Structure of ferric pseudobactin: a siderophore from a plant growth promoting Pseudomonas

Abstract

Plant growth promoting Pseudomonas B10 and its yellow-green, fluorescent Fe transport agent (siderophore) pseudobactin enhance the growth of the potato and control certain phytopathogenic microorganisms. The structure of pseudobactin was determined by single-crystal X-ray diffraction methods using counter data. The structure consisted of a linear hexapeptide, L-Lys-D-threo-.beta.-OH-Asp-L-Ala-D-allo-Thr-L-Ala-D-N.delta.-OH-Orn, in which the N.delta.-OH N of the ornithine was cyclized with the C-terminal carboxyl group and the N.epsilon.-amino group of the lysine was linked via an amide bond to a fluorescent quinoline derivative. The Fe-chelating groups consisted of a hydroxamate group derived from N.delta.-hydroxyornithine, an .alpha.-hydroxy acid derived from .beta.-hydroxyaspartic acid and an o-dihydroxy aromatic group derived from the quinoline moiety. The combination of metal-chelating ligands and the alternating L- and D-amino acids was unusual. Pseudobactin crystallized as a single coordination isomer with the .LAMBDA. absolute configuration. This is the first structural determination of a fluorescent siderophore. In the crystal structure, ferric pseudobactin formed a dimer, which constituted the asymmetric unit. The asymmetric unit also contained 26 water molecules. The molecules in the dimer were related by a pseudo-2-fold symmetry axis. Red-brown crystals of ferric pseudobactin (C42H57N12O16Fe.cntdot.13H2O), obtained from pyridine-acetic acid buffer solution equilibrated with water, conformed to space group I2 with a = 29.006 (23) .ANG., b= 14.511 (13) .ANG., c = 28.791 (21) .ANG. and .beta. = 96.06 (5).degree. at -135 (2) .degree.C. For 8 molecules/unit cell, the calculated density was 1.38 g/cm3; the observed density was 1.40 g/cm3. The structure was refined by least-squares methods with anisotropic thermal parameters for all nonhydrogen atoms to a final R factor of 0.08 (8989 observed reflections).