Oxidative phosphorylation compartmentation

FIGURE 18.16 Compartmentalization of glycolysis, the citric acid cycle, and oxidative phosphorylation. 

Pathways are compartmentalized within the cell. Glycolysis, glycogenesis, glycogenolysis, the pentose phosphate pathway, and fipogenesis occur in the cytosol. The mitochondrion contains the enzymes of the citric acid cycle, P-oxidation of fatty acids, and of oxidative phosphorylation. The endoplasmic reticulum also contains the enzymes for many other processes, including protein synthesis, glycerofipid formation, and dmg metabolism. 

The accumulation of the precursors for fatty acid synthesis is a wonderful example of the coordinated use of multiple processes to fulfdl a biochemical need. The citric acid cycle, subcellular compartmentalization, and the pentose phosphate pathway provide the carbon atoms and reducing power, whereas glycolysis and oxidative phosphorylation provide the ATP to meet the needs for fatty acid synthesis. 

As described in Chapter 18, oxidative phosphorylation, the predominant means of generating ATP from fuel molecules, is compartmentalized into... 

Like most bacteria, the Microtox strain has many metabolic pathways which function in respiration, oxidative phosphorylation, osmotic stabilization, and transport of chemicals and nutrients into and out of the cell, and which are located within or near the cytoplasmic membrane. The luciferase pathway [9], which functions as a shunt for electrons directly to oxygen at the level of reduced flavin mono-nucleotide, is also located within the cell membrane complex. This, coupled with lack of membrane-aided compartmentalization of internal functions, gives many target sites at or near the cytoplasmic membrane. These factors all contribute to a rapid response of the organisms to a broad spectrum of toxic substances. 

A system with definite inside and outside compartments (closed vesicles) is essential for oxidative phosphorylation. The process does not occur in soluble preparations or in membrane fragments without compartmentalization. 

Anabolic and catabolic pathways may be segregated into different cellular compartments. In liver cells, the biosynthesis of fatty acids from acetyl-CoA occurs in the cytosol where the enzymes for both biosynthesis and the generation of NADPH are located. The degradation of fatty acids occurs within the mitochondria where the appropriate enzymes and the apparatus for oxidative phosphorylation are located (Section 9.5). Compartmentalization of metabolic pathways necessitates the provision of mechanisms by which... 

Of all the intracellular organelles, the mitochondrion has been the most extensively studied with respect to the compartmentation of compounds within its boimdaries. In part, this results from the ease of separation of mitochondria from mammalian tissues (most notably the liver), as well as from the key role mitochondria play in a number of metabolic processes. The mitochondrial membrane is capable of transporting metabolites on specific transporters and of segregating metabolites from the cytosol. It is important to note that some metabolites apparently move across the mitochondrial membrane in an unspecific or non-carrier-linked manner. For example, ketone bodies, water, CO2, and oxygen appear to freely diffuse into and out of mitochondria. In the following sections we will discuss specific aspects of the transport mechanisms, followed by a more general discussion of their role in regulating major metabolic pathways. We will start with the most important result of intracellular compartmentation—oxidative phosphorylation—as viewed by the chemiosmotic theory. 

A large number of cellular processes and biosythetic pathways of eukaryotic cells are compartmentalized and restricted to specific membranes. Mitochondria and chloroplasts are two cases in point. In prokaryotic cells, many of the same functions are performed by a single membrane. The transport of metabolites and ions, oxidative phosphorylation, photosynthesis, phospholipid biosynthesis, and the synthesis of cell-wall constituents are a few examples of processes carried out by enzyme systems localized in the bacterial plasma membrane (Chapters... 

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