Membrane protein synthesis,

Docosahexaenoic acid has the potential to affect cell function by modulating two aspects of membrane proteins synthesis (particularly via transcriptional control) (Sellmayer et al., 1997) and structure (within the lipid milieu of the membrane). Although understanding the effect of DHA on proteins is still in its infancy, two examples, rhodopsin and protein kinase C (PKC), have been particularly well studied. The effect of DHA on rhodopsin is reviewed in detail in Chapter 2. 

Stock, A., Koshland, D.E., Jr. and Stock, J. (1985). Homologies between Salmonella typhimurium CheY protein and proteins involved in the regulation of chemotaxis, membrane protein synthesis, and sporulation. Proc. Natl. Acad. Sci. 

Clearly, except in the case of night blindness, there is no way to correlate the symptoms observed in vitamin A deficiency with molecular alterations or, for that matter, with a specific molecular role of vitamin A in metabolism. Coenzyme roles in hydroxysteroid dehydrogenase, squalene cyclodehydrase, sulfate transferase, codeine demethylase, and gulonolactone oxidase have been proposed but not established conclusively. Effects on membrane, protein synthesis, and electron transport have been described, but none satisfactorily explains the pathogenesis of vitamin A deficiency [122]. 

Bioenergetics Enzyme Mechanisms Gene Expression, Regulation of Ion Transport Across Biological Membranes Protein Synthesis... 

The elucidation of the control mechanism of membrane protein synthesis could be rendered easier if appropriate conditional mutants, such as temperature-sensitive mutants for membrane protein synthesis or regulatory mutants, would be available. Such mutants could prove to be priceless tools if they could be obtained for specific proteins, such as the different components of the L protein group or the hypothetic repressor for the synthesis of these proteins. At the time of this writing, temperature-sensitive mutants for membrane proteins required for photosystem II activity have been obtained and are under investigation (P. Benoun, personal communication). Temperature-sensitive mutants of C. reinhardi for all the cyto-... 

Table I summarizes data on the inhibition of synthesis of the individual outer-membrane proteins by the different antibiotics tested. In contrast to kasugomycin, tetracycline did not show clear differential inhibitory effects. On the other hand, chloramphenicol did reveal some differential inhibitory effects on the individual outer membrane protein synthesis, although no significant differential effects on overall envelope and cytoplasmic protein synthesis was detected (Fig. 3C). At concentrations of...

The mitochondrial complex that carries out ATP synthesis is called ATP synthase or sometimes FjFo-ATPase (for the reverse reaction it catalyzes). ATP synthase was observed in early electron micrographs of submitochondrial particles (prepared by sonication of inner membrane preparations) as round, 8.5-nm-diameter projections or particles on the inner membrane (Figure 21.23). In micrographs of native mitochondria, the projections appear on the matrixfacing surface of the inner membrane. Mild agitation removes the particles from isolated membrane preparations, and the isolated spherical particles catalyze ATP hydrolysis, the reverse reaction of the ATP synthase. Stripped of these particles, the membranes can still carry out electron transfer but cannot synthesize ATP. In one of the first reconstitution experiments with membrane proteins, Efraim Racker showed that adding the particles back to stripped membranes restored electron transfer-dependent ATP synthesis. 

ATP synthase actually consists of two principal complexes. The spheres observed in electron micrographs make up the Fj unit, which catalyzes ATP synthesis. These Fj spheres are attached to an integral membrane protein aggregate called the Fq unit. Fj consists of five polypeptide chains named a, j3, y, 8, and e, with a subunit stoichiometry ajjSaySe (Table 21.3). Fq consists of three hydrophobic subunits denoted by a, b, and c, with an apparent stoichiometry of ajbgCg.ig- Fq forms the transmembrane pore or channel through which protons move to drive ATP synthesis. The a, j3, y, 8, and e subunits of Fj contain 510, 482, 272, 146, and 50 amino acids, respectively, with a total molecular mass... 

What molecular architecture couples the absorption of light energy to rapid electron-transfer events, in turn coupling these e transfers to proton translocations so that ATP synthesis is possible Part of the answer to this question lies in the membrane-associated nature of the photosystems. Membrane proteins have been difficult to study due to their insolubility in the usual aqueous solvents employed in protein biochemistry. A major breakthrough occurred in 1984 when Johann Deisenhofer, Hartmut Michel, and Robert Huber reported the first X-ray crystallographic analysis of a membrane protein. To the great benefit of photosynthesis research, this protein was the reaction center from the photosynthetic purple bacterium Rhodopseudomonas viridis. This research earned these three scientists the 1984 Nobel Prize in chemistry. 

The insulin receptor is a transmembrane receptor tyrosine kinase located in the plasma membrane of insulin-sensitive cells (e.g., adipocytes, myocytes, hepatocytes). It mediates the effect of insulin on specific cellular responses (e.g., glucose transport, glycogen synthesis, lipid synthesis, protein synthesis). 

Cyanobacteria, prokaryotic algae that perform oxygenic photosynthesis, respond to a decrease in ambient growth temperature by desaturating the fatty acids of membrane lipids to compensate for the decrease in the molecular motion of the membrane lipids at low temperatures. During low-temperature acclimation of cyanobacterial cells, the desaturation of fatty acids occurs without de novo synthesis of fatty acids [110, 111]. All known cyanobacterial desaturases are intrinsic membrane proteins that act on acyl-Hpid substrates. 

The membranes of the endoplasmic reticulum contain the enzyme system for acylglycerol synthesis, and the ribosomes are responsible for protein synthesis. 

Diphtheria toxin, an exotoxin of Corynebacterium diphtheriae infected with a specific lysogenic phage, catalyzes the ADP-ribosylation of EF-2 on the unique amino acid diphthamide in mammalian cells. This modification inactivates EF-2 and thereby specifically inhibits mammalian protein synthesis. Many animals (eg, mice) are resistant to diphtheria toxin. This resistance is due to inability of diphtheria toxin to cross the cell membrane rather than to insensitivity of mouse EF-2 to diphtheria toxin-catalyzed ADP-ribosylation by NAD. 

The eytoplasm is a viscous fluid and contains within it systems of paramount importance. These are the nucleus, responsible for the genehc make-up of the cell, and the ribosomes, whieh are the site of protein synthesis, hi addihon are found granules of reserve material suehas polylydioxybutyric add, an energy reserve, and polyphosphate or volutin granules, the exact funchon of which has not yet been elucidated. The prokaiyohc nueleus or bacterial chromosome exists in the cytoplasm in the form of a loop and is not surrounded by a nuclear membrane. Bacteria cany other chromosomal elements episomes, which are portions of the main chromosome that have become isolated firm it, and plasmids, whieh may be called miniature chromosomes. These are small annular pieees of DNA whieh carry a limited amount of genetic information. 

Like other cells, a neuron has a nucleus with genetic DNA, although nerve cells cannot divide (replicate) after maturity, and a prominent nucleolus for ribosome synthesis. There are also mitochondria for energy supply as well as a smooth and a rough endoplasmic reticulum for lipid and protein synthesis, and a Golgi apparatus. These are all in a fluid cytosol (cytoplasm), containing enzymes for cell metabolism and NT synthesis and which is surrounded by a phospholipid plasma membrane, impermeable to ions and water-soluble substances. In order to cross the membrane, substances either have to be very lipid soluble or transported by special carrier proteins. It is also the site for NT receptors and the various ion channels important in the control of neuronal excitability. 

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