Endoplasmic reticulum membrane-bound proteins

The reaction is catalysed by UDP-glucuronosyltrans-ferases (UGTs). These enzymes are endoplasmic reticulum, membrane-bound proteins, which utilize UDP-GlcA as a sugar donor and transfer glucuronic acid to available substrates, a process that forms p-D-glucuronides. These metabolites can be formed from a wide variety of chemicals... 

Fig. 1.6. Schematic representation of protein biosynthesis by rough endoplasmic reticulum membrane-bound polysomes (according to Blobel and Dobberstein, 1975a). Codons corresponding to signal peptides after initiation codon AUG are indicated by a zig-zag, the signal sequence is indicated by a dashed line (see text).

Protein glycosylation occurs mainly on serine and asparagine residues,but can also occur on hydroxylysine and hydroxyproline (Scheme 15). Glycosylation is very important in the endoplasmic reticulum and Golgi apparatus and can be involved in cell signaling. Many of the membrane-bound proteins and excreted proteins are glycosylated. Protein glycosylation is important in all forms of eukaryotes. ... 

Secretory and membrane-tai eted proteins are synthesized as larger precursors on endoplasmic reticulum (ER)—bound ribosomes, which inject the protein across or into the ER membrane as it is translated. 

The core protein is synthesized on and enters the rough endoplasmic reticulum (RER). The protein is then glycosylated by membrane-bound transferases as it moves through the ER. 

The amino acids of a protein control its location in the cell. Some proteins are water soluble, whereas others are bound to the ceil membrane (plasma membrane), the mitochondrial membrane, and the membranes of the endoplasmic reticulum and nucleus. The association of a protein with a membrane is maintained by a stretch of lipophilic amino acids. Insertion of this stretch into the membrane occurs as the protein is synthesized. Water-soluble proteins are formed on ribosomes that "float" free in the cytoplasm. Membrane-bound proteins are formed on ribosomes that associate with the endoplasmic reticulum (ER). As the amino acids are polymerized in the vicinity of the F,R, a stretch of lipophilic acids becomes inserted into the membrane of the FR. This anchoring of the protein is maintained when it is shuttled from its location in the ER to its desired location in the plasma membrane. 

The protein defective in Menkes disease is a membrane-bound protein consisting of 1500 amino acids. Evidence suggests that this protein occurs in the membrane of the endoplasmic reticulum, not in the plasma membrane. The mutations in the gene coding for Menkes protein that are responsible for the disease take a number of forms. The gene, as studied in hundreds of human subjects, may contain insertions (an extra nucleotide), deletions (one less nucleotide), conversions of an amino acid s codon to a stop codon (resulting in a truncated protein), and other types of mutations. 

There is evidence supporting a role for hepatic damage by intravascular proteases in the pathogenesis of neonatal hepatic disease, as reviewed in the previous edition of this textbook,The AAT deposits in the hepatic endoplasmic reticulum do not bind normally to calnexin, one of the chaperones for protein synthesis and release it is known that proteolytic enzymes reduce the activity of intracellular, membrane-bound proteins involved in metabolic processes. An additional component in congenital and neonatal hepatic disease may be exposure to maternal estrogens, which increase susceptibility to damage from hepatitis viral infections and some toxins. 

Early work on protein secretion, largely performed by Palade and coworkers in the late 1960s, focused on the organellar route of a secreted protein from its synthesis to its release into the extracellular fluid (Palade, 1975). Their work showed that exported proteins are synthesized on polysomes bound to the membrane of the rough endoplasmic reticulum (RER). These proteins are not found in the cytoplasm, but are immediately sequestered in the lumen of the endoplasmic reticulum (ER) they are subsequently transported to the Golgi apparatus and then to secretory vesicles, from which they are secreted. This export route is called the Palade pathway. 

The sialyltransferases are membrane-bound proteins located in the endoplasmic reticulum (ER) and in the Golgi apparatus. Information about their sequence homology is limited, but they do appear to share a common topography [35]. A catalytic domain resides at the C-terminus followed by an N-terminal segment that anchors the enzyme into the ER or Golgi membrane. Soluble, catalytically active sialyltransferases that lack the anchor segment have been isolated from milk, serum, and other body fluids, suggesting that this N-terminal anchor is not necessary for the enzyme to retain catalytic activity. However, the ability to obtain from natural sources quantities of most sialyltransferases that would be needed for synthesis applications is hampered by low tissue concentrations and difficult purifications. 

A pathway analogous to the DsbA-DsbB system was recently shown to operate in the endoplasmic reticulum of eukaryotes (Frand and Kaiser, 1998 1999 Pollard et al., 1998). This pathway includes the membrane-bound protein EROl, which keeps protein disulfide isomerase (PDI) in an oxidized state in vivo. Consequently, PDI plays a role somewhat similar to that of DsbAin catalyzing the formation of disulfide bonds during the folding of secreted proteins. The mechanisms driving oxidative protein folding in eukaryotes are described in more detail in another chapter in this volume. 

The enzymes responsible for the conversion of lanosterol to cholesterol, as were those for the conversion of farnesyl pyrophosphate to squalene and lanosterol, are all integral membrane-bound proteins of the endoplasmic reticulum. Many have resisted solubiUzation, some have been partially purified, and several have been obtained as pure proteins. As a consequence, much of the enzymological and mechanistic studies have been done on impure systems and one would anticipate a more detailed and improved understanding of these events as more highly purified enzymes become available. Many approaches have been taken to establish the biosynthetic route that sterols follow to cholesterol. Some examples are synthesis of potential intermediates, the use of inhibitors, both of sterol transformations and of the electron transfer systems, and by isotope dilution experiments. There is good evidence that the enzymes involved in these transformations do not have strict substrate specificity. As a result, many compounds that have been found to be converted to intermediates or to cholesterol may not be true intermediates. In addition, there is structural similarity between many of the intermediates so that alternate pathways and metabolites are possible. For example, it has been shown that side-chain saturation can be either the first or the last reaction in the sequence. Fig. 21 shows a most probable series of intermediates for this biosynthetic pathway. 

Vitamin K is essential for activity of vitamin K-dependent y-glutamyl carboxyiase, which is responsible for post-translational modification of glutamyl residues (Glu) to y-carboxylated glutamyl residues (Gla) producing a small family of vitamin K-dependent proteins (VKD proteins). It is membrane bound and carboxylates the proteins as they emerge from the endoplasmic reticulum. The VKD proteins associated with blood clotting are well established but recent research has revealed VKD proteins associated with bone metabolism (bone Gla protein (BGP) and matrix Gla protein (MGP)). 

In contrast to prokaryotic P450s, eukaryotic P450s ate generally membrane-bound proteins. Most eukaryotic P450s are incorporated into the endoplasmic reticulum. However, several mammalian P450s that participate in the synthesis of... 

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