Copper metallochaperones

A novel near-infrared turn-on and ratiometric fluorescent probe capable of copper(ii) ion determination in living cells

A near-infrared ratiometric fluorescent probe CR-Ac based on a coumarin-benzopyrylium platform has been developed for selective detection of Cu2+. The cell imaging data revealed the capabilities of CR-Ac in monitoring the dynamic changes of subcellular Cu2+ and the quantification of Cu2+ levels in living cells.

Cu(I) Controls Conformational States in Human Atox1 Metallochaperone: An EPR and Multiscale Simulation Study

Atox1 is a human copper metallochaperone that is responsible for transferring copper ions from the main human copper transporter, hCtr1, to ATP7A/B in the Golgi apparatus. Atox1 interacts with the Ctr1 C-terminal domain as a dimer, although it transfers the copper ions to ATP7A/B in a monomeric form. The copper binding site in the Atox1 dimer involves Cys12 and Cys15, while Lys60 was also suggested to play a role in the copper binding. We recently showed that Atox1 can adopt various conformational states, depending on the interacting protein. In the current study, we apply EPR experiments together with hybrid quantum mechanics–molecular mechanics molecular dynamics simulations using a recently developed semiempirical density functional theory approach, to better understand the effect of Atox1’s conformational states on copper coordination. We propose that the flexibility of Atox1 occurs owing to protonation of one or more of the cysteine residues, and that Cys15 is an important residue for Atox1 dimerization, while Cys12 is a critical residue for Cu(I) binding. We also show that Lys60 electrostatically stabilizes the Cu(I)–Atox1 dimer.

Metallothioneins regulate ATP7A trafficking and control cell viability during copper deficiency and excess

Copper (Cu) is an essential, yet potentially toxic nutrient, as illustrated by inherited diseases of copper deficiency and excess. Elevated expression of the ATP7A Cu exporter is known to confer copper tolerance, however, the contribution of metal-binding metallothioneins is less clear. In this study, we investigated the relative contributions of ATP7A and the metallothioneins MT-I and MT-II to cell viability under conditions of Cu excess or deficiency. Although the loss of ATP7A increased sensitivity to low Cu concentrations, the absence of MTs did not significantly affect Cu tolerance. However, the absence of all three proteins caused a synthetic lethal phenotype due to extreme Cu sensitivity, indicating that MTs are critical for Cu tolerance only in the absence of ATP7A. A lack of MTs resulted in the trafficking of ATP7A from the trans-Golgi complex in a Cu-dependent manner, suggesting that MTs regulate the delivery of Cu to ATP7A. Under Cu deficiency conditions, the absence of MTs and / or ATP7A enhanced cell proliferation compared to wild type cells, suggesting that these proteins compete with essential Cu-dependent pathways when Cu is scarce. These studies reveal new roles for ATP7A and metallothioneins under both Cu deficiency and excess.

Chromatographic detection of low-molecular-mass metal complexes in the cytosol of Saccharomyces cerevisiae

Fluorescence-based chelators are commonly used to probe labile low-molecular-mass (LMM) metal pools in the cytosol of eukaryotic cells, but such chelators destroy the complexes of interest during detection. The objective of this study was to use chromatography to directly detect such complexes. Towards this end, 47 batches of cytosol were isolated from fermenting S. cerevisiae yeast cells and passed through a 10 kDa cut-off membrane. The metal contents of the cytosol and resulting flow-through solution (FTS) were determined. FTSs were applied to a size-exclusion LC column located in an anaerobic refrigerated glove box. The LC system was coupled to an online inductively-coupled-plasma mass spectrometer (ICP-MS) for detection of individual metals. Iron-detected chromatograms of cytosolic FTSs from WT cells exhibited 2-4 major species with apparent masses between 500-1300 Da. Increasing the iron concentration in the growth medium 40-fold increased the overall intensity of these peaks. Approximately 3 LMM cytosolic copper complexes with apparent masses between 300-1300 Da were also detected; their LC intensities were weak, but these increased with increasing concentrations of copper in the growth medium. Observed higher-mass copper-detected peaks were tentatively assigned to copper-bound metallothioneins Cup1 and Crs5. FTSs from strains in which Cup1 or the Cox17 copper chaperone were deleted altered the distribution of LMM copper complexes. LMM zinc- and manganese-detected species were also present in cytosol, albeit at low concentrations. Supplementing the growth medium with zinc increased the intensity of the zinc peak assigned to Crs5 but the intensities of LMM zinc complexes were unaffected. Phosphorus-detected chromatograms were dominated by peaks at apparent masses 400-800 Da, with minor peaks at 1000-1500 Da in some batches. Sulfur chromatograms contained a low-intensity peak that comigrated with a glutathione standard; quantification suggested a GSH concentration in the cytosol of ca. 13 mM. A second LMM sulfur peak that migrated at an apparent mass of 100 Da was also evident.

Functional characterization and expression patterns of PnATX genes under different abiotic stress treatments in Populus

The copper chaperone ATX1 has been investigated previously in the herbaceous plants Arabidopsis and rice. However, the molecular mechanisms of ATX1 underlying copper transport and functional characteristics in the woody plant Populus are poorly understood. In this study, PnATX1 and PnATX2 of Populus simonii × P. nigra were identified and characterized. Sequence analysis showed that PnATXs contained the metal-binding motif MXCXXC in the N-terminus and a lysine-rich region. Phylogenetic analysis of ATX protein sequences revealed that PnATXs were clustered in the same group as AtATX1. PnATX proteins were localized in the cytoplasm and nucleus. Tissue-specific expression analysis showed that PnATX1 and PnATX2 were expressed in all analyzed tissues and, in particular, expressed to a higher relative expression level in young leaves. Quantitative real-time PCR analysis indicated that each PnATX gene was differentially expressed in different tissues under treatments with copper, zinc, iron, jasmonate and salicylic acid (SA). The copper-response element GTAC, methyl jasmonate and salicylic acid responsiveness elements and other cis-acting elements were identified in the PnATX1 and PnATX2 promoters. Expression of β-glucuronidase driven by the PnATX1 promoter was observed in the apical meristem of 7-day-old Arabidopsis transgenic seedlings, and the signal strength was not influenced by deficient or excessive copper conditions. Both PnATX1 and PnATX2 functionally rescued the defective phenotypes of yeast atx1Δ and sod1Δ strains. Under copper excess and deficiency conditions, transgenic Arabidopsis atx1 mutants harboring 35S::PnATX constructs exhibited root length and fresh weight similar to those of the wild type and higher than those of Arabidopsis atx1 mutants. Superoxide dismutase activity decreased in transgenic lines compared with that of atx1 mutants, whereas peroxidase and catalase activities increased significantly under excess copper. The results provide a basis for elucidating the role of Populus PnATX genes in copper homeostasis.

Comparison of the pH-dependent formation of His and Cys heptapeptide complexes of nickel(II), copper(II), and zinc(II) as determined by ion mobility-mass spectrometry

The analog methanobactin (amb) peptide with the sequence ac-His₁ -Cys₂ -Gly₃ -Pro₄ -Tyr₅ -His₆ -Cys₇ (amb_5A ) will bind the metal ions of zinc, nickel, and copper. To further understand how amb_5A binds these metals, we have undertaken a series of studies of structurally related heptapeptides where one or two of the potential His or Cys binding sites have been replaced by Gly, or the C-terminus has been blocked by amidation. The studies were designed to compare how these metals bind to these sequences in different pH solutions of pH 4.2 to 10 and utilized native electrospray ionization (ESI) with ion mobility-mass spectrometry (IM-MS) which allows for the quantitative analysis of the charged species produced during the reactions. The native ESI conditions were chosen to conserve as much of the solution-phase behavior of the amb peptides as possible and an analysis of how the IM-MS results compare with the expected solution-phase behavior is discussed. The oligopeptides studied here have applications for tag-based protein purification methods, as therapeutics for diseases caused by elevated metal ion levels or as inhibitors for metal-protein enzymes such as matrix metalloproteinases.

A novel colorimetric and "turn-off" fluorescent probe based on catalyzed hydrolysis reaction for detection of Cu2+ in real water and in living cells

A novel and highly selective fluorescent 1,8-naphthalimide-based probe, 3, was designed and synthesized for rapid Cu²⁺ detection in a CH₃CN-H₂O (3:1, v/v, pH = 7.4) solution by means of a distinct hydrolysis mechanism via its Cu²⁺-promoting feature. Upon treatment with Cu²⁺, the fluorescence response of probe 3 at 550 nm abruptly decreased, which was visible to the naked eye, and this response was accompanied by a clear change of the color of the solution; the color changed from the original yellow color to colorless. This color change occurred due to the Cu²⁺-promoted hydrolysis of 3, which yielded a fluorescence-quenched product. It is inspiring that probe 3 exhibited excellent sensitivity, a short response time and strong anti-interference recognition. Compared with the allowable amount of Cu²⁺ (∼20 μM) in drinking water, the detection limit of 3 for Cu²⁺ is calculated to be 9.15 nM, which is much lower than the amount defined by standards. The probe can be successfully applied for the determination of Cu²⁺ in real aqueous samples. Furthermore, probe 3 can be used as a fluorescent sensor to detect Cu²⁺ in biological environments, demonstrating its low toxicity to organisms and good cell permeability in live cell imaging.

Fluorescence "Turn On-Off" Sensing of Copper (II) Ions Utilizing Coumarin-Based Chemosensor: Experimental Study, Theoretical Calculation, Mineral and Drinking Water Analysis

Herein, we report the preparation of a fluorescent sensor based on coumarin derivative for copper (II) ion sensing in CH₃CN/HEPES media. 6,7-dihydroxy-3-(4-(trifluoro)methylphenyl)coumarin (HMAC) sensor was fabricated and analyzed by spectroscopic techniques. The sensor demonstrates "turn on-off" fluorescence quenching in the presence of copper (II) ions at 458 nm. A clear complex between the chemosensor HMAC and copper (II) ions was characterized by ESI-MS as well as the Job's method. Also, the limit of detection (LOD, 3σ/k) value was determined as 24.5 nM in CH₃CN/HEPES (95/5, v/v) buffer media (pH = 7.0). This value is lower than the admissible level of copper (II) ions in drinking water (maximum 31.5 μM) reported by EU Water Framework Directive (WFD) and World Health Organization (WHO) guidelines. The theoretical calculations (density functional theory, DFT) have been performed for the geometric optimized structures. As a final stage, real sample analyses have successfully been performed by using HMAC, as well as ICP-OES method. The relative standard deviation for copper (II) in mineral and drinking water samples has been determined to be below 0.15% and recovery values are in the range of 95.48-109.20%.

Single-cell tracking demonstrates copper chaperone Atox1 to be required for breast cancer cell migration

Copper ions are needed for several hallmarks of cancer. However, the involved pathways, mechanisms, and copper-binding proteins are mostly unknown. We recently found that cytoplasmic Antioxidant 1 copper chaperone (Atox1), which is up-regulated in breast cancer, is localized at the lamellipodia edges of aggressive breast cancer cells. To reveal molecular insights into a putative role in cell migration, we here investigated breast cancer cell (MDA-MB-231) migration by video microscopy as a function of Atox1. Tracking of hundreds of individual cells (per condition) over a 9-h time series revealed that cell migration velocity and directionality are significantly reduced upon Atox1 silencing in the cells. Because silencing of the copper transporter ATP7A also reduced cell migration, these proteins appear to be on the same pathway, suggesting that their well-known copper transport activity is involved. In-cell proximity ligation assays demonstrated that Atox1, ATP7A, and the proenzyme of lysyl oxidase (LOX; copper-loaded via ATP7A) are all in close proximity and that LOX activity is reduced upon Atox1 silencing in the cells. Since LOX is an established player in cancer cell migration, our results imply that Atox1 mediates breast cancer cell migration via coordinated copper transport in the ATP7A-LOX axis. Because individual cell migration is an early step in breast cancer metastasis, Atox1 levels in tumor cells may be a predictive measure of metastasis potential and serve as a biomarker for copper depletion therapy.

Monoisoamyl DMSA reduced copper-induced neurotoxicity by lowering 8-OHdG level, amyloid beta and Tau protein expressions in Sprague-Dawley rats

Copper dyshomeostasis has long been linked with several neurodegenerative disorders.

Pyrene-appended bipyridine hydrazone ligand as a turn-on sensor for Cu2+ and its bioimaging application

A pyrene-appended bipyridine hydrazone-based ligand, HL, was synthesized and characterized by spectroscopic methods. Upon complexation with Cu(ii), HL formed a hexanuclear paddlewheel metal–organic macrocycle (MOM) via self-assembly with a high association constant with the molecular formula of [Cu₆L₆(NO₃)₆]. Intermolecular and intramolecular π–π interactions were demonstrated in this hexanuclear Cu(ii) complex. Further, it was observed that HL had the potential to detect a trace level of Cu(ii) ion selectively among a wide range of biologically relevant metal ions in aqueous medium at physiological pH. Using HL, it was feasible to sense copper(ii) ions in living cells due to its good cell permeability and high solubility under physiological conditions along with its high IC₅₀ value. The low detection limit, high sensitivity and good reproducibility make this Cu–sensor very promising. The complex (MOM) formed between the ligand and Cu(ii) was found to be 1 : 1 on the basis of fluorescence titrations and was confirmed by ESI-MS. Moreover, single-crystal study of the hexanuclear self-assembled fluorescent species provided better insight into its chemistry, e.g. coordination environment and binding mode, unlike most of the metal sensors due to the lack of a single-crystal structure of the metal sensor complex. Cytotoxicity assay and bioimaging were performed in living cells (Vero cells), giving green fluorescent images. Fluorescence lifetime measurements and theoretical calculations were carried out. The morphology and topographic details on the surface of the metal–organic macrocycle (MOM) were studied by field-emission scanning electron microscopy (FESEM).

A pyrene-based “turn-on” Cu(ii) sensor provides a chemiluminescent Cu₆ metal organic macrocycle (MOM) applicable for live cell imaging.

Inhibiting the copper efflux system in microbes as a novel approach for developing antibiotics

Five out of six people receive at least one antibiotic prescription per year. However, the ever-expanding use of antibiotics in medicine, agriculture, and food production has accelerated the evolution of antibiotic-resistant bacteria, which, in turn, made the development of novel antibiotics based on new molecular targets a priority in medicinal chemistry. One way of possibly combatting resistant bacterial infections is by inhibiting the copper transporters in prokaryotic cells. Copper is a key element within all living cells, but it can be toxic in excess. Both eukaryotic and prokaryotic cells have developed distinct copper regulation systems to prevent its toxicity. Therefore, selectively targeting the prokaryotic copper regulation system might be an initial step in developing next-generation antibiotics. One such system is the Gram-negative bacterial CusCFBA efflux system. CusB is a key protein in this system and was previously reported to play an important role in opening the channel for efflux via significant structural changes upon copper binding while also controlling the assembly and disassembly process of the entire channel. In this study, we aimed to develop novel peptide copper channel blockers, designed by in silico calculations based on the structure of CusB. Using a combination of magnetic resonance spectroscopy and various biochemical methods, we found a lead peptide that promotes copper-induced cell toxicity. Targeting copper transport in bacteria has not yet been pursued as an antibiotic mechanism of action. Thus, our study lays the foundation for discovering novel antibiotics.

Enzyme Mimicking Based on the Natural Melanin Particles from Human Hair

Natural enzymes are mainly composed by the protein part and metallic cofactor part, both of which work cooperatively to achieve high catalytic activity. Here, natural melanin particles (NMPs) were extracted from human hair and further bound with metal ions to mimic natural enzymes. The different metal-bound NMPs (M-NMPs) exhibited different enzyme-like activities with great promise in diverse biomedical applications. It was found that Fe-bound NMPs (Fe-NMPs) showed outstanding peroxidase (POD)-like activity that possessed potential in antibacterial applications, and Mn-bound NMPs (Mn-NMPs) displayed catalase (CAT)-like activity with a remarkable radiotherapy sensitization effect in cancer therapy. Besides, Cu-bound NMPs (Cu-NMPs) could serve as combined POD, superoxide dismutase (SOD), and CAT alternatives, which exhibited prominent reactive oxygen species (ROS) scavenging ability, revealing great potential in anti-inflammation. The versatile enzyme-like activities of M-NMPs derived from hair might give extensive perspective for designing biomedical materials and provide a promising tool in solving biomedical problems.

Restoration of myocellular copper-trafficking proteins and mitochondrial copper enzymes repairs cardiac function in rats with diabetes-evoked heart failure

Diabetes impairs systemic copper regulation, and acts as a major independent risk factor for heart failure (HF) wherein mitochondrial dysfunction is a key pathogenic process. Here we asked whether diabetes might alter mitochondrial structure/function and thus impair cardiac performance by damaging myocellular pathways that mediate cell-copper homeostasis. We measured activity of major mitochondria-resident copper-enzymes cytochrome c oxidase (mt-Cco) and superoxide dismutase 1 (mt-Sod1); expression of three main mitochondrial copper-chaperones [Cco copper chaperone 17 (Cox17), Cox11, and mitochondria-resident copper chaperone for Sod1 (mt-Ccs)]; of copper-dependent Cco-assembly protein Sco1; and regulation of mitochondrial biogenesis, in left-ventricular (LV) tissue from groups of non-diabetic-control, untreated-diabetic, and divalent-copper-selective chelator-treated diabetic rats. Diabetes impaired LV pump function; ∼halved LV-copper levels; substantively decreased myocellular expression of copper chaperones, and enzymatic activity of mt-Cco and mt-Sod1. Divalent-copper chelation with triethylenetetramine improved cardiac pump function, restored levels of myocardial copper, the copper chaperones, and Sco1; and enzymatic activity of mt-Cco and mt-Sod1. Copper chelation also restored expression of the key mitochondrial biogenesis regulator, peroxisome-proliferator-activated receptor gamma co-activator-1α (Pgc-1α). This study shows for the first time that altered myocardial copper-trafficking is a key pathogenic process in diabetes-evoked HF. We also describe a novel therapeutic effect of divalent-copper-selective chelation, namely restoration of myocellular copper trafficking, which is thus revealed as a potentially tractable target for novel pharmacological intervention to improve cardiac function.

A pathway for assembling [4Fe-4S]2+ clusters in mitochondrial iron-sulfur protein biogenesis

During its late steps, the mitochondrial iron-sulfur cluster (ISC) assembly machinery leads to the formation of [4Fe-4S] clusters. In vivo studies revealed that several proteins are implicated in the biosynthesis and trafficking of [4Fe-4S] clusters in mitochondria. However, they do not provide a clear picture into how these proteins cooperate. Here, we showed that three late-acting components of the mitochondrial ISC assembly machinery (GLRX5, BOLA3, and NFU1) are part of a ISC assembly pathway leading to the synthesis of a [4Fe-4S]²⁺ cluster on NFU1. We showed that the [2Fe-2S]²⁺ GLRX5-BOLA3 complex transfers its cluster to monomeric apo NFU1 to form, in the presence of a reductant, a [4Fe-4S]²⁺ cluster bound to dimeric NFU1. The cluster formation on NFU1 does not occur with [2Fe-2S]²⁺ GLRX5, and thus, the [4Fe-4S] cluster assembly pathway is activated only in the presence of BOLA3. These results define NFU1 as an 'assembler' of [4Fe-4S] clusters, that is, a protein able of converting two [2Fe-2S]²⁺ clusters into a [4Fe-4S]²⁺ cluster. Finally, we found that the [4Fe-4S]²⁺ cluster bound to NFU1 has a coordination site which is easily accessible to sulfur-containing ligands, as is typically observed in metallochaperones. This finding supports a role for NFU1 in promoting rapid and controlled cluster-exchange reaction.

The classic metal-sensing transcription factor MTF1 promotes myogenesis in response to copper

Metal-regulatory transcription factor 1 (MTF1) is a conserved metal-binding transcription factor in eukaryotes that binds to conserved DNA sequence motifs, termed metal response elements. MTF1 responds to both metal excess and deprivation, protects cells from oxidative and hypoxic stresses, and is required for embryonic development in vertebrates. To examine the role for MTF1 in cell differentiation, we use multiple experimental strategies [including gene knockdown (KD) mediated by small hairpin RNA and clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9), immunofluorescence, chromatin immunopreciptation sequencing, subcellular fractionation, and atomic absorbance spectroscopy] and report a previously unappreciated role for MTF1 and copper (Cu) in cell differentiation. Upon initiation of myogenesis from primary myoblasts, both MTF1 expression and nuclear localization increased. Mtf1 KD impaired differentiation, whereas addition of nontoxic concentrations of Cu+-enhanced MTF1 expression and promoted myogenesis. Furthermore, we observed that Cu+ binds stoichiometrically to a C terminus tetra-cysteine of MTF1. MTF1 bound to chromatin at the promoter regions of myogenic genes, and Cu addition stimulated this binding. Of note, MTF1 formed a complex with myogenic differentiation (MYOD)1, the master transcriptional regulator of the myogenic lineage, at myogenic promoters. These findings uncover unexpected mechanisms by which Cu and MTF1 regulate gene expression during myoblast differentiation.-Tavera-Montañez, C., Hainer, S. J., Cangussu, D., Gordon, S. J. V., Xiao, Y., Reyes-Gutierrez, P., Imbalzano, A. N., Navea, J. G., Fazzio, T. G., Padilla-Benavides, T. The classic metal-sensing transcription factor MTF1 promotes myogenesis in response to copper.

The effects of copper on photosynthesis and biomolecules yield in Chlorolobion braunii

Our knowledge of the effects of copper on microalgal physiology is largely based on studies conducted with high copper concentrations; much less is known when environmentally relevant copper levels come into question. Here, we evaluated the physiology of Chlorolobion braunii exposed to free copper ion concentrations between 5.7 × 10^-9 and 5.0 × 10^-6  mol · L^-1 , thus including environmentally relevant values. Population growth and maximum photosynthetic quantum yield of PSII were determined daily during the 96 h laboratory controlled experiment. Exponentially-growing cells (48 h) were analyzed for effective quantum yield and rapid light curves (RLC), and total lipids, proteins, carbohydrates, chlorophyll a and carotenoids were determined. The results showed that growth rates and population density decreased gradually as copper increased in experiment, but the photosynthetic parameters (maximum and effective quantum yields) and photochemical quenching (qP) decreased only at the highest free copper concentration tested (5.0 × 10^-6 mol · L^-1 ); nonphotochemical quenching (NPQ) increased gradually with copper increase. The RLC parameters Ek and rETRmax were inversely proportional to copper concentration, while α and Im decreased only at 5.0 × 10^-6 mol · L^-1 . The effects of copper in biomolecules yield (mg · L^-1 ) varied depending on the biomolecule. Lipid yield increased at free copper concentration as low as 2.5 × 10^-8 mol · L^-1 , but proteins and carbohydrates were constant throughout.

Metalation and Maturation of Zinc Ectoenzymes: A Perspective

Numerous zinc ectoenzymes are folded and activated in the compartments of the early secretory pathway, such as the ER and the Golgi apparatus, before reaching their final destination. During this process, zinc must be incorporated into the active site; therefore, metalation of the nascent protein is indispensable for the expression of the active enzyme. However, to date, the molecular mechanism underlying this process has been poorly investigated. This is in sharp contrast to the physiological and pathophysiological roles of zinc ectoenzymes, which have been extensively investigated over the past decades. This manuscript concisely outlines the present understanding of zinc ectoenzyme activation through metalation by zinc and compares this with copper ectoenzyme activation, in which elaborate copper metalation mechanisms are known. Moreover, based on the comparison, several hypotheses are discussed. Approximately 80 years have passed since the first zinc enzyme was identified; therefore, it is necessary to improve our understanding of zinc ectoenzymes from a biochemical perspective, which will further our understanding of their biological roles.

Serum, erythrocyte and urinary concentrations of iron, copper, selenium and zinc do not change during an incremental test to exhaustion in either normothermic or hyperthermic conditions

AIM

The aim of this study was to evaluate the effect of the performance of an incremental exercise test until exhaustion in normothermic and hyperthermic conditions on serum, erythrocyte and urine concentrations of Iron (Fe), Copper (Cu), Selenium (Se) and Zinc (Zn).

METHODS

Nineteen adult males (age: 22.58 ± 1.06 years) performed two maximum incremental exercise tests on a cycloergometer in normothermia (22 ± 2 °C) and hyperthermia (42±2 °C) separated by 48 h. Urine, serum and erythrocyte samples were collected before and after each test.

RESULTS

Serum Se (p < 0.01) and Cu (p < 0.05) levels were altered after each test, but the significance disappeared with the correction for haematocrit. The rest of the values did not undergo alterations in either condition.

CONCLUSIONS

It seems that a higher stimulus is necessary to obtain changes in these minerals. The study reveals the need to correct serum concentrations concerning possible changes in these volumes after an acute effort.

The Design and Synthesis of Fluorescent Coumarin Derivatives and Their Study for Cu2+ Sensing with an Application for Aqueous Soil Extracts

A series of fluorescent coumarin derivatives 2a–e were systematically designed, synthesized and studied for their Cu²⁺ sensing performance in aqueous media. The sensitivities and selectivities of the on-to-off fluorescent Cu²⁺ sensing signal were in direct correlation with the relative arrangements of the heteroatoms within the coordinating moieties of these coumarins. Probes 2b and 2d exhibited Cu²⁺ concentration dependent and selective fluorescence quenching, with linear ranges of 0–80 μM and 0–10 μM, and limits of detection of 0.14 μM and 0.38 μM, respectively. Structural changes of 2b upon Cu²⁺ coordination were followed by fluorescence titration, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), mass spectrometry, and single crystal X-ray diffraction on the isolated Cu²⁺-coumarin complex. The results revealed a 1:1 stoichiometry between 2b and Cu²⁺, and that the essential structural features for Cu²⁺-selective coordination are the coumarin C=O and a three-bond distance between the amide NH and heterocyclic N. Probe 2b was also used to determine copper (II) levels in aqueous soil extracts, with recovery rates over 80% when compared to the standard soil analysis method: inductively coupled plasma-mass spectrometry (ICP-MS).

Relationships Between Copper-Related Proteomes and Lifestyles in β Proteobacteria

Copper is an essential transition metal whose redox properties are used for a variety of enzymatic oxido-reductions and in electron transfer chains. It is also toxic to living beings, and therefore its cellular concentration must be strictly controlled. We have performed in silico analyses of the predicted proteomes of more than one hundred species of β proteobacteria to characterize their copper-related proteomes, including cuproproteins, i.e., proteins with active-site copper ions, copper chaperones, and copper-homeostasis systems. Copper-related proteomes represent between 0 and 1.48% of the total proteomes of β proteobacteria. The numbers of cuproproteins are globally proportional to the proteome sizes in all phylogenetic groups and strongly linked to aerobic respiration. In contrast, environmental bacteria have considerably larger proportions of copper-homeostasis systems than the other groups of bacteria, irrespective of their proteome sizes. Evolution toward commensalism, obligate, host-restricted pathogenesis or symbiosis is globally reflected in the loss of copper-homeostasis systems. In endosymbionts, defense systems and copper chaperones have disappeared, whereas residual cuproenzymes are electron transfer proteins for aerobic respiration. Lifestyle is thus a major determinant of the size and composition of the copper-related proteome, and it is particularly reflected in systems involved in copper homeostasis. Analyses of the copper-related proteomes of a number of species belonging to the Burkholderia, Bordetella, and Neisseria genera indicates that commensals are in the process of shedding their copper-homeostasis systems and chaperones to greater extents yet than pathogens.

Biochemical pathway for the biosynthesis of the CuA center in bacterial cytochrome c oxidase

The di-copper center Cu_A is an essential metal cofactor in cytochrome oxidase (Cox) of mitochondria and many prokaryotes, mediating one-electron transfer from cytochrome c to the site for oxygen reduction. Cu_A is located in subunit II (CoxB) of Cox and protrudes into the periplasm of Gram-negative bacteria or the mitochondrial intermembrane space. How the two copper ions are brought together to build CoxB·Cu_A is the subject of this review. It had been known that the reductase TlpA and the metallochaperones ScoI and PcuC are required for Cu_A formation in bacteria, but the mechanism of copper transfer has emerged only recently for the Bradyrhizobium diazoefficiens system. It consists of the following steps: (a) TlpA keeps the active site cysteine pair of CoxB in its dithiol state as a prerequisite for metal insertion; (b) ScoI·Cu²⁺ rapidly forms a transient complex with apo-CoxB; (c) PcuC, loaded with Cu¹⁺ and Cu²⁺ , dissociates this complex to CoxB·Cu²⁺ , and a second PcuC·Cu¹⁺ ·Cu²⁺ transfers Cu¹⁺ to CoxB·Cu²⁺ , yielding mature CoxB·Cu_A . Variants of this pathway might exist in other bacteria or mitochondria.

Folding of copper proteins: role of the metal?

Copper is a redox-active transition metal ion required for the function of many essential human proteins. For biosynthesis of proteins coordinating copper, the metal may bind before, during or after folding of the polypeptide. If the metal binds to unfolded or partially folded structures of the protein, such coordination may modulate the folding reaction. The molecular understanding of how copper is incorporated into proteins requires descriptions of chemical, thermodynamic, kinetic and structural parameters involved in the formation of protein-metal complexes. Because free copper ions are toxic, living systems have elaborate copper-transport systems that include particular proteins that facilitate efficient and specific delivery of copper ions to target proteins. Therefore, these pathways become an integral part of copper protein folding in vivo. This review summarizes biophysical-molecular in vitro work assessing the role of copper in folding and stability of copper-binding proteins as well as protein-protein copper exchange reactions between human copper transport proteins. We also describe some recent findings about the participation of copper ions and copper proteins in protein misfolding and aggregation reactions in vitro.

Copper homeostasis as target of both consolidated and innovative strategies of anti-tumor therapy

BACKGROUND

Copper was reported to be involved in the onset and progression of cancer. Proteins in charge of copper uptake and distribution, as well as cuproenzymes, are altered in cancer. More recently, proteins involved in signaling cascades, regulating cell proliferation, and anti-apoptotic protein factors were found to interact with copper. Therefore, therapeutic strategies using copper complexing molecules have been proposed for cancer therapy and used in clinical trials.

OBJECTIVES

This review will focus on novel findings about the involvement of copper and cupro-proteins in cancer dissemination process, epithelium to mesenchymal transition and vascularization. Particularly, implication of well-established (e.g. lysil oxidase) or newly identified copper-binding proteins (e.g. MEMO1), as well as their interplay, will be discussed. Moreover, we will describe recently synthesized copper complexes, including plant-derived ones, and their efficacy in contrasting cancer development.

CONCLUSIONS

The research on the involvement of copper in cancer is still an open field. Further investigation is required to unveil the mechanisms involved in copper delivery to the novel copper-binding proteins, which may identify other possible gene and protein targets for cancer therapy.

A Histidine Residue and a Tetranuclear Cuprous-thiolate Cluster Dominate the Copper Loading Landscape of a Copper Storage Protein from Streptomyces lividans

The chemical basis for protecting organisms against the toxic effect imposed by excess cuprous ions is to constrain this through high-affinity binding sites that use cuprous-thiolate coordination chemistry. In bacteria, a family of cysteine rich four-helix bundle proteins utilise thiolate chemistry to bind up to 80 cuprous ions. These proteins have been termed copper storage proteins (Csp). The present study investigates cuprous ion loading to the Csp from Streptomyces lividans (SlCsp) using a combination of X-ray crystallography, site-directed mutagenesis and stopped-flow reaction kinetics with either aquatic cuprous ions or a chelating donor. We illustrate that at low cuprous ion concentrations, copper is loaded exclusively into an outer core region of SlCsp via one end of the four-helix bundle, facilitated by a set of three histidine residues. X-ray crystallography reveals the existence of polynuclear cuprous-thiolate clusters culminating in the assembly of a tetranuclear [Cu₄ (μ₂ -S-Cys)₄ (Ν^δ1 -His)] cluster in the outer core. As more cuprous ions are loaded, the cysteine lined inner core of SlCsp fills with cuprous ions but in a fluxional and dynamic manner with no evidence for the assembly of further intermediate polynuclear cuprous-thiolate clusters as observed in the outer core. Using site-directed mutagenesis a key role for His107 in the efficient loading of cuprous ions from a donor is established. A model of copper loading to SlCsp is proposed and discussed.

Arabidopsis thaliana Hcc1 is a Sco-like metallochaperone for CuA assembly in Cytochrome c Oxidase

The assembly of the Cu_A site in Cytochrome c Oxidase (COX) is a critical step for aerobic respiration in COX-dependent organisms. Several gene products have been associated with the assembly of this copper site, the most conserved of them belonging to the Sco family of proteins, which have been shown to perform different roles in different organisms. Plants express two orthologs of Sco proteins: Hcc1 and Hcc2. Hcc1 is known to be essential for plant development and for COX maturation, but its precise function has not been addressed until now. Here, we report the biochemical, structural and functional characterization of Arabidopsis thaliana Hcc1 protein (here renamed Sco1). We solved the crystal structure of the Cu⁺¹ -bound soluble domain of this protein, revealing a tri coordinated environment involving a CxxxCx_n H motif. We show that AtSco1 is able to work as a copper metallochaperone, inserting two Cu⁺¹ ions into the Cu_A site in a model of CoxII. We also show that AtSco1 does not act as a thiol-disulfide oxido-reductase. Overall, this information sheds new light on the biochemistry of Sco proteins, highlighting the diversity of functions among them despite their high structural similarities. DATABASE: PDB entry 6N5U (Crystal structure of Arabidopsis thaliana ScoI with copper bound).

Execution of julolidine based derivative as bifunctional chemosensor for Zn2+ and Cu2+ ions: Applications in bio-imaging and molecular logic gate

A simple julolidine based chemosensor (JT) was designed and synthesized by single condensation step. JT displayed excellent selectivity and sensitivity with on-off responses towards Zn²⁺ and Cu²⁺ over other biologically relevant metal ions in aqueous media. Upon addition of Zn²⁺ ions, JT exhibited a significant blue shift in emission followed by turn-on enhancement while with Cu²⁺, the fluorescence intensity of JT was completely vanished. The 1:1 binding affinity between JT and Zn²⁺/Cu²⁺ was proposed by Job's plot analysis. The detection limit for Zn²⁺ and Cu²⁺ ions reached at 3.5 × 10^-8 M and 1.46 × 10^-6 M, respectively. The sensing mechanism of JT with Zn²⁺/Cu²⁺ was supported by DFT calculations. Based on photophysical studies and its reversibility environment with EDTA, molecular logic gates were fabricated. Furthermore, JT was successfully established to detect intracellular Zn²⁺ ions in live cells by turn-on response.

Biochemistry of Copper Site Assembly in Heme-Copper Oxidases: A Theme with Variations

Copper is an essential cofactor for aerobic respiration, since it is required as a redox cofactor in Cytochrome c Oxidase (COX). This ancient and highly conserved enzymatic complex from the family of heme-copper oxidase possesses two copper sites: Cu_A and Cu_B. Biosynthesis of the oxidase is a complex, stepwise process that requires a high number of assembly factors. In this review, we summarize the state-of-the-art in the assembly of COX, with special emphasis in the assembly of copper sites. Assembly of the Cu_A site is better understood, being at the same time highly variable among organisms. We also discuss the current challenges that prevent the full comprehension of the mechanisms of assembly and the pending issues in the field.

Copper-zinc superoxide dismutase (Sod1) activation terminates interaction between its copper chaperone (Ccs) and the cytosolic metal-binding domain of the copper importer Ctr1

Copper-zinc superoxide dismutase (Sod1) is a critical antioxidant enzyme that rids the cell of reactive oxygen through the redox cycling of a catalytic copper ion provided by its copper chaperone (Ccs). Ccs must first acquire this copper ion, directly or indirectly, from the influx copper transporter, Ctr1. The three proteins of this transport pathway ensure careful trafficking of copper ions from cell entry to target delivery, but the intricacies remain undefined. Biochemical examination of each step in the pathway determined that the activation of the target (Sod1) regulates the Ccs·Ctr1 interaction. Ccs stably interacts with the cytosolic C-terminal tail of Ctr1 (Ctr1c) in a copper-dependent manner. This interaction becomes tripartite upon the addition of an engineered immature form of Sod1 creating a stable Cu(I)-Ctr1c·Ccs·Sod1 heterotrimer in solution. This heterotrimer can also be made by the addition of a preformed Sod1·Ccs heterodimer to Cu(I)-Ctr1c, suggestive of multiple routes to the same destination. Only complete Sod1 activation (i.e. active site copper delivery and intra-subunit disulfide bond formation) breaks the Sod1·Ccs·Ctr1c complex. The results provide a new and extended view of the Sod1 activation pathway(s) originating at cellular copper import.

Copper relay path through the N-terminus of Wilson disease protein, ATP7B

In human cells, copper (Cu) ions are transported by the cytoplasmic Cu chaperone Atox1 to the Wilson disease protein (ATP7B) in the Golgi for loading of Cu-dependent enzymes. ATP7B is a membrane-spanning protein which, in contrast to non-mammalian homologs, has six cytoplasmic metal-binding domains (MBDs). To address the reason for multiple MBDs, we introduced strategic mutations in which one, two or three MBDs had been blocked for Cu binding via cysteine-to-serine mutations (but all six MBDs are present in all) in a yeast system that probes Cu flow through Atox1 and ATP7B. The results, combined with earlier work, support a mechanistic model in which MBD1-3 forms a regulatory unit of ATP7B Cu transport. Cu delivery via Atox1 to this unit, followed by loading of Cu in MBD3, promotes release of inhibitory interactions. Whereas the Cu site in MBD4 can be mutated without a large effect, an intact Cu site in either MBD5 or MBD6 is required for Cu transport. All MBDs, expressed as single-domain proteins, can replace Atox1 and deliver Cu to full-length ATP7B. However, only MBD6 can deliver Cu to truncated ATP7B where all six MBDs are removed, suggesting a docking role for this structural unit.

Copper chaperone ATOX1 is required for MAPK signaling and growth in BRAF mutation-positive melanoma

Copper (Cu) is a tightly regulated micronutrient that functions as a structural or catalytic cofactor for specific proteins essential for a diverse array of biological processes. While the study of the extremely rare genetic diseases, Menkes and Wilson, has highlighted the requirement for proper Cu acquisition and elimination in biological systems for cellular growth and proliferation, the importance of dedicated Cu transport systems, like the Cu chaperones ATOX1 and CCS, in the pathophysiology of cancer is not well defined. We found that ATOX1 was significantly overexpressed in human blood, breast, and skin cancer samples, while CCS was significantly altered in human brain, liver, ovarian, and prostate cancer when compared to normal tissue. Further analysis of genetic expression data in Cancer Cell Line Encyclopedia (CCLE) revealed that ATOX1 is highly expressed in melanoma cell lines over other cancer cell lines. We previously found that Cu is required for BRAF^V600E-driven MAPK signaling and melanomagenesis. Here we show that genetic loss of ATOX1 decreased BRAF^V600E-dependent growth and signaling in human melanoma cell lines. Pharmacological inhibition of ATOX1 with a small molecule, DCAC50, decreased the phosphorylation of ERK1/2 and reduced the growth of BRAF mutation-positive melanoma cell lines in a dose-dependent manner. Taken together, these results suggest that targeting the Cu chaperone ATOX1 as a novel therapeutic angle in BRAF^V600E-driven melanomas.

Identification of novel rice (Oryza sativa) HPP and HIPP genes tolerant to heavy metal toxicity

HPP (heavy metal associated plant protein) and HIPP (heavy metal associated isoprenylated plant protein) are a group of metal-binding metallochaperones playing crucial roles in metal homeostasis and detoxification. Up to now, only few of them have been functionally identified in plants. Here, we identified 54 HPP and HIPP genes in rice genome. Analysis of the transcriptome datasets of the rice genome exposed to cadmium (Cd) revealed 17 HPP/HIPP genes differentially expressed, with 11 being upregulated (>2 fold change, p < 0.05). Comprehensive analysis of transcripts by qRT-PCR showed that both types of genes displayed diverse expression pattern in rice under excess manganese (Mn), copper (Cu) and Cd stress. Multiple genomic analyses of HPPs/HIPPs including phylogenesis, conserved domains and motifs, genomic arrangement and genomic and tandem duplication were performed. To identify the role of the genes, OsHIPP16, OsHIPP34 and OsHIPP60 were randomly selected to express in yeast (Saccharomyces cerevisiae) mutants pmrl, cup2, ycf1 and zrc1, exhibiting sensitivity to Mn, Cu, Cd and Zn toxicity, respectively. Complementation test showed that the transformed cells accumulated more metals in the cells, but their growth status was improved. To confirm the functional role, two mutant oshipp42 lines defective in OsHIPP42 expression were identified under metal stress. Under normal condition, no difference of growth between the oshipp42 mutant and wild-type plants was observed. Upon excess Cu, Zn, Cd and Mn, the oshipp42 lines grew weaker than the wild-type. Our work provided a novel source of heavy metal-binding genes in rice that can be potentially used to develop engineered plants for phytoremediation in heavy metal-contaminated soils.

Unraveling the Impact of Cysteine-to-Serine Mutations on the Structural and Functional Properties of Cu(I)-Binding Proteins

Appropriate maintenance of Cu(I) homeostasis is an essential requirement for proper cell function because its misregulation induces the onset of major human diseases and mortality. For this reason, several research efforts have been devoted to dissecting the inner working mechanism of Cu(I)-binding proteins and transporters. A commonly adopted strategy relies on mutations of cysteine residues, for which Cu(I) has an exquisite complementarity, to serines. Nevertheless, in spite of the similarity between these two amino acids, the structural and functional impact of serine mutations on Cu(I)-binding biomolecules remains unclear. Here, we applied various biochemical and biophysical methods, together with all-atom simulations, to investigate the effect of these mutations on the stability, structure, and aggregation propensity of Cu(I)-binding proteins, as well as their interaction with specific partner proteins. Among Cu(I)-binding biomolecules, we focused on the eukaryotic Atox1-ATP7B system, and the prokaryotic CueR metalloregulator. Our results reveal that proteins containing cysteine-to-serine mutations can still bind Cu(I) ions; however, this alters their stability and aggregation propensity. These results contribute to deciphering the critical biological principles underlying the regulatory mechanism of the in-cell Cu(I) concentration, and provide a basis for interpreting future studies that will take advantage of cysteine-to-serine mutations in Cu(I)-binding systems.

Structural basis and mechanism for metallochaperone-assisted assembly of the CuA center in cytochrome oxidase

The mechanisms underlying the biogenesis of the structurally unique, binuclear Cu^1.5+•Cu^1.5+ redox center (Cu_A) on subunit II (CoxB) of cytochrome oxidases have been a long-standing mystery. Here, we reconstituted the CoxB•Cu_A center in vitro from apo-CoxB and the holo-forms of the copper transfer chaperones ScoI and PcuC. A previously unknown, highly stable ScoI•Cu²⁺•CoxB complex was shown to be rapidly formed as the first intermediate in the pathway. Moreover, our structural data revealed that PcuC has two copper-binding sites, one each for Cu¹⁺ and Cu²⁺, and that only PcuC•Cu¹⁺•Cu²⁺ can release CoxB•Cu²⁺ from the ScoI•Cu²⁺•CoxB complex. The CoxB•Cu_A center was then formed quantitatively by transfer of Cu¹⁺ from a second equivalent of PcuC•Cu¹⁺•Cu²⁺ to CoxB•Cu²⁺. This metalation pathway is consistent with all available in vivo data and identifies the sources of the Cu ions required for Cu_A center formation and the order of their delivery to CoxB.

Silver Ions as a Tool for Understanding Different Aspects of Copper Metabolism

In humans, copper is an important micronutrient because it is a cofactor of ubiquitous and brain-specific cuproenzymes, as well as a secondary messenger. Failure of the mechanisms supporting copper balance leads to the development of neurodegenerative, oncological, and other severe disorders, whose treatment requires a detailed understanding of copper metabolism. In the body, bioavailable copper exists in two stable oxidation states, Cu(I) and Cu(II), both of which are highly toxic. The toxicity of copper ions is usually overcome by coordinating them with a wide range of ligands. These include the active cuproenzyme centers, copper-binding protein motifs to ensure the safe delivery of copper to its physiological location, and participants in the Cu(I) ↔ Cu(II) redox cycle, in which cellular copper is stored. The use of modern experimental approaches has allowed the overall picture of copper turnover in the cells and the organism to be clarified. However, many aspects of this process remain poorly understood. Some of them can be found out using abiogenic silver ions (Ag(I)), which are isoelectronic to Cu(I). This review covers the physicochemical principles of the ability of Ag(I) to substitute for copper ions in transport proteins and cuproenzyme active sites, the effectiveness of using Ag(I) to study copper routes in the cells and the body, and the limitations associated with Ag(I) remaining stable in only one oxidation state. The use of Ag(I) to restrict copper transport to tumors and the consequences of large-scale use of silver nanoparticles for human health are also discussed.

Copper trafficking in eukaryotic systems: current knowledge from experimental and computational efforts

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The main copper transporter, Ctr1, can transfer Cu(I) in the cell, through two different intracellular domains.

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Conformational flexibility of the copper metallochaperone Atox1 controls copper transfer mechanism in the cell.

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Each metal binding domain in ATP7B has a specific role.

Copper plays a vital role in fundamental cellular functions, and its concentration in the cell must be tightly regulated, as dysfunction of copper homeostasis is linked to severe neurological diseases and cancer. This review provides a compendium of current knowledge regarding the mechanism of copper transfer from the blood system to the Golgi apparatus; this mechanism involves the copper transporter hCtr1, the metallochaperone Atox1, and the ATPases ATP7A/B. We discuss key insights regarding the structural and functional properties of the hCtr1-Atox1-ATP7B cycle, obtained from diverse studies relying on distinct yet complementary biophysical, biochemical, and computational methods. We further address the mechanistic aspects of the cycle that continue to remain elusive. These knowledge gaps must be filled in order to be able to harness our understanding of copper transfer to develop therapeutic approaches with the capacity to modulate copper metabolism.

Ratiometric two-photon microscopy reveals attomolar copper buffering in normal and Menkes mutant cells

Copper is controlled by a sophisticated network of transport and storage proteins within mammalian cells, yet its uptake and efflux occur with rapid kinetics. Present as Cu(I) within the reducing intracellular environment, the nature of this labile copper pool remains elusive. While glutathione is involved in copper homeostasis and has been assumed to buffer intracellular copper, we demonstrate with a ratiometric fluorescent indicator, crisp-17, that cytosolic Cu(I) levels are buffered to the vicinity of 1 aM, where negligible complexation by glutathione is expected. Enabled by our phosphine sulfide-stabilized phosphine (PSP) ligand design strategy, crisp-17 offers a Cu(I) dissociation constant of 8 aM, thus exceeding the binding affinities of previous synthetic Cu(I) probes by four to six orders of magnitude. Two-photon excitation microscopy with crisp-17 revealed rapid, reversible increases in intracellular Cu(I) availability upon addition of the ionophoric complex CuGTSM or the thiol-selective oxidant 2,2'-dithiodipyridine (DTDP). While the latter effect was dramatically enhanced in 3T3 cells grown in the presence of supplemental copper and in cultured Menkes mutant fibroblasts exhibiting impaired copper efflux, basal Cu(I) availability in these cells showed little difference from controls, despite large increases in total copper content. Intracellular copper is thus tightly buffered by endogenous thiol ligands with significantly higher affinity than glutathione. The dual utility of crisp-17 to detect normal intracellular buffered Cu(I) levels as well as to probe the depth of the labile copper pool in conjunction with DTDP provides a promising strategy to characterize perturbations of cellular copper homeostasis.

The essential metals for humans: a brief overview

The human body needs about 20 essential elements in order to function properly and among them, for certain, 10 are metal elements, though for every metal we do need, there is another one in our body we could do without it. Until about 1950 poor attention was given to the so-called "inorganic elements" and while researches on "organic elements" (C, N, O and H) and organic compounds were given high priority, studies on essential inorganic elements were left aside. Base on current knowledge it is ascertained today that metals such as Na, K, Mg, Ca, Fe, Mn, Co, Cu, Zn and Mo are essential elements for life and our body must have appropriate amounts of them. Here a brief overview to highlight their importance and current knowledge about their essentiality.

Novel fluorescent probes for relay detection copper/citrate ion and application in cell imaging

Two novel fluorescent probes, 2‑(2'‑hydroxyphenyl)‑4‑(2'‑hydroxymethyl‑8‑quinolinamino)methyloxazole (L1), and 2‑(2'‑hydroxyphenyl)‑4‑(2'‑methyl‑8‑quinolinamino)methyloxazole (L2), exhibited colorimetric and "turn off" fluorometric response to Cu²⁺ ion in DMSO/H₂O solution (v/v = 1/1, 0.01 M, Tris-HCl buffer, pH 7.20) and the corresponding detection limit were found to be 2.14 × 10^-8 and 2.70 × 10^-8 M, which were much lower than drinking water permission concentrations by the United States Environmental Protection Agency (U.S. EPA) and World Health Organization (WHO). The L1-Cu²⁺ and L2-Cu²⁺ complexes ensemble detected citrate anions (CA) sequentially through fluorescence recovery response due to the extrusion of Cu²⁺ ion from the complexes. The binding processes were investigated by UV-vis, fluorescence, IR and DFT calculation. Furthermore, the vivo sensitivity experiments of Cu²⁺ ion and CA was demonstrated through fluorescence imaging in living cells.

Study on the interaction between pyridoxal and CopC by multi-spectroscopy and docking methods

The interaction between pyridoxal hydrochloride (HQ) and apoCopC was investigated using Fourier transform infrared spectroscopy (FTIR), isothermal titration calorimetry (ITC), circular dichroism (CD), fluorescence spectroscopy, three-dimensional (3D) fluorescence spectroscopy, fluorescence lifetime, TNS fluorescence and docking methods. FTIR, CD, TNS fluorescence and fluorescence lifetime experiments suggested that the apoCopC conformation was altered by HQ with an increase in the random coil content and a reduction in the β-sheet content. In addition, the data from fluorescence spectroscopy, 3D fluorescence spectroscopy and molecular docking revealed that the binding site of HQ was located in the hydrophobic area of apoCopC, and a redshift of the HQ fluorescence spectra was observed. Furthermore, ITC and fluorescence quenching data manifested that the binding ratio of HQ and apoCopC was 1:1, and the forming constant was calculated to be (7.06 ± 0.21) × 10⁵ M^-1. The thermodynamic parameters ΔH and ΔS suggested that the formation of a CopC-HQ complex depended on the hydrophobic force. Furthermore, the average binding distance between tryptophan in apoCopC and HQ was determined by means of Förster non-radioactive resonance energy transfer and molecular docking. The results agreed well with each other. As a redox switch in the modulation of copper, the interaction of apoCopC with small molecules will affect the action of the redox switch. These findings could provide useful information to illustrate the copper regulation mechanism.