Beyond copper: examining the significance of His-mutations in mycobacterial GroEL1 HRCT for Ni(II) complex stability and formation

Recently, we have studied the coordination chemistry of the Cu(II)-histidine-rich C-terminal tail (HRCT) complex of the mycobacterial GroEL1 protein. The structure of this domain differs significantly compared to the well-known methionine-glycine-rich GroEL chaperonin - it was predicted that mycobacterial GroEL1 could play a significant role in the metal homeostasis of Mycobacteria, especially copper. However, we found that this particular domain's pattern also repeats in a number of Ni(II)-binding proteins. Here, we present the studies concerning the properties of GroEL1 HRCT as a ligand for Ni(II) ions. For this purpose, we chose eight model peptides: L1 - Ac-DHDHHHGHAH, L2 - Ac-DKPAKAEDHDHHHGHAH, and 6 mutants of the latter in the pH range of 2-11. We examined the stoichiometry, stability, and spectroscopic features of copper complexes. We noticed that similar to the Cu(II)-complex, the presence of a Lys5 residue significantly increases the stability of the system. The impact of His mutations was also examined and carefully studied using NMR spectroscopy. His9 and His13 are the crucial residues for Ni(II) binding, whereas His12 has minimal relevance in complex formation.

Exploring binding preferences: Cu(II), Ni(II), and Zn(II) complexes of mycobacterial GroEL1 His-rich and Glu/His-rich domains

Mycobacterial histidine-rich GroEL1 protein significantly differs from the well-known methionine-glycine-rich GroEL chaperonin and most preferably participates in Cu(II) homeostasis. Some GroEL1 proteins, however, do not possess six but only three histidine residues and more acidic residues that can function as binding sites for metal ions. To evaluate the importance of this difference, we examined and compared the properties of GroEL1 His-rich or Glu/His-rich C-terminal domains as ligands for Cu(II), Ni(II), and Zn(II) ions. We studied the stoichiometry, stability, and binding sites of Cu(II)/Ni(II)/Zn(II) complexes of two model peptides: XEN = Ac-DKPEEEEDGHGHAH (M. xenopi) and ABS = Ac-DKPAEEADHGHGHHGHAH (M. abscessus) in the pH range 2-11. In the case of Cu(II), Ni(II), and Zn(II) complexes of XEN and ABS, ABS always formed more stable complexes. For XEN, there seemed to be no preference for Ni(II) or Zn(II) ions. In contrast, for ABS, Zn(II) formed a complex that was slightly more stable than the one formed by Ni(II). This may be due to the 6 His residues, which preferentially interact with Zn(II) rather than Ni(II). The study identified that an equilibrium of complexes-known as polymorphism-may occur in ABS complexes. Therefore, distinct sets of histidine residues may be involved in metal binding.

Histidine-Rich C-Terminal Tail of Mycobacterial GroEL1 and Its Copper Complex─The Impact of Point Mutations

The mycobacterial histidine-rich GroEL1 protein differs significantly compared to the well-known methionine/glycine-rich GroEL chaperonin. It was predicted that mycobacterial GroEL1 can play a significant role in the metal homeostasis of Mycobacteria but not, as its analogue, in protein folding. In this paper, we present the properties of the GroEL1 His-rich C-terminus as a ligand for Cu(II) ions. We studied the stoichiometry, stability, and spectroscopic features of copper complexes of the eight model peptides: L1─Ac-DHDHHHGHAH, L2─Ac-DKPAKAEDHDHHHGHAH, and six mutants of L2 in the pH range of 2-11. We revealed the impact of adjacent residues to the His-rich fragment on the complex stability: the presence of Lys and Asp residues significantly increases the stability of the system. The impact of His mutations was also examined: surprisingly, the exchange of each single His to the Gln residue did not disrupt the ability of the ligand to provide three binding sites for Cu(II) ions. Despite the most possible preference of the Cu(II) ion for the His9-His13 residues (Ac-DKPAKAEDHDHHH-) of the model peptide, especially the His11 residue, the study shows that there is not only one possible binding mode for Cu(II). The significance of this phenomenon is very important for the GroEL1 function─if the single mutation occurs naturally, the protein would be still able to interact with the metal ion.

Zn(II) and Cd(II) Complexes of AMT1/MAC1 Homologous Cys/His-Rich Domains: So Similar yet So Different

Infections caused by Candida species are becoming seriously dangerous and difficult to cure due to their sophisticated mechanisms of resistance. The host organism defends itself from the invader, e.g., by increasing the concentration of metal ions. Therefore, there is a need to understand the overall mechanisms of metal homeostasis in Candida species. One of them is associated with AMT1, an important virulence factor derived from Candida glabrata, and another with MAC1, present in Candida albicans. Both of the proteins possess a homologous Cys/His-rich domain. In our studies, we have chosen two model peptides, L680 (Ac-₁₀ACMECVRGHRSSSCKHHE₂₇-NH₂, MAC1, Candida albicans) and L681 (Ac-₁₀ACDSCIKSHKAAQCEHNDR₂₈-NH₂, AMT1, Candida glabrata), to analyze and compare the properties of their complexes with Zn(II) and Cd(II). We studied the stoichiometry, thermodynamic stability, and spectroscopic parameters of the complexes in a wide pH range. When competing for the metal ion in the equimolar mixture of two ligands and Cd(II)/Zn(II), L680 forms more stable complexes with Cd(II) while L681 forms more stable complexes with Zn(II) in a wide pH range. Interestingly, a Glu residue was responsible for the additional stability of Cd(II)-L680. Despite a number of scientific reports suggesting Cd(II) as an efficient surrogate of Zn(II), we showed significant differences between the Zn(II) and Cd(II) complexes of the studied peptides.

Coordination Properties of the Zinc Domains of BigR4 and SmtB Proteins in Nickel Systems─Designation of Key Donors

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.

The histidine-rich α5 domains of SmtB (L4, Mycobacterium tuberculosis) and BigR4 (L1, Mycobacterium smegmatis) were studied as ligands for Ni(II). Point mutation analysis of L1 revealed that His102 and 106 preferably bind metal ions. The general Ni(II)-binding motif for both of the ligands was established as HX3HX3DX3HX2ED. L1 forms more stable complexes than L4 due to the stabilizing effect of arginine residues.

Sometimes less is more-the impact of the number of His residues on the stability of Zn(II)-SmtB and BigR4 α-5 domain complexes

The increasing number of antibiotic-resistant pathogens has become one of the major health problems of modern times, including infections caused by Mycobacterium tuberculosis. One of the possible mammalian immune system responses to mycobacterial infection is the increase of the zinc(ii) concentration in phagosomes to a toxic level. The mycobacterial SmtB protein belongs to the family of ArsR/SmtB transcription regulators. In the presence of high concentrations of metals, SmtB dissociates from DNA and activates the expression of metal efflux proteins. In this work, we focus on the α5 zinc(ii) binding domains of SmtB/BigR4 proteins (the latter being the SmtB homolog from non-pathogenic M. smegmatis) and two mutants of BigR4. We will be taking a closer look at the coordination modes and thermodynamic properties of their zinc(ii) complexes. The study points out the specificity of metal-ligand interactions and describes the effect of mutations on the coordination properties of the studied systems. The stabilities of the zinc(ii) complexes were determined by potentiometry. The coordination sites were determined by NMR experiments and DFT calculations. The comparison of complex stabilities reveals that the Zn(ii)-BigR4 species are more stable than the Zn(ii)-SmtB complexes. His mutations strongly affect the stability of the complexes and the coordination modes of the metal ion. Exchanging one of the histidines for alanine causes, surprisingly, an increase in the stability of zinc(ii) complexes with the studied domain. This was confirmed by potentiometric and DFT methods. This work can be considered as a bioinorganic introduction to the discovery of new strategies in M. tuberculosis infection treatment based on zinc(ii)-sensitive mechanisms.

[Iatrogenic retinal breaks during vitrectomy: retrospective study]

PURPOSE

To establish the incidence and characteristics of iatrogenic retinal breaks occurring during vitrectomy and to propose a protocol for their management.

METHODS

Retrospective study of 195 consecutive vitrectomies performed in 2001. Minimum follow up was 3 months for all cases.

RESULTS

13 patients with iatrogenic retinal breaks (6.6%) have been detected. In 8 cases the breaks affected the peripheral retina, in 4 cases they were located in the posterior pole and there was one retinal dialysis. In 4 cases there were more than one retinal break. 9 patients (70%) were vitrectomized for diabetic retinal complications. In 12 cases, the breaks were detected during surgery and treated with endolaser and/or cryoretinopexy (3 cases located inferiorly). Nevertheless 7 cases (54%) developed a retinal detachment, 2 of which could be reattached.

CONCLUSIONS

Among iatrogenic retinal breaks produced during vitrectomy, the peripheral ones are slightly more frequent than the breaks in the posterior pole. Iatrogenic breaks occur more frequently in diabetic patients, having a bad prognosis and often developing a retinal detachment despite endolaser or cryoretinopexy.