Specificity water-mediated contact

Conformational changes of trp (tryptophane) repressor and water-mediated contacts to DNA. trp Repressor is a 108 amino acids long polypeptide folded into 6 a-helices A to F. It was investigated by high resolution X-ray diffraction studies as apo repressor, as binary complex with its corepressor tryptophan, and as ternary complex bound to its specific operator DNA, a palindromic self-complementary 18-mer with overhanging T [712, 713] ... 

The trp repressor-operator complex exhibits no direct hydrogen bonds from the protein to the DNA bases but has three water molecules that mediate the necessary contacts for specific recognition between protein and DNA, so exemplifying well the importance water has in the process of recognition (26). Water-mediated links are also important in nuclear hormone receptors such as the estrogen receptor-DNA complex. 

The restriction endonuclease Mspl makes specific contacts with all eight bases in the four base pair recognition sequence (CCGG), by six direct and five water-mediated hydrogen bonds and 13 water-mediated links to the phosphates (30). Numerous... 

The lac repressor headpiece in complex with its operator (Figure Id) was simulated using distance restraints from NMR studies, with a 50 ps piece of unrestrained trajectory. From this simulation the specific contacts between the protein and the DNA were found to be formed mainly by non-polar contacts, and to a lesser extent through water mediated hydrogen bonds. 

Lipids are transported between membranes. As indicated above, lipids are often biosynthesized in one intracellular membrane and must be transported to other intracellular compartments for membrane biogenesis. Because lipids are insoluble in water, special mechanisms must exist for the inter- and intracellular transport of membrane lipids. Vesicular trafficking, cytoplasmic transfer-exchange proteins and direct transfer across membrane contacts can transport lipids from one membrane to another. The best understood of such mechanisms is vesicular transport, wherein the lipid molecules are sorted into membrane vesicles that bud out from the donor membrane and travel to and then fuse with the recipient membrane. The well characterized transport of plasma cholesterol into cells via receptor-mediated endocytosis is a useful model of this type of lipid transport. [9, 20]. A brain specific transporter for cholesterol has been identified (see Chapter 5). It is believed that transport of cholesterol from the endoplasmic reticulum to other membranes and of glycolipids from the Golgi bodies to the plasma membrane is mediated by similar mechanisms. The transport of phosphoglycerides is less clearly understood. Recent evidence suggests that net phospholipid movement between subcellular membranes may occur via specialized zones of apposition, as characterized for transfer of PtdSer between mitochondria and the endoplasmic reticulum [21]. 

The sensitivity and selectivity of olfaction and contact chemosensation are due (1) in the brain, to the existence of a neuronal network of neurons tuned to a specific chemical stimulus, and (2) in the periphery, to the existence of olfactory/ chemosensory receptor neurons housed in sensory microorgans called sensilla. The sensilla can best be viewed as simple cuticular porous extrusions that increase the surface that captures airborne odorants or chemicals dissolved in water droplets. They contain the receptive olfactory or chemosensory structures (Schneider, 1969). The olfactory sensilla are most numerous on the antennae and mediate the reception of sex pheromones and plant volatiles, as well as other odorants. Low volatility pheromones may also be detected by contact chemoreceptors on... 

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