The stability of the molybdenum-azotochelin complex and its effect on siderophore production in Azotobacter vinelandii |
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Authors: | A-K Duhme R C Hider M J Naldrett R N Pau |
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Institution: | Anorganisch-chemisches Institut der Universit?t, Wilhelm-Klemm-Strasse 8, D-48149 Münster, Germany e-mail: duhmea@nwz.uni-muenster.de, DE Department of Pharmacy, King's College London, Manresa Road, London SW3?6LX, UK, GB Nitrogen Fixation Laboratory, John Innes Centre, Norwich Research Park, Norwich NR4?7UH, UK, GB
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Abstract: | Azotobacter vinelandii produces five siderophores with different metal binding properties, depending on the concentrations of Fe(III) and molybdate
in the growth medium. The three lower protonation constants of the unusual bis(catecholamide) siderophore azotochelin (L)
were determined by a simultaneous spectrophotometric and potentiometric titration as log K
5=3.65(5), log K
4=7.41(3) and log K
3=8.54(4). The metal-ligand equilibrium constant for MoO2(L)]3– was obtained from analysis of the absorbance concentration data: at 20 °C and pH 6.6, log K
eq=4(1). Based on an average log K
a value of 12.1 for the two basic phenolic oxygens of azotochelin, the equilibrium formation constant was converted into the
conventional formation constant K
f(MoL) = MoO2L3
–]/MoO2
2+]L5
–] = 1035 M–1. To assess the influence of molybdenum-siderophore interactions on metal uptake in A. vinelandii, the dose-response effect of molybdate in the growth medium on siderophore biosynthesis was followed by UV-vis spectroscopy
and HPLC. It could be shown that the formation of molybdenum siderophore complexes clearly reduces the concentration of free
siderophores available for iron solubilization. Furthermore, in media with initial molybdate concentrations up to 100 μM,
the molybdenum azotochelin complex is the predominant molybdenum species, suggesting that azotochelin might also possess sequestering
activity towards molybdenum. Even higher molybdate levels result in a complete repression of the synthesis of the tetradentate
siderophore azotochelin, while they initiate the alternative release of the more efficient iron chelator, the hexadentate
siderophore protochelin.
Received: 20 April 1998 / Accepted: 29 June 1998 |
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Keywords: | Molybdenum Azotochelin Azotobacter vinelandii Siderophore Metal uptake |
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