Eukaryotic cells, attachment

Airother interesting facet of lipid anchors is that they are transient. Lipid anchors can be reversibly attached to and detached from proteins. This provides a switching device for altering the affinity of a protein for the membrane. Reversible lipid anchoring is one factor in the control of signal transduction pathways in eukaryotic cells (Chapter 34). 

The answer is b. (Hardman, p 1131.) Chloramphenicol inhibits protein synthesis in bacteria and, to a lesser extent, in eukaryotic cells. The drug binds reversibly to the. 505 ribosomal subunit and prevents attachment of aminoacybtransfer RNA (tRNA) to its binding site. The amino acid substrate is unavailable for peptidyl transferase and peptide bond formation. 

Of the extremely diverse examples of protein modifications observed in eukaryotic cells, the modifications by lipid (and glycolipid) molecules are of special interest because lipid-attached proteins can be anchored at the membrane, although all of these proteins are not always anchored. So far, three groups of membrane anchoring proteins have been noted (Fig. 5). 

Goehring, a. S., Rivers, D. M., and Sprague, G. F., Jr., Attachment of the ubiquitin-related protein Urmlp to the antioxidant protein Ahplp, Eukaryot. Cell, 2003, 2, 930. 

Thus, the fat globules are surrounded, at least initially, by a membrane typical of eukaryotic cells. Membranes are a conspicuous feature of all cells and may represent 80% of the dry weight of some cells. They serve as barriers separating aqueous compartments with different solute composition and as the structural base on which many enzymes and transport systems are located. Although there is considerable variation, the typical composition of membranes is about 40% lipid and 60% protein. The lipids are mostly polar (nearly all the polar lipids in cells are located in the membranes), principally phospholipids and cholesterol in varying proportions. Membranes contain several proteins, perhaps up to 100 in complex membranes. Some of the proteins, referred to as extrinsic or peripheral, are loosely attached to the membrane surface and are easily removed by mild extraction procedures. The intrinsic or integral proteins, about 70% of the total protein, are tightly bound to the lipid portion and are removed only by severe treatment, e.g. by SDS or urea. 

The geometry of cell construction provides another important aspect of metabolic control. In a bacterium, the periplasmic space (Fig. 8-28) provides a compartment that is separate from the cytosol. Some enzymes are localized in this space and do not mix with those within the cell. Other enzymes are fixed within or attached to the membrane. Eukaryotic cells have more compartments nuclei, mitochondria (containing both matrix and intermembrane spaces), lysosomes, microbodies, plastids, and vacuoles. Within the cytosol the tubules and vesicles of the endoplasmic reticulum (ER) separate off other membrane-bounded compartments. The rate of transport of metabolites through the membranes between compartments is limited and often is controlled tightly. 

A number of different proteolytic systems are thought to be responsible for the degradation of soluble proteins in the cytoplasm of eukaryotic cells. One of the best understood is that which involves ATP and the protein ubiquitin. Ubiquitin is a small protein of only 76 residues. It occurs universally in eukaryotic cells and is highly conserved in sequence only three residues distinguish the ubiquitin in yeast and humans. The covalent attachment of ubiquitin to proteins is thought to tag them for subsequent hydrolysis by cellular proteases. 

Since DNA is a highly charged polyanion, it is always hydrated by water molecules [in the dry state (under moist air) it contains 12 water molecules per nucleotide subunit]. In a cellular environment, proteins (histones in eukaryotic cells) are always attached to DNA or are at least surrounded by proteins as in viruses. In order to attack DNA, radicals have to be sufficiently mobile in such a partially hydrophilic environment. For this reason, typical lipid radicals confined to the membranes will not be discussed here, although one must keep in mind that small fragments of free-radical nature maybe able to escape the lipid environment and can, in principle, also react with DNA. 

To evaluate the actual protein levels ofpeptide synthetases with respect to translation, posttranslational processing, stability, and localization. These aspects are especially related to the compartmentation of eukaryotic cells. So far, evidence for a vacuolar attachment of ACV synthetase in P. chrysogenum has been obtained [95], but it has not been fully substantiated in further studies [96], The synthetase is now considered to be cytoplasmatic. 

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