Kinetic analysis of copper transfer from a chaperone to its target protein mediated by complex formation

Cu Homeostasis in Bacteria: The Ins and Outs

Copper (Cu) is an essential trace element for all living organisms and used as cofactor in key enzymes of important biological processes, such as aerobic respiration or superoxide dismutation. However, due to its toxicity, cells have developed elaborate mechanisms for Cu homeostasis, which balance Cu supply for cuproprotein biogenesis with the need to remove excess Cu. This review summarizes our current knowledge on bacterial Cu homeostasis with a focus on Gram-negative bacteria and describes the multiple strategies that bacteria use for uptake, storage and export of Cu. We furthermore describe general mechanistic principles that aid the bacterial response to toxic Cu concentrations and illustrate dedicated Cu relay systems that facilitate Cu delivery for cuproenzyme biogenesis. Progress in understanding how bacteria avoid Cu poisoning while maintaining a certain Cu quota for cell proliferation is of particular importance for microbial pathogens because Cu is utilized by the host immune system for attenuating pathogen survival in host cells.

Characterization of the copper resistance mechanism and bioremediation potential of an Acinetobacter calcoaceticus strain isolated from copper mine sludge

Bioremediation is one of the most effective ways for removal of heavy metals and restoration of contaminated sites. This study investigated the copper (Cu) resistance mechanism and bioremediation potential of an Acinetobacter calcoaceticus strain KW3 isolated from sludge of Cu mine. The effect of Cu concentrations on the bacterial growth, biomass, and adsorption capacity, as well as the effect of contact time on the adsorption process was evaluated in a batch biosorption test. The strain exhibited strong tolerance of Cu, and the minimal inhibitory concentration was around 400 mg Cu²⁺ L^-1, at which the maximum adsorption capacity was 14.1 mg g^-1 dry cell mass. Cell walls and intracellular soluble components adsorbed 51.2% and 46.6% of Cu²⁺, respectively, suggesting that the strain not only adsorbed Cu²⁺ on the surface but also metastasized ions into cells. The adsorption and kinetic data were well fitted with Freundlich isotherm and Pseudo-second-order models, suggesting that cell surface had a high affinity for Cu²⁺ and the chemisorption could be the main adsorption mechanism. Scanning electron microscope and Fourier transform infrared spectroscopy analysis revealed that hydroxyl, carboxylic, amide, sulfate, and phosphate on cell walls might be involved in the biosorption process. Two-dimensional gel electrophoresis and MALDI-TOF/TOF mass spectrometry revealed that some oxidoreductases, in particular Cu resistance protein A (CopA) expression levels, were upregulated. Antioxidant defense and P_1B-type ATPases CopA efflux might play a crucial role in maintaining cellular homeostasis and intracellular detoxification. To our knowledge, this is the first time that Cu resistance mechanisms, especially intracellular enzymatic mechanisms, were identified in an A. calcoaceticus KW3 strain.

The Cu chaperone CopZ is required for Cu homeostasis in Rhodobacter capsulatus and influences cytochrome cbb3 oxidase assembly

Cu homeostasis depends on a tightly regulated network of proteins that transport or sequester Cu, preventing the accumulation of this toxic metal while sustaining Cu supply for cuproproteins. In Rhodobacter capsulatus, Cu-detoxification and Cu delivery for cytochrome c oxidase (cbb₃ -Cox) assembly depend on two distinct Cu-exporting P_1B -type ATPases. The low-affinity CopA is suggested to export excess Cu and the high-affinity CcoI feeds Cu into a periplasmic Cu relay system required for cbb₃ -Cox biogenesis. In most organisms, CopA-like ATPases receive Cu for export from small Cu chaperones like CopZ. However, whether these chaperones are also involved in Cu export via CcoI-like ATPases is unknown. Here we identified a CopZ-like chaperone in R. capsulatus, determined its cellular concentration and its Cu binding activity. Our data demonstrate that CopZ has a strong propensity to form redox-sensitive dimers via two conserved cysteine residues. A ΔcopZ strain, like a ΔcopA strain, is Cu-sensitive and accumulates intracellular Cu. In the absence of CopZ, cbb₃ -Cox activity is reduced, suggesting that CopZ not only supplies Cu to P_1B -type ATPases for detoxification but also for cuproprotein assembly via CcoI. This finding was further supported by the identification of a ~150 kDa CcoI-CopZ protein complex in native R. capsulatus membranes.

Generation of 34S-substituted protein-bound [4Fe-4S] clusters using 34S-L-cysteine

The ability to specifically label the sulphide ions of protein-bound iron-sulphur (FeS) clusters with ³⁴S isotope greatly facilitates structure-function studies. In particular, it provides insight when using either spectroscopic techniques that probe cluster-associated vibrations, or non-denaturing mass spectrometry, where the ∼+2 Da average increase per sulphide enables unambiguous assignment of the FeS cluster and, where relevant, its conversion/degradation products. Here, we employ a thermostable homologue of the O-acetyl-l-serine sulfhydrylase CysK to generate ³⁴S-substituted l-cysteine and subsequently use it as a substrate for the l-cysteine desulfurase NifS to gradually supply ³⁴S^2- for in vitro FeS cluster assembly in an otherwise standard cluster reconstitution protocol.

Mass spectrometric studies of Cu(I)-binding to the N-terminal domains of B. subtilis CopA and influence of bacillithiol

CopA is a Cu(I)-exporting transmembrane P_1B-type ATPase from Bacillus subtilis. It contains two N-terminal cytoplasmic domains, CopAab, which bind Cu(I) with high affinity and to form higher-order complexes with multiple Cu(I) ions. To determine the precise nature of these species, electrospray ionisation mass spectrometry (ESI-MS) under non-denaturing conditions was employed. Up to 1 Cu per CopAab resulted in Cu coordination to one or both CopAab domains. At >1 Cu/CopAab, two distinct dimeric charge state envelopes were observed, corresponding to distinct conformations, each with Cu₆(CopAab)₂ as its major form. The influence of the physiologically relevant low molecular weight thiol bacillithiol (BSH) on Cu(I)-binding to CopAab was assessed. Dimeric CopAab persisted in the presence of BSH, with previously undetected Cu₇(CopAab)₂ and Cu₆(CopAab)₂(BSH) forms apparent.

A Copper Relay System Involving Two Periplasmic Chaperones Drives cbb3-Type Cytochrome c Oxidase Biogenesis in Rhodobacter capsulatus

PccA and SenC are periplasmic copper chaperones required for the biogenesis of cbb₃-type cytochrome c oxidase ( cbb₃-Cox) in Rhodobacter capsulatus at physiological Cu concentrations. However, both proteins are dispensable for cbb₃-Cox assembly when the external Cu concentration is high. PccA and SenC bind Cu using Met and His residues and Cys and His residues as ligands, respectively, and both proteins form a complex during cbb₃-Cox biogenesis. SenC also interacts directly with cbb₃-Cox, as shown by chemical cross-linking. Here we determined the periplasmic concentrations of both proteins in vivo and analyzed their Cu binding stoichiometries and their Cu(I) and Cu(II) binding affinity constants ( K_D) in vitro. Our data show that both proteins bind a single Cu atom with high affinity. In vitro Cu transfer assays demonstrate Cu transfer both from PccA to SenC and from SenC to PccA at similar levels. We conclude that PccA and SenC constitute a Cu relay system that facilitates Cu delivery to cbb₃-Cox.

The metal chaperone Atox1 regulates the activity of the human copper transporter ATP7B by modulating domain dynamics

The human transporter ATP7B delivers copper to the biosynthetic pathways and maintains copper homeostasis in the liver. Mutations in ATP7B cause the potentially fatal hepatoneurological disorder Wilson disease. The activity and intracellular localization of ATP7B are regulated by copper, but the molecular mechanism of this regulation is largely unknown. We show that the copper chaperone Atox1, which delivers copper to ATP7B, and the group of the first three metal-binding domains (MBD1-3) are central to the activity regulation of ATP7B. Atox1-Cu binding to ATP7B changes domain dynamics and interactions within the MBD1-3 group and activates ATP hydrolysis. To understand the mechanism linking Atox1-MBD interactions and enzyme activity, we have determined the MBD1-3 conformational space using small angle X-ray scattering and identified changes in MBD dynamics caused by apo-Atox1 and Atox1-Cu by solution NMR. The results show that copper transfer from Atox1 decreases domain interactions within the MBD1-3 group and increases the mobility of the individual domains. The N-terminal segment of MBD1-3 was found to interact with the nucleotide-binding domain of ATP7B, thus physically coupling the domains involved in copper binding and those involved in ATP hydrolysis. Taken together, the data suggest a regulatory mechanism in which Atox1-mediated copper transfer activates ATP7B by releasing inhibitory constraints through increased freedom of MBD1-3 motions.

Orchestration of dynamic copper navigation – new and missing pieces

A general principle in all cells in the body is that an essential metal – here copper – is taken up at the plasma membrane, directed through cellular compartments for use in specific enzymes and pathways, stored in specific scavenging molecules if in surplus, and finally expelled from the cells.