A low-temperature heteronuclear NMR study of two exchanging conformations of metal-bound pyoverdin PaA from Pseudomonas aeruginosa

Under iron-deficient conditions, the Gram-negative bacterium Pseudomonas aeruginosa ATCC 15692 secretes a peptidic siderophore, pyoverdin PaA, composed of an aromatic chromophore derived from 2,3-diamino-6,7-dihydroxyquinoline and a partially cyclized octapeptide, D-Ser-L-Arg-D-Ser-L-FoOHOrn-(L-Lys-L-FoOHOrn-L-Thr-L-Thr) (FoOHOrn: delta N-formyl-delta N-hydroxyornithine), in which the C-terminal carboxyl group forms a peptidic bond with the primary amine of the L-Lys side chain. Ferric iron is chelated by the catechol group on the chromophore and the two hydroxyornithine side chains. In aqueous solution, the (1)H-NMR spectrum of pyoverdin PaA-Ga(III), in which Ga(III) is used instead of Fe(III) for spectroscopic purposes, showed clear evidence of exchange broadening, preventing further structural characterization. The use of cryo-solvents allowed measurements to be made at temperatures as low as 253 K where two distinct conformations with roughly equivalent populations could be observed. (13)C and (15)N labeling of pyoverdin PaA enabled complete assignment of both forms of pyoverdin PaA-Ga(III) at 253 and 267 K, using triple-resonance multidimensional NMR experiments commonly applied to doubly labeled proteins.