Chemistry and biology of pyoverdines, Pseudomonas primary siderophores

Abstract

Pyoverdine is the generic name given to a vast family of fluorescent green-yellowish pigments produced by Pseudomonas species. Pseudomonas aeruginosa is an opportunistic pathogen, particularly infecting humans with compromised natural defenses. These infections result in significantly higher morbidity, longer hospitalization, increased mortality rates and excess health care costs. P. aeruginosa is very difficult to eradicate because of an intrinsic coupled with an adaptive resistance to a wide variety of classical antibiotics. When subjected to iron starvation conditions, Pseudomonas bacteria synthesize pyoverdines, their primary siderophores, to acquire iron from the extracellular medium. These molecules are not only powerful iron(III) scavengers but efficient iron(III) transporters as well. Three distinct structural parts constitute pyoverdines, i.e. (i) the fluorescent chromophore, deriving from a dihydroxyquinoline, attached via its carbonyl group to (ii) a type-specific peptide composed of 6 to 14 amino acids and (iii) a small side chain corresponding to a carboxylic acid derivative. Their chemical structure show three bidentate chelating sites including a catechol and two hydroxamates, leading to an octahedral geometry when complexed to iron(III). While the chromophore group is common to all pyoverdines, their peptide moiety differs among strains and species by the number, length, composition and configuration of amino acids. Following chelation with iron(III), the newly formed pyoverdine-Fe complex is recognized by a specific outer membrane transporter, namely FpvA, and reenters the cell where the iron is released from the pyoverdine into the periplasm for further incorporation into bacterial proteins. The remaining apo-pyoverdine is then recycled and secreted back to the extracellular medium by efflux pumps. Besides, the role of pyoverdines in P. aeruginosa is not only limited to scavenge iron from the bacterial environment. Indeed, these siderophores act as signal molecules for the production of acute virulence factors and are involved in biofilm formation as well. The ongoing expanding pathogenicity of P. aeruginosa has become a major public health problem, and finding alternative strategies to classical antibiotics is urgently needed. Pyoverdines along with the iron pathway recently gained interest among academical researchers as potential new approaches to "fight" the bacteria. This review describes the classification of the nearly 60 pyoverdines identified so far, their structural and chemical properties and their (bio)synthesis. The different mechanisms underlying the steps of a pyoverdine's life in Pseudomonas are detailed as well: the affinity by which a pyoverdine chelates iron(III), the description of the interactions inducing the siderophore-receptor recognition, the specific transport of the pyoverdine-Fe(III) complex. As pyoverdine production and severe infections are linked, we will also report on situations where pyoverdines are considered as being P. aeruginosa Achilles heel: the propensity of FpvA to transport exo-pyoverdines, organic synthesis of pyoverdines and analogs, grafting of antibiotics on pyoverdines in a Trojan Horse strategy.