A reevaluation of iron binding by Mycobactin J

Abstract

The complex stability constant (log β) and the free iron concentration (pM) are used to compare the relative strength of iron binding by siderophores. Direct measurements of these thermodynamic parameters are often not possible for siderophores due to very large log β values ranging from 30 to 50. Instead, siderophore iron(III)-binding constants are determined by competitive experiments with other strong chelators with known values, such as EDTA. Iron(III) binding constants of water-insoluble siderophores, such as the mycobactins produced by the mycobacterium family, have never been directly measured. Since mycobactins contain two hydroxamic acid binding motifs, their log β values have been assumed to be comparable to those of other hydroxamate-based siderophores like desferrioxamine B, at ~ 30. However, exochelin MN, another mycobacterial siderophore that contains two hydroxamic acid moieties, has a log β of 39.1 and a pM of 31.1, which makes it among the strongest siderophores known. We have found that mycobactin J, the amphiphilic siderophore of Mycobacterium paratuberculosis, can remove iron(III) from TrenCAM (log β = 43.6) within 1 min in methanol. This surprising result indicates that log β for mycobactin J is ~ 43 and the ligand exchange kinetics in methanol is fast. The results imply that mycobactins are capable of removing iron quickly from very strongly binding siderophores in a cellular milieu. We propose a model mechanism for iron acquisition by pathogenic mycobacteria in vivo. This model explains how the host iron captured by siderophores can be returned to the invading pathogen even in the absence of active uptake mechanisms.

Mycobacterium marinum produces distinct mycobactin and carboxymycobactin siderophores to promote growth in broth and phagocytes

“…However, MBT associated with membranes might also contribute to iron acquisition. Interestingly, MBT (i.e., its gallium complex) was reported to also localise to host cells lipid droplets, which accumulated in the vicinity of phagosomes containing Escherichia coli (Luo, Fadeev, & Groves, ; McQueen & Groves, ). Because mycobacteria utilise lipid droplets to harvest and metabolise lipids (Barisch & Soldati, ), it is possible that iron‐loaded MBT can be transported to MCVs along this pathway.…”

Alison Butler: papers in celebration of her 2018 ACS Alfred Bader Award in Bioorganic or Bioinorganic Chemistry

“…In the reviews, the chemical biology, coordination chemistry and reactivity of β-hydroxyaspartate siderophores, bis-hydroxamate macrocyclic siderophores and catecholate siderophores are covered by Hardy and Butler [7], Codd et al [8] and McRose, Seyedsayamdost and Morel [9], respectively. Groves and coauthors [10] find that the stability constant for the amphiphilic mycobactin J siderophore of Mycobacterium paratuberculosis is much higher than previously reported, which when considered along with the observed rapid exchange kinetics suggests that mycobactins are capable of removing iron quickly from very high-affinity siderophores in cellular environments. The mechanism of Fe(III)-amphi-enterobactin uptake by Vibrio campbellii and V. harveyi strains is shown by Naka et al [11] to occur via a novel outer membrane receptor, FapA, which can also recognize enterobactin.…”

Chemical Interactions of Cryptic Actinomycete Metabolite 5‐Alkyl‐1,2,3,4‐tetrahydroquinolines through Aggregate Formation

“…Mycobacteria typically produce two sets of siderophores:water-soluble molecules and lipid-soluble ones. [13,14] Hydrophilic siderophores are thought to capture cytosolic iron ions and pass them to the lipid-soluble ones associated with lipid droplets in macrophage cells,t hus mycobacteria can collect iron efficiently during infection. Another example is hydroxyalkylquinolines synthesized from the same biosynthetic pathway in Burkholderia thailandensis.…”

“…Variations in the length and degree of unsaturation of the fatty acid give the metabolites a broad membrane affinity, which, through cell‐association, may prevent diffusion of siderophores in the marine environment. Mycobacteria typically produce two sets of siderophores: water‐soluble molecules and lipid‐soluble ones . Hydrophilic siderophores are thought to capture cytosolic iron ions and pass them to the lipid‐soluble ones associated with lipid droplets in macrophage cells, thus mycobacteria can collect iron efficiently during infection.…”