Iron uptake and molecular recognition in Pseudomonas putida: Receptor mapping with ferrichrome and its biomimetic analogs

The presence of an Fe³⁺-ferrichrome uptake system in fluorescent Pseudomonas spp. was demonstrated, and its structural requirements were mapped in Pseudomonas putida with the help of biomimetic ferrichrome analogs. Growth tests, ⁵⁵Fe³⁺ uptake, and competition experiments demonstrated that the synthetic L-alanine derivative B5 inhibits the action of ferrichrome but does not facilitate Fe³⁺ transport, while the enantiomeric D-Ala derivative B6 fails to compete with ferrichrome. Contraction of the molecule's envelope by replacing L-Ala by glycine provided a synthetic carrier, B9, which fully simulates ferrichrome as a growth promoter. Sodium azide inhibited ⁵⁵Fe³⁺ uptake of the Gly derivative B9, suggesting an active transport process. These data demonstrate the chiral discrimination of the ferrichrome receptor and its sensitivity to subtle structural changes. They further confirm that receptor binding is a necessary but not sufficient condition for Fe³⁺ uptake to occur and suggest that binding to the receptor and transport proteins might rely on different recognition patterns.