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Interactions of Zn2+ ions with proteins and nucleic acids
DVOŘÁKOVÁ, Kateřina
Interaction of Zn2+ ions with proteins and nucleic acids Zinc is an important metal in biological systems. It is a strong Lewis acid, forms a stable Zn2+ ion and can exist in several coordination geometries. Zinc is required for the activity of more than 300 enzymes. In proteins zinc can either participate directly in chemical catalysis or can be important for maintaining protein structure and stability. In this work a dataset of high quality (resolution better than 3 Ǻ) crystal structures deposited in the Protein Data Bank on internet have been examined to identify typical zinc binding sites and to establish their coordination geometries. Totally 135 zinc binding sites found in 74 structures have been analyzed. The most frequent coordinating atoms are sulphur in cystein, NE2 and ND1 nitrogens in histidine followed by oxygen atoms in carboxylates of aspartic and glutamic acids. 67% of zinc binding sites were found to be four-coordinated with tetrahedral coordination geometry. In contrast Mg2+ cations show a clear preference to bind to oxygen atoms and to form octahedral cavities. We have not found any structure with a direct zinc coordination to DNA except the 1ZQT structure which was however excluded due to its low resolution (> 3 {\AA}). Similarly only three zinc binding sites have been found in RNA structures which were however determined in the protein free environment. All other zinc binding sites have been located in proteins even in the presence of the nucleic acids.
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Changes in cytokinin contents and metabolism in spinach in response to zinc excess in soil
Žižková, E. ; Gajdošová, Silvia ; Pavlíková, D. ; Pavlík, Milan ; Száková, J. ; Motyka, Václav
Changes in endogenous cytokinin (CK) contents and metabolism in spinach (Spinacia oleracea L.) responding to excessive amounts of zinc in soil were studied. Application of three zinc rates in soil (250, 500, 750 mg.kg-1) resulted in a progressive (1.7- to 3.3-times) increase of endogenous CKs in spinach leaves. This increase was predominantly due to accumulation of inactive and storage CK forms (N- and O-glucosides) and was accompanied with an enhanced activity of cytokinin oxidase/dehydrogenase (CKX), the key CK degradative enzyme, especially on the highest zinc rate (750 mg.kg-1 soil). On this rate, maximum endogenous CK concentrations in both control and stressed plants were found at the first and the last samplings, i.e. in the course of very young leaf development and just before or during flowering. The CKX activity of stressed plants exceeded that of controls during the whole development and reached the maxima at the last sampling of treated plants.
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Heterodifúze 65Zn v binárních slitinách Mg - x Al av komerční slitině AZ91
Čermák, Jiří ; Stloukal, Ivo
Bulk diffusion coefficient of Zn, D, in Mg – x Al model alloys (x = 0, 1.8, 3.9 and 9 at. %), in commercial alloy AZ91 and in intermetallic compound Mg17Al12 intermetallic was measured by residual activity and serial sectioning methods. Composition dependence of D was obtained, that shows a slight increase of D with increasing Al concentration. Measurements were done at several diffusion temperatures, which enabled to evaluate the pre-exponential factor and the activation enthalpy of Zn diffusion.
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Difúze 65Zn ve dvoufázové eutektické slitině Mg - 33 wt.% Al
Čermák, Jiří ; Stloukal, Ivo
Diffusion of zinc in a two-phase eutectic alloy Mg – 33 wt.% Al and in its components, Mg – Al solid solution and in the intermetallic compound Mg17Al12, was studied. The measurement of Zn diffusion coefficient was carried out by radiotracer technique in temperature interval 498 – 848 K. Zinc is believed to simulate the migration behavior of magnesium. It was observed that the interphase boundaries do not act as short-circuit paths. The study of the stability of the eutectic structure enabled to estimate the mean specific energy of the (Mg)/ Mg17A12 phase interface.
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