate, succinyl-CoA, or oxaloacetate. Thus, in nonphotosynthetic eulcaiyotes, the mitochondrion is the site of most energy-yielding... 

The carbamoyl phosphate, which functions as an activated carbamoyl group donor, now enters the urea cycle. The cycle has four enzymatic steps. First, carbamoyl phosphate donates its carbamoyl group to ornithine to form citrulline, with the release of Pj (Fig. 18-10, step ). Ornithine plays a role resembling that of oxaloacetate in the citric acid cycle, accepting material at each turn of the cycle. The reaction is catalyzed by ornithine transcarbamoylase, and the citrulline passes from the mitochondrion to the cytosol. 

As we noted in Chapter 16, the enzymes of many metabolic pathways are clustered (p. 605), with the product of one enzyme reaction being channeled directly to the next enzyme in the pathway. In the urea cycle, the mitochondrial and cytosolic enzymes appear to be clustered in this way. The citrulline transported out of the mitochondrion is not diluted into the general pool of metabolites in the cytosol but is passed directly to the active site of argininosuccinate synthetase. This channeling between enzymes continues for argininosuccinate, arginine, and ornithine. Only urea is released into the general cytosolic pool of metabolites. 

In hepatocytes and adipocytes, cytosolic NADPH is largely generated by the pentose phosphate pathway (see Fig. 14-21) and by malic enzyme (Fig. 21-9a). The NADP-linked malic enzyme that operates in the carbon-assimilation pathway of C4 plants (see Fig. 20-23) is unrelated in function. The pyruvate produced in the reaction shown in Figure 21-9a reenters the mitochondrion. In hepatocytes and in the mammary gland of lactating animals, the NADPH required for fatty acid biosynthesis is supplied primarily by the pentose phosphate pathway (Fig. 21-9b). 

What are the molar concentrations of the electron carriers in mitochondrial membranes In one experiment, cytochrome b was found in rat liver mitochondria to the extent of 0.28 pmol/g of protein. If we take a total mitochondrion as about 22% protein, the average concentration of the cytochrome would be 0.06 mM. Since all the cytochromes are concentrated in the inner membranes, which may account for 10% or less of the volume of the mitochondrion, the concentration of cytochromes may approach 1 mM in these membranes. This is sufficient to ensure rapid reactions with substrates. 

Most biochemical reactions are integrated into multistep pathways using several enzymes. For example, the breakdown of glucose into C02 and H20 involves a series of reactions that begins in the cytosol and continues to completion in the mitochondrion. A complex series of reactions like this is referred to as a biochemical pathway (fig. 1.19). 

Summary diagram of the breakdown of glucose to carbon dioxide and water in a eukaryotic cell. As depicted here, the process starts with the absorption of glucose at the plasma membrane and its conversion into glucose-6-phosphate. In the cytosol, this six-carbon compound is then broken down by a sequence of enzyme-catalyzed reactions into two molecules of the three-carbon compound pyruvate. After absorption by the mitochondrion, pyruvate is broken down to carbon dioxide and water by a sequence of reactions that requires molecular oxygen. 

To understand why isocitrate dehydrogenase is so intensely regulated we must consider reactions beyond the TCA cycle, and indeed beyond the mitochondrion (fig. 13.15). Of the two compounds citrate and isocitrate, only citrate is transported across the barrier imposed by the mitochondrial membrane. Citrate that passes from the mitochondrion to the cytosol plays a major role in biosynthesis, both because of its immediate regulatory properties and because of the chain of covalent reactions it initiates. In the cytosol citrate undergoes a cleavage reaction in which acetyl-CoA is produced. The other cleavage product, oxaloacetate, can be utilized directly in various biosynthetic reactions or it can be converted to malate. The malate so formed can be returned to the mitochondrion, or it can be converted in the cytosol to pyruvate, which also results in the reduction of NADP+ to NADPH. The pyruvate is either utilized directly in biosynthetic processes, or like malate, can return to the mitochondrion. 

The acetyl-CoA produced in the cytosol from citrate breakdown is used in biosynthetic reactions, including the synthesis of lipids, some amino acids, cofactors, and pigments. This is the only source of acetyl-CoA in the cytosol because the acetyl-CoA produced in the mitochondrion cannot diffuse across the mitochondrial membrane. The NADPH produced indirectly in the cytosol from the citrate is also of great importance in biosynthetic reactions. 

Current estimates are that three protons move into the matrix through the ATP-synthase for each ATP that is synthesized. We see below that one additional proton enters the mitochondrion in connection with the uptake of ADP and Pi and export of ATP, giving a total of four protons per ATP. How does this stoichiometry relate to the P-to-O ratio When mitochondria respire and form ATP at a constant rate, protons must return to the matrix at a rate that just balances the proton efflux driven by the electron-transport reactions. Suppose that 10 protons are pumped out for each pair of electrons that traverse the respiratory chain from NADH to 02, and 4 protons move back in for each ATP molecule that is synthesized. Because the rates of proton efflux and influx must balance, 2.5 molecules of ATP (10/4) should be formed for each pair of electrons that go to 02. The P-to-O ratio thus is given by the ratio of the proton stoichiometries. If oxidation of succinate extrudes six protons per pair of electrons, the P-to-O ratio for this substrate is 6/4, or 1.5. These ratios agree with the measured P-to-O ratios for the two substrates. 

In eukaryotes, most of the reactions of aerobic energy metabolism occur in mitochondria. An inner membrane separates the mitochondrion into two spaces the internal matrix space and the intermembrane space. An electron-transport system in the inner membrane oxidizes NADH and succinate at the expense of 02, generating ATP in the process. The operation of the respiratory chain and its coupling to ATP synthesis can be summarized as follows ... 

The acetyl-CoA used as substrate for fatty acid synthases in the cytosol originates from the mitochondrion. This acetyl-CoA condenses with C02 to form malonyl-CoA, which eliminates (XL after an initial condensation reaction. After three more steps a two-carbon unit is added to a growing fatty acid chain. This cycle repeats itself many... 

Acyl-CoA is not transported across the inner membrane of the mitochondrion. Instead, the acyl-CoA reacts with carnitine to yield the acyl-carnitine derivative. This reaction is catalyzed by carnitine acyltransferase I, which is located on the outer mitochondrial membrane. The acyl-carnitine is transported across the inner membrane by a specific carrier protein. Once inside the matrix of the mitochondrion, the acyl-carnitine is converted back to its acyl-CoA... 

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