BACTERIAL IRON TRANSPORT - COORDINATION PROPERTIES OF PYOVERDIN PAA, A PEPTIDIC SIDEROPHORE OF PSEUDOMONAS-AERUGINOSA

Pyoverdin PaA is a siderophore excreted by Pseudomonas aeruginosa, a common and pathogenic bacterium. It belongs to a family of fluorescent iron(III) biological ligands. Its chemical structure shows three bidentate coordination sites, two hydroxamic acids and a dihydroxyquinoline-type function bound to a peptidic chain. Spectrophotometric, potentiometric and cyclic voltammetric measurements allowed the determination of the acid-base functions of the free siderophore as well as the iron(III) and iron(II) coordination properties. Pyoverdin PaA forms neutral and strong ferric complexes at physiological pH. The thermodynamic stability of its ferric and ferrous complexes is very similar to that of linear trihydroxamate siderophores, such as ferrioxamine B (Desferal) and coprogen, in spite of its anchored structure and of a catechol-type binding site. As for trihydroxamate ligands, the reduction potential was found to be accessible to physiological reductant systems and an iron(III) release mechanism via a reduction step could be proposed. Kinetic studies carried out by either classical or stopped-flow spectrophotometry have provided the kinetic parameters related to the formation and the dissociation of the ferric pyoverdin PaA complexes in acidic conditions. Stepwise mechanisms revealed the flexibility of this strong ligand. The binding of the terminal hydroxamic acid of pyoverdin PaA is proposed to be the rate limiting step of the iron(III) coordination process. The dissociation mechanism showed an unfolding of the siderophore leading to protonated ferric intermediate species corresponding to the successive protonation of the binding sites. Accessible reduction potential to physiological reductants, fast iron(III) uptake kinetics and efficient assistance of the protons to the iron(III) release mechanism are favorable features for iron biological transport by pyoverdin PaA.