The stability of the molybdenum-azotochelin complex and its effect on siderophore production in Azotobacter vinelandii

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 degrees 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-]/[MoO22+][L5-] = 10(35) 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 mu M, 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.