The catecholate siderophores of Azotobacter vinelandii: their affinity for iron and role in oxygen stress management

Abstract

In iron-limited medium, Azotobacter vinelandii strain UW produces three catecholate siderophores: the tricatecholate protochelin, the dicatecholate azotochelin and the monocatecholate aminochelin. Each siderophore was found to bind Fe³⁺ preferentially to Fe²⁺, in a ligand:Fe ratio of 1:1, 3:2 and 3:1, respectively. Protochelin had the highest affinity for Fe³⁺, with a calculated proton-independent solubility coefficient of 10⁴³⁹, comparable to ferrioxamine B. Iron-limited wild-type strain UW grown under N₂-fixing or nitrogen-sufficient conditions hyper-produced catecholate siderophores in response to oxidative stress caused by high aeration. In addition, superoxide dismutase activity was greatly diminished in iron-limited cells, whereas catalase activity was maintained. The ferredoxin I (Fdl)-negative A. vinelandii strain LM100 also hyper-produced catecholates, especially protochelin, under oxidative stress conditions, but had decreased activities of both superoxide dismutase and catalase, and was about 10 times more sensitive to paraquat than strain UW. Protochelin and azotochelin held Fe³⁺ firmly enough to prevent its reduction by.O^- ₂ and did not promote the generation of hydroxyl radical by the Fenton reaction. Ferric-aminochelin was unable to resist reduction by O^- ₂ and was a Fenton catalyst. These data suggest that under iron-limited conditions, A. vinelandii suffers oxidative stress caused by.O^- ₂. The catecholate siderophores azotochelin, and especially protochelin, are hyper-produced to offer chemical protection from oxidative damage catalysed by.O^- ₂ and Fe³⁺. The results are also consistent with Fdl being required for oxidative stress management in A. vinelandii.