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Content available remote Mechanism of ligand binding – PDZ domain taken as example
Abstract: The mechanism of specific ligand binding by proteins is discussed using the PDZ domain complexing the pentapeptide. This process is critical for clustering the membrane ion channel. The traditional model based on the Beta-sheet extension by complexed pentapeptide is interpreted as a hydrophobic core extension supported by additional Beta-strand generated by complexed pentapeptide. The explanation is based on the fuzzy oil drop model applied to the crystal structure of PDZ-pentapeptide.
The fuzzy oil drop model was applied to analyze the structure of macromomycin, the apoprotein of the antitumor antibiotic auromomycin, revealing the differentiation of β-structural fragments present in β-sandwich. The seven-stranded antiparallel β-barrel and two antiparallel β-sheet ribbons represent the highly ordered geometry of the structure. However, participation in hydrophobic core formation appears different. The structure of the complete domain represents the status of the irregular hydrophobic core; however, some β-structural fragments appear to represent the hydrophobicity density distribution accordant with the idealized distribution of hydrophobicity as expected using the fuzzy oil drop model. Four β-structural fragments generating one common layer appear to be unstable in respect to the general structure of the hydrophobic core. This area is expected to be more flexible than other parts of the molecule. The protein binds the ligand – chromophore, two 2-methyl-2,4-pentanediol – in a well- defined cleft. The presence of this cleft makes the general structure of the hydrophobic core irregular (as it may be interpreted using the fuzzy oil drop model). Two short loops generated by two SS bonds fit very well to the general distribution of hydrophobicity density as expected for the model. No information about the potential amyloidogenic character of this protein is given in the literature; however, the specificity of the hydrophobicity distribution profile is found to be highly similar to the one observed in transthyretin (Banach M, Konieczny L, Roterman I. The fuzzy oil drop model, based on hydrophobicity density distribution, generalizes the influence of water environment on protein structure and function. J Theor Biol 2014;359:6–17), suggesting a possible tendency to turn to the amyloid form. A detailed analysis of macromomycin will be given, and a comparable analysis with other proteins of β-sandwich or β-barrel will be presented.
Content available remote Dializa równowagowa. Metoda badania selektywności oddziaływań ligand.DNA
Equilibrium or competition dialysis is a powerful tool for binding study of ligands that are expected to bind to nucleic acids with selectivity related to their structure or sequence. In the equilibrium dialysis experiment, a set of nucleic acid samples that differ in structure and sequence is dialyzed against a test ligand solution. After equilibration, the concentration of ligand bound to each structure or sequence is determined by UV-Vis absorption or fluorescence spectroscopy in each dialysis unit. Since all nucleic acid samples are in equilibrium with the same free ligand concentration, the amount of bound ligand is directly related to the ligand binding affinity. Thus, equilibrium provides a direct measure of selectivity and identifies the nucleic acid sample, which is preferred by a particular ligand. We describe here the principles and practice of the method. Examples of an application of the method are limited to the discovery of small molecules that selectively recognize the unique structural features of G-quadruplexes. There are proofs for important functional roles of G-quadruplex structures in biology (maintenance of telomeres, transcriptional regulation, and modulation of mRNA translation). G-quadruplex DNA can exist in a variety of structural forms that may possess numerous potential binding sites for small molecules. Therefore equilibrium dialysis provides a useful tool for discovery of new mall-molecule therapeutic agents targeting G-quadruplexes.
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