Endoplasmic reticulum membrane-associated

Calcium ions (Ca ) are important for the mediation of hepatic injury. Cytosolic free calcium is maintained at relatively low concentrations compared to the extracellular levels. The majority of intracellular calcium is sequestered within the mitochondria and endoplasmic reticulum. Membrane associated calcium and magnesium ATPases are responsible for maintaining the calcium gradient (Farrell et ah, 1990). Significant and persistent increases in the intracellular calcium result from nonspecific increases in permeability of the plasma membrane, mitochondrial membranes, and membranes of the smooth endoplasmic reticulum. Calcium pumps in the mitochondrial membrane require NADPH, thus depletion of available NADPH can cause calcium release from mitochondria (Cullen, 2005). 

Scheme 8. A convergent synthesis70 of endoplasmic reticulum membrane-associated oligosaccharide 50.

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

In the family of cation pumps, only the Na,K-ATPase and H,K-ATPase possess a p subunit glycoprotein (Table II), while the Ca-ATPase and H-ATPase only consist of an a subunit with close to 1 000 amino acid residues. It is tempting to propose that the p subunit should be involved in binding and transport of potassium, but the functional domains related to catalysis in Na,K-ATPase seem to be contributed exclusively by the a subunit. The functional role of the P subunit is related to biosynthesis, intracellular transport and cell-cell contacts. The P subunit is required for assembly of the aj8 unit in the endoplasmic reticulum [20]. Association with a j8 subunit is required for maturation of the a subunit and for intracellular transport of the xP unit to the plasma membrane. In the jSl-subunit isoform, three disulphide... 

Craig S, Staehelin LA. High pressure freezing of intact plant tissues. Evaluation and characterization of novel features of the endoplasmic reticulum and associated membrane systems. Eur J Cell Biol 1988 46 81-93. 

Mayer TU, Braun T, Jentsch, S (1998) Role of the proteasome in membrane extraction of a short-lived ER-transmembrane protein. EMBO ] 17 3251-3257 McCracken AA, Brodsky JL (1996) Assembly of ER-associated protein degradation in vitro dependence on cytosol, calnexin, and ATP. J Cell Biol 132 291-298 McDonald HB, Byers B (1997) A proteasome cap subunit required for spindle pole body duplication in yeast. J Cell Biol 137 539-553 McGee TP, Cheng HH, Kumagai H, Omura S, Simoni RD (1996) Degradation of 3-hydroxy-3-methylg utaryl-CoA reductase in endoplasmic reticulum membranes is accelerated as a result of increased susceptibility to proteolysis. J Biol Chem 271 25630-25638... 

Hrazdina G, Zobel AM, Hoch HC. 1987. Biochemical, immunological, and immuno-cytochemical evidence for the association of chalcone synthase with endoplasmic reticulum membranes. Proc Natl Acad Sci USA 84 8966-8970. 

RNA polymerase III (RNA Pol III) is also located in the nucleoplasm. It transcribes the genes for tRNA, 5S rRNA, U6 snRNA, and the 7S RNA associated with the signal recognition particle (SRP) involved in the translocation of proteins across the endoplasmic reticulum membrane (see Topic H4). 

Figure 17. Analysis of phosphoenzyme intermediates of SR Ca2+-ATPase mutants with alterations to carboxylate-containing residues in the transmembrane sector. Wild-type or mutant Ca2+-ATPases expressed in the endoplasmic reticulum membranes of COS-1 cells were phosphorylated with [y-32P] ATP (panel a) or [32P]P (panels b and c). Following acid-quench of the phosphorylated intermediate, the samples were subjected to SDS-polyacrylamide gel electrophoresis under acid pH conditions and the dried gels were autoradiographed to visualize the radioactivity associated with the covalently bound phosphate. Panel a shows the Ca2+-concentration dependence of phosphorylation from ATP. The Glu309- Lys mutant is unable to phosphorylate, even at 12.5 mM Ca2+. In the wild-type Ca2+-ATPase the phosphorylation reaction is fully saturated at 10 pM Ca2+. Panel b shows lack of Ca2+ inhibition of backdoor phosphorylation from P in the mutants. E indicates the presence of ECTA to chelate Ca2+ (normally a requirement for phosphorylation by the backdoor route). C indicates the...

Phenylalanine ammonia lyase (PAL), the first enzyme of the biosynthetic sequence, and a flavonoid glucosyltransferase, the last enzyme, appear to be located in the lumen of the membranes. Cinnamate 4-hydroxy-lase is membrane embedded, while other enzyme activities appear to be weakly associated with the cytoplasmic phase of endoplasmic reticulum membranes (Hrazdina... 

As in morphine biosynthesis, the knowledge of the enzyme sequences allows a more detailed understanding of the localization of the enzymes (104). Strictosidine synthase (Fig. 2b) seems to be localized to the vacuole (105), and strictosidine glu-cosidase is believed to be associated with the membrane of the endoplasmic reticulum (73, 106). Tabersonine-16-hydroxylase is associated with the endoplasmic reticulum membrane (98) N-methyl transferase activity is believed to be associated... 

The pathways diverge at phosphatidate. In the synthesis of triacylglycerols, phosphatidate is hydrolyzed by a specific phosphatase to give a diacylglycerol (DAG). This intermediate is acylated to a triacylglycerol in a reaction that is catalyzed by diglyceride acyltransferase. Both enzymes are associated in a triacylglycerol synthetase complex that is bound to the endoplasmic reticulum membrane. 

The hydroperoxide cleavage enzyme from cucumber fruit is optimally active at pH 6.5, is very heat-labile, and attacks both 9- and 13-hydroperoxide isomers with equal facility. Subcellular localization studies have shown that the enzyme is associated mainly with the plasmalemma, Golgi body, and endoplasmic reticulum membranes (Wardale et al., 1978). 

Hydroxyl radicals damage the mitochondrial (chromoplastic), microsomal (peroxysomal) and endoplasmic reticulum membranes. The damage associated with many degenerative processes (diseases) includes certain forms of cancer and occurs even during the normal aging of organisms. Hydroxyl radicals operate mainly at the place of formation, where they attack unsaturated fatty acid (RH) bound in the membrane phospholipids. Unsaturated fatty... 

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