Bacteria Endoplasmic reticulum

The cytochromes are iron-containing hemoproteins in which the iron atom oscillates between Fe + and Fe + during oxidation and reduction. Except for cytochrome oxidase (previously described), they are classified as dehydrogenases. In the respiratory chain, they are involved as carriers of electrons from flavoproteins on the one hand to cytochrome oxidase on the other (Figure 12-4). Several identifiable cytochromes occur in the respiratory chain, ie, cytochromes b, Cp c, a, and (cytochrome oxidase). Cytochromes are also found in other locations, eg, the endoplasmic reticulum (cytochromes P450 and h, and in plant cells, bacteria, and yeasts. 

Topical eukaryotic cells (Fig. 1-7) are much larger than prokaryotic cells—commonly 5 to 100 pm in diameter, with cell volumes a thousand to a million times larger than those of bacteria. The distinguishing characteristics of eukaryotes are the nucleus and a variety of membrane-bounded organelles with specific functions mitochondria, endoplasmic reticulum, Golgi complexes, and lysosomes. Plant cells also contain vacuoles and chloroplasts (Fig. 1-7). Also present in the cytoplasm of many cells are granules or droplets containing stored nutrients such as starch and fat. 

Five kinds of phospholipid predominate phosphatidylcholine, phosphatidylethanolamine, phosphatidyl-serine, phosphatidylglycerols, and sphingomyelin. Usually there are also small amounts of phosphatidyli-nositol. The major phospholipid in animal cells is phosphatidylcholine, but in bacteria it is phosphatidylethanolamine. The phospholipids of E. coli consist of 80% phosphatidylethanolamine, 15% phosphati-dylglycerol, and 5% diphosphatidylglycerol (cardio-lipin). Significant amounts of cardiolipin are found only in bacteria and in the inner membrane of mitochondria. Sphingomyelin is almost absent from mitochondria, endoplasmic reticulum, or nuclear membranes. 

Although it is primarily an intracellular compound, glutathione is secreted by epithelial and other cells. It may regulate the redox state of proteins in plasma and other extracellular fluids as well as within cells. In addition, glutathione released from the liver may be an important source of cysteine for other tissues. In the endoplasmic reticulum and the periplasm of bacteria glutathione functions in crosslinking thiol groups in newly formed proteins (Eq. 10-9). 

Cytochromes b, a, and o. Protoheme-containing cytochromes b are widely distributed.127,128 There are at least five of them in E. coli. Whether in bacteria, mitochondria, or chloroplasts, the cytochromes b function within electron transport chains, often gathering electrons from dehydrogenases and passing them on to c-type cytochromes or to iron-sulfur proteins. Most cytochromes b are bound to or embedded within membranes of bacteria, mitochondria, chloroplasts, or endoplasmic reticulum (ER). For example, cyto-... 

Hydrocarbons yield more energy upon combustion than do most other organic compounds, and it is, therefore, not surprising that one important type of food reserve, the fats, is essentially hydrocarbon in nature. In terms of energy content the component fatty acids are the most important. Most aerobic cells can oxidize fatty acids completely to C02 and water, a process that takes place within many bacteria, in the matrix space of animal mitochondria, in the peroxisomes of most eukaryotic cells, and to a lesser extent in the endoplasmic reticulum. 

The oxidative processes of cells have been hard to study, largely because the enzymes responsible are located in or on cell membranes. In bacteria the sites of electron transport and oxidative phosphorylation are on the inside of the plasma membrane or on membranes of mesosomes. In eukaryotes they are found in the inner membranes of the mitochondria and, to a lesser extent, in the endoplasmic reticulum. For this reason we should probably start with a closer look at mitochondria, the "power plants of the cell."... 

The processes by which proteins are selected for secretion into the periplasmic space of bacteria or into the vesicles of the endoplasmic reticulum of eukaryotic cells are similar and have been discussed in Chapter 10 (pp. 519-521). However, some details are still being worked out. The first step in translocation is binding of the N terminus of a protein that is emerging from a ribosome to the signal recognition parti-cje 563,564 j le core 0f this particle has a universally conserved structure consisting of two proteins and an RNA molecule.565 571 b... 

Complexes of PolyP and PHB, similar to those in bacteria, were found in the membranes of the endoplasmic reticulum and mitochondria of animal cells (Reusch, 1989, 1999a, 2000 Reusch and Sadoff, 1988), which suggests their participation in the processes of transmembrane transfer. The most intriguing report was that the Ca-ATPase purified from human erythrocytes contains PolyPs and PHBs and that the plasma membrane Ca2+-ATPase may function as a polyphosphate kinase this exhibits ATP-polyphosphate transferase and polyphosphate-ADP transferase activities. These findings suggest a novel supramolecular structure for the functional Ca2+-ATPase and a new mechanism of uphill Ca2+ extrusion coupled with ATP hydrolysis (Reusch et al, 1997). 

The most well understood pathway is the one that delivers secretory and membrane proteins to the endoplasmic reticulum (ER) membrane in eukaryotic cells and to the inner membrane in bacteria. In both kinds of cells, the pivotal role is played by the so-called Sec61 (in eukaryotes) or SecYEG (in prokaryotes) translocon, a multisubunit translocation channel that provides a conduit for soluble proteins to cross the membrane. The same translocon also serves to integrate membrane proteins into the lipid bilayer. 

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