Bacterial Hydroxamate Siderophores

Exochelins (322,323) are peptidic siderophores imm Mycobacterium spp. (see also below mycobactins). Exochelin MS (16) from M. smegmatis comprises (3-Ala and three W -OHOm units, which are linked by their atoms to acyl groups thus forming hydroxamic acids. 

Exochelin MN (17) from M. neoaurum contains W -methyl-W -hydroxy-Om linked by its to (3-Ala and by its carboxyl group to of Om, which in turn is bound amidically to cOHOm all amino acids are l configured. 

The Ee chelating properties of exochelin MN (17) were investigated in detail (pK values, chelation constants, redox equilibria, etc.) (87). In one publication (128) siderophores from Mycobacterium tuberculosis otherwise referred to as carboxymycobactins (see below Sect. 2.8) were also named exochelins. 

Vicibactin (18) (previously called hydroxamate K (61a)) from Rhizobium legu-minosarum is a macrocyclic trilactone consisting of A -acetyl-Ai -hydroxy-D-Om and (/ )-3-hydroxybutyric acid (91). 

Vicibactin 7101 from a mutant strain lacks the N-acetyl groups but shows comparable siderophore activity as demonstrated by uptake studies (91). 

---

Edwards, D. C., and Myneni, C. B. (2006). Near-edge x-ray absorption fine structure spectroscopy of bacterial hydroxamate siderophores in aqueous solutions. J. Phys. Chem. A 110, 11809-11818. 

N, 6-N-di(2,3-dihydroxybenzoyl)-L-lysine (58) is a siderophore produced by Azotobacter vinelandii which has only two catechol groups. However, of the catecholate siderophores by far the best studied is enterobactin. A major difference between hydroxamate and catecholate siderophores occurs in their utilization as transport agents. For the former, the iron complex is taken up by the bacterial cell, the iron released, and the hydroxamate siderophore re-secreted for additional iron chelation. In contrast, enterobactin is destroyed by enzymatic hydrolysis within the cell and therefore the ligand is not recycled. This hydrolysis of the amide linkages of the iron(III) enterobactin lowers the redox potential of the chelate complex sufficiently to allow iron reduction — and thus uptake of iron into the cell metabolism (59, 60). 

---