The increasing number of
antibiotic-resistant pathogens has become one of the foremost health problems of modern times. One of the most lethal and multidrug-resistant bacteria is Mycobacterium tuberculosis (Mtb), which causes
tuberculosis (TB). TB continues to engulf health systems due to the significant development of bacterial multidrug-resistant strains. Mammalian immune system response to mycobacterial
infection includes, but is not limited to, increasing the concentration of
zinc(II) and other divalent
metal ions in phagosome vesicles up to toxic levels.
Metal ions are necessary for the survival and virulence of bacteria but can be highly toxic to organisms if their concentrations are not strictly controlled. Therefore, understanding the mechanisms of how bacteria use
metal ions to maintain their optimum concentrations and survive under lethal environmental conditions is essential. The mycobacterial SmtB
protein, one of the
metal-dependent transcription regulators of the ArsR/SmtB family, dissociates from
DNA in the presence of high concentrations of metals, activating the expression of
metal efflux
proteins. In this work, we explore the properties of α5
metal-binding domains of SmtB/BigR4
proteins (the latter being the SmtB homolog from nonpathogenic Mycobacterium smegmatis), and two mutants of BigR4 as
ligands for
nickel(II)
ions. The study focuses on the specificity of
metal-
ligand interactions and describes the effect of mutations on the coordination properties of the studied systems. The results of this research reveal that the Ni(II)-BigR4 α5 species are more stable than the Ni(II)-SmtB α5 complexes. His mutations, exchanging one of the histidines for
alanine, cause a decrease in the stability of Ni(II) complexes. Surprisingly, the lack of His102 resulted also in increased involvement of
acidic amino acids in the coordination. The results of this study may help to understand the role of critical mycobacterial virulence factor─SmtB in
metal homeostasis. Although SmtB prefers Zn(II) binding, it may also bind
metal ions that prefer other coordination modes, for example, Ni(II). We characterized the properties of such complexes in order to understand the nature of mycobacterial SmtB when acting as a
ligand for
metal ions, given that
nickel and
zinc ArsR family
proteins possess analogous
metal-binding motifs. This may provide an introduction to the design of a new antimicrobial strategy against the pathogenic bacterium M.
tuberculosis.