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

Pyoverdin PaA is a siderophore excreted by Pseudomonas aeruginosa, a c ommon and pathogenic bacterium. It belongs to a family of fluorescent iron(III) biological ligands. Its chemical structure shows three biden tate coordination sites, two hydroxamic acids and a dihydroxyquinoline -type function bound to a peptidic chain. Spectrophotometric, potentio metric 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 neutra l and strong ferric complexes at physiological pH. The thermodynamic s tability of its ferric and ferrous complexes is very similar to that o f linear trihydroxamate siderophores, such as ferrioxamine B (Desferal ) and coprogen, in spite of its anchored structure and of a catechol-t ype binding site. As for trihydroxamate ligands, the reduction potenti al was found to be accessible to physiological reductant systems and a n iron(III) release mechanism via a reduction step could be proposed. Kinetic studies carried out by either classical or stopped-flow spectr ophotometry have provided the kinetic parameters related to the format ion and the dissociation of the ferric pyoverdin PaA complexes in acid ic conditions. Stepwise mechanisms revealed the flexibility of this st rong ligand. The binding of the terminal hydroxamic acid of pyoverdin PaA is proposed to be the rate limiting step of the iron(III) coordina tion process. The dissociation mechanism showed an unfolding of the si derophore leading to protonated ferric intermediate species correspond ing to the successive protonation of the binding sites. Accessible red uction potential to physiological reductants, fast iron(III) uptake ki netics and efficient assistance of the protons to the iron(III) releas e mechanism are favorable features for iron biological transport by py overdin PaA.