The Mycobactin Biosynthesis Pathway: A Prospective Therapeutic Target in the Battle against Tuberculosis

The alarming rise in drug-resistant clinical cases of tuberculosis (TB) has necessitated the rapid development of newer chemotherapeutic agents with novel mechanisms of action. The mycobactin biosynthesis pathway, conserved only among the mycolata family of actinobacteria, a group of intracellularly surviving bacterial pathogens that includes Mycobacterium tuberculosis, generates a salicyl-capped peptide mycobactin under iron-stress conditions in host macrophages to support the iron demands of the pathogen. This in vivo essentiality makes this less explored mycobactin biosynthesis pathway a promising endogenous target for novel lead-compounds discovery. In this Perspective, we have provided an up-to-date account of drug discovery efforts targeting selected enzymes (MbtI, MbtA, MbtM, and PPTase) from the mbt gene cluster (mbtA-mbtN). Furthermore, a succinct discussion on non-specific mycobactin biosynthesis inhibitors and the Trojan horse approach adopted to impair iron metabolism in mycobacteria has also been included in this Perspective.

The kinetics of dimethylhydroxypyridinone interactions with iron(iii) and the catalysis of iron(iii) ligand exchange reactions: implications for bacterial iron transport and combination chelation therapies

Many microbes acquire environmental Fe by secreting organic chelators, siderophores, which possess the characteristics of a high and specific binding affinity for iron(iii) that results in the formation of thermodynamically stable, and kinetically inert iron(iii) complexes. Mechanisms to overcome the kinetic inertness include the labilization of iron(iii) by means of ternary complex formation with small chelators. This study describes a kinetic investigation of the labilization of iron(iii) between two stable binding sites, the prototypical siderophore ferrioxamine B and EDTA, by the bidentate siderophore mimic, 1,2-dimethyl-3-hydroxy-4-pyridinone (L1, H(DMHP)). The proposed mechanism is substantiated by investigating the iron(iii) exchange reaction between ferrioxamine B and H(DMHP) to form Fe(DMHP)3, as well as the iron(iii) exchange from Fe(DMHP)3 to EDTA. It is also shown that H(DMHP) is a more effective catalyst for the iron(iii) exchange reaction than bidentate hydroxamate chelators reported previously, supporting the hypothesis that chelator structure and iron(iii) affinity influence low denticity ligand facilitated catalysis of iron(iii) exchange reactions. The results are also discussed in the context of the design and use of combination chelator therapies in the treatment of Fe overload in humans.

Trace metal complexation by the triscatecholate siderophore protochelin: structure and stability

Although siderophores are generally viewed as biological iron uptake agents, recent evidence has shown that they may play significant roles in the biogeochemical cycling and biological uptake of other metals. One such siderophore that is produced by A. vinelandii is the triscatecholate protochelin. In this study, we probe the solution chemistry of protochelin and its complexes with environmentally relevant trace metals to better understand its effect on metal uptake and cycling. Protochelin exhibits low solubility below pH 7.5 and degrades gradually in solution. Electrochemical measurements of protochelin and metal-protochelin complexes reveal a ligand half-wave potential of 200 mV. The Fe(III)Proto(3-) complex exhibits a salicylate shift in coordination mode at circumneutral to acidic pH. Coordination of Mn(II) by protochelin above pH 8.0 promotes gradual air oxidation of the metal center to Mn(III), which accelerates at higher pH values. The Mn(III)Proto(3-) complex was found to have a stability constant of log β(110) = 41.6. Structural parameters derived from spectroscopic measurements and quantum mechanical calculations provide insights into the stability of the Fe(III)Proto(3-), Fe(III)H(3)Proto, and Mn(III)Proto(3-) complexes. Complexation of Co(II) by protochelin results in redox cycling of Co, accompanied by accelerated degradation of the ligand at all solution pH values. These results are discussed in terms of the role of catecholate siderophores in environmental trace metal cycling and intracellular metal release.