Arsenylation of nucleoside diphosphates in Rhodospirillum rubrum chromatophores |
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Authors: | Luit Slooten Adriaan Nuyten |
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Affiliation: | Vrije Universiteit Brussel, Faculty of Sciences, Laboratory for Biophysics, Pleinlaan 2, 1050 Brussels Belgium |
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Abstract: | Rhodospirillum rubrum chromatophores catalyze the formation of ADP-arsenate during illumination with ADP, Mg2+ and arsenate. The reaction was measured with (1) firefly luciferase, (2) a coupled enzyme assay involving hexokinase and glucose-6-phosphate dehydrogenase, and (3) a glass electrode. ADP-arsenate hydrolyzed spontaneously with rate constants ranging from 14 to 43 min?1. Magnesium, arsenate and phosphate accelerated hydrolysis of ADP-arsenate. From a comparison of the three methods, with ADP as the substrate, it is estimated that φR (i.e., the ratio between the quantum yields of ADP-arsenate and ATP for light emission from luciferase) is 0.19–0.23. Furthermore, arsenylation rates were 46–52% of phosphorylation rates in experiments with 30 μ M ADP and 0.8 mM arsenate or phosphate. Similarly, the Vapp for arsenylation of GDP or IDP was 47–59% of the Vapp for phosphorylation during measurements in the presence of 1 mM arsenate or phosphate. The Kapp(GDP) was higher during arsenylation than during phosphorylation; the Kapp(IDP) was about the same during arsenylation as during phosphorylation. It is suggested that a shift in the equilibrium of substrates and products on the enzyme, toward hydrolysis, is the main cause of the relatively low arsenylation rates. |
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Keywords: | Arsenylation ATP synthase Luciferase ADP - arsenate (R. rubrum) BChl bacteriochlorophyll Tricine |
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