Energy metabolism oxidative phosphorylation

The life of a cell is maintained by the continuous activity of a myriad of biochemical reactions that provide metabolic energy, synthesize (and degrade) structural and functional molecules such as proteins, nucleic acids and lipids, and drive cellular dynamic functions such as contraction, locomotion, and cytokinesis. These reactions are organized into networks or modules that have specific functions such as protein synthesis or production of energy by oxidative phosphorylation. Then, when a cell is in a stable state, these reaction networks must also operate stably i.e., energy is continuously generated and... 

In animal cells, fatty acids are degraded both in mitochondria and peroxisomes, whereas in lower eukaryotes, P-oxidation is confined to peroxisomes. Mitochondrial P-oxidation provides energy for oxidative phosphorylation and generates acetyl-CoA for ketogenesis in liver. The oxidation of fatty acids with odd numbers of carbon atoms also yields propi-onyl-CoA that is metabolized to succinate. 

The combustion of the acetyl groups of acetyl-CoA by the citric acid cycle and oxidative phosphorylation to produce COg and HgO represents stage 3 of catabolism. The end products of the citric acid cycle, COg and HgO, are the ultimate waste products of aerobic catabolism. As we shall see in Chapter 20, the oxidation of acetyl-CoA during stage 3 metabolism generates most of the energy produced by the cell. 

The processes of electron transport and oxidative phosphorylation are membrane-associated. Bacteria are the simplest life form, and bacterial cells typically consist of a single cellular compartment surrounded by a plasma membrane and a more rigid cell wall. In such a system, the conversion of energy from NADH and [FADHg] to the energy of ATP via electron transport and oxidative phosphorylation is carried out at (and across) the plasma membrane. In eukaryotic cells, electron transport and oxidative phosphorylation are localized in mitochondria, which are also the sites of TCA cycle activity and (as we shall see in Chapter 24) fatty acid oxidation. Mammalian cells contain from 800 to 2500 mitochondria other types of cells may have as few as one or two or as many as half a million mitochondria. Human erythrocytes, whose purpose is simply to transport oxygen to tissues, contain no mitochondria at all. The typical mitochondrion is about 0.5 0.3 microns in diameter and from 0.5 micron to several microns long its overall shape is sensitive to metabolic conditions in the cell. 

This potential, or protonmotive force as it is also called, in turn drives a number of energy-requiring functions which include the synthesis of ATP, the coupling of oxidative processes to phosphorylation, a metabohc sequence called oxidative phosphorylation and the transport and concentration in the cell of metabolites such as sugars and amino acids. This, in a few simple words, is the basis of the chemiosmotic theory linking metabolism to energy-requiring processes. 

Cells that do not have mitochondria (such as red blood cells) must use glucose for energy since they have no TCA cycle or oxidative phosphorylation. Without a constant glucose supply, these cells would die. The brain relies heavily on glucose metabolism for energy however, the brain can adapt to use alternative energy sources if glucose is not available. 

Mitochondrial oligomycin-sensitive mg2+ATPase is thought to play a major role in oxidative phosphorylation (Boyer et al. 1977). It has been suggested that impairment of mitochondrial energy metabolism by chlordecone may contribute to the decreases in body weight observed following exposure to this chemical (Desaiah 1981). 

Enzymatic catalysis of reactions. Important enzymes are located in membranes at the interface between the lipid and aqueous phases. This is where reactions with apolar substrates occur. Examples include lipid biosynthesis (see p. 170) and the metabolism of apolar xenobiotics (see p. 316). The most important reactions in energy conversion—i.e., oxidative phosphorylation (see... 

Using the transport systems in the membranes, cells regulate their volume, internal pH value, and ionic environment. They concentrate metabolites that are important for energy metabolism and biosynthesis, and exclude toxic substances. Transport systems also serve to establish ion gradients, which are required for oxidative phosphorylation and stimulation of muscle and nerve cells, for example (see p. 350). 

For many years, niclosamide (Niclocide) was widely used to treat infestations of cestodes. Niclosamide is a chlorinated salicylamide that inhibits the production of energy derived from anaerobic metabolism. It may also have adenosine triphosphatase (ATPase) stimulating properties. Inhibition of anaerobic incorporation of inorganic phosphate into ATP is detrimental to the parasite. Niclosamide can uncouple oxidative phosphorylation in mammalian mitochondria, but this action requires dosages that are higher than those commonly used in treating worm infections. 

Metabolic effects Salicylates cause uncoupling of oxidative phosphorylation which leads to conversion of energy into heat and may thus produce hyperpyrexia and increased protein catabolism. Larger dose produces hyperglycemia and glycosuria in normal individual while in diabetic patient it produces hypoglycemia which may be due to an enhanced peripheral utilization of glucose and inhibition of... 

---