A direct spectrophotometric method for the simultaneous determination of zinc and cobalt in metalloproteins using 4-(2-pyridylazo)resorcinol

A metal ion mediated functional dichotomy encodes plasticity during translation quality control

Proofreading during translation of the genetic code is a key process for not only translation quality control but also for its modulation under stress conditions to provide fitness advantage. A major class of proofreading modules represented by editing domains of alanyl-tRNA synthetase (AlaRS-Ed) and threonyl-tRNA synthetase (ThrRS-Ed) features a common fold and an invariant Zn2+ binding motif across life forms. Here, we reveal the structural basis and functional consequence along with the necessity for their operational dichotomy, i.e., the metal ion is ubiquitous in one and inhibitor for the other. The universally conserved Zn2+ in AlaRS-Ed protects its proofreading activity from reactive oxygen species (ROS) to maintain high fidelity Ala-codons translation, necessary for cell survival. On the other hand, mistranslation of Thr-codons is well tolerated by the cells, thereby allowing for a ROS-based modulation of ThrRS-Ed's activity. A single residue rooted over ~3.5 billion years of evolution has been shown to be primarily responsible for the functional divergence. The study presents a remarkable example of how protein quality control is integrated with redox signalling through leveraging the tunability of metal binding sites from the time of last universal common ancestor (LUCA).

Molecular insights into LINC complex architecture through the crystal structure of a luminal trimeric coiled-coil domain of SUN1

The LINC complex, consisting of interacting SUN and KASH proteins, mechanically couples nuclear contents to the cytoskeleton. In meiosis, the LINC complex transmits microtubule-generated forces to chromosome ends, driving the rapid chromosome movements that are necessary for synapsis and crossing over. In somatic cells, it defines nuclear shape and positioning, and has a number of specialised roles, including hearing. Here, we report the X-ray crystal structure of a coiled-coiled domain of SUN1's luminal region, providing an architectural foundation for how SUN1 traverses the nuclear lumen, from the inner nuclear membrane to its interaction with KASH proteins at the outer nuclear membrane. In combination with light and X-ray scattering, molecular dynamics and structure-directed modelling, we present a model of SUN1's entire luminal region. This model highlights inherent flexibility between structured domains, and raises the possibility that domain-swap interactions may establish a LINC complex network for the coordinated transmission of cytoskeletal forces.

Native mass spectrometric studies of IscSU reveal a concerted, sulfur-initiated mechanism of iron-sulfur cluster assembly

Iron-sulfur (Fe-S) clusters are cofactors essential for life. Though the proteins that function in the assembly of Fe-S clusters are well known, details of the molecular mechanism are less well established. The Isc (iron-sulfur cluster) biogenesis apparatus is widespread in bacteria and is the closest homologue to the human system. Mutations in certain components of the human system lead to disease, and so further studies of this system could be important for developing strategies for medical treatments. We have studied two core components of the Isc biogenesis system: IscS, a cysteine desulfurase; and IscU, a scaffold protein on which clusters are built before subsequent transfer onto recipient apo-proteins. Fe2+-binding, sulfur transfer, and formation of a [2Fe-2S] was followed by a range of techniques, including time-resolved mass spectrometry, and intermediate and product species were unambiguously identified through isotopic substitution experiments using 57Fe and 34S. Under cluster synthesis conditions, sulfur adducts and the [2Fe-2S] cluster product readily accumulated on IscU, but iron adducts (other than the cluster itself) were not observed at physiologically relevant Fe2+ concentrations. Our data indicate that either Fe2+ or sulfur transfer can occur first, but that the transfer of sulfane sulfur (S0) to IscU must occur first if Zn2+ is bound to IscU, suggesting that it is the key step that initiates cluster assembly. Following this, [2Fe-2S] cluster formation is a largely concerted reaction once Fe2+ is introduced.

High-throughput detection of metal contamination in HTS outputs

Large compound libraries utilised for HTS often include metal contaminated compounds which can interfere with assay signal or target biology, and therefore appear as hits. Pursuit of these compounds can divert considerable time and resource away from more propitious hits, yet there is currently no established method of detecting metal impurities in a rapid and effective manner. Here we describe the development and application of a high-throughput method to identify metal contaminants using acoustic mist ionisation mass spectrometry (AMI-MS). Although metals species by themselves are not detectable by AMI-MS, we have identified two compounds that chelate metal ions and enable their detection. 6-(diethylamino)-1,3,5-triazine-2,4(1H,3H)-dithione (DMT) and 1-(3-{[4-(4-cyanophenyl)-1-piperidinyl]carbonyl}-4-methylphenyl)-3-ethylthiourea (TU) can form complexes with a range of metal ions. Using a collection of metal catalysts, we have developed two metal chelator assays that collectively allow for the detection of Ag, Au, Co, Cu, Fe, Pd, Pt and Zn. We employed these assays to profile the hit outputs of a Zn liable target, and a Pd liable target, and identified significant quantities of metal contaminated compounds in the HTS outputs. This work provides a method of rapidly identifying metal impurities in hit compounds and has become part of an established workflow in triaging HTS outputs at AstraZeneca, facilitating faster identification of robust lead series.

Physico-analytical studies on some heterocyclic azo dyes and their metal complexes with some transition metals

In this investigation, the azo dyes; 2-(3′-phenyl-5′-pyrazolyl azo) schaffer acid (la) and 2-(3′-phenyl-5′-pyrazolyl azo) resorcinol (Ib); were prepared through diazotizing 3-phenyl-5-aminopyrazole (PAP) and coupling the resulting diazonium salt with Schäffer acid and resorcinol respectively. The prepared azo dyes are characterized using both IR spectra and the elemental analysis (C, H, N and S). The prepared azo dyes are used as chromogenic reagents for the spectrophotometric determination of copper (II), nickel (II), cobalt (II) and zinc (II) ions. The conditional acid dissociation constants of these azo dyes (la and Ib) and the stability constants of its metal ion complexes have been determined by spectro-analytical methods. The effect of pH, time, organic solvent and the foreign ions on the spectrophotometric determination of these ions and their complexes with the azo dyes under study were studied. The stoichiometric ratio (M:L) of the formed complexes was also determined. The molar absorptivity, the Sandell's sensitivity values, the obeyance of Beers law and the stability constants of the formed complexes have been also determined and discussed.

Development and validation of a simple and rapid ICP-OES method for quantification of elemental impurities in voriconazole drug substance

Background

The aim of the present study was to develop and validate an inductively coupled plasma optical emission spectroscopic (ICP–OES) method for quantification of elemental impurities, i.e., Lead, Palladium, and Zinc, in voriconazole drug substance, and this method was employed for the regular sample analysis of Lead, Palladium, and Zinc in voriconazole drug substance for pharmaceutical use. The method has been validated using RF power of 1150 W, auxiliary gas of 0.5 L/min, and nebulizer flow of 0.4 L/min and plasma view at axial mode for Lead and Palladium and radial mode for Zinc. The wavelength was monitored for Lead, Palladium, and Zinc at 220.3 nm, 340.4 nm, and 213.8 nm respectively.

Results

The method is selective and is capable of detecting desired elemental impurities with regulatory acceptance limits in the presence of other elements. The validation experiments involve the demonstration of system suitability, specificity, LOD and LOQ, linearity, precision, and accuracy experiments. The linearity results obtained ˃ 0.9990 for all three impurities.

Conclusion

The proposed method is simple, sensitive quality control tool for the simultaneous quantitative determination of Lead, Palladium, and Zinc at low levels in voriconazole drug substance.

Graphical abstract

A molecular mechanism for LINC complex branching by structurally diverse SUN-KASH 6:6 assemblies

The Linker of Nucleoskeleton and Cytoskeleton (LINC) complex mechanically couples cytoskeletal and nuclear components across the nuclear envelope to fulfil a myriad of cellular functions, including nuclear shape and positioning, hearing, and meiotic chromosome movements. The canonical model is that 3:3 interactions between SUN and KASH proteins underlie the nucleocytoskeletal linkages provided by the LINC complex. Here, we provide crystallographic and biophysical evidence that SUN-KASH is a constitutive 6:6 complex in which two constituent 3:3 complexes interact head-to-head. A common SUN-KASH topology is achieved through structurally diverse 6:6 interaction mechanisms by distinct KASH proteins, including zinc-coordination by Nesprin-4. The SUN-KASH 6:6 interface provides a molecular mechanism for the establishment of integrative and distributive connections between 3:3 structures within a branched LINC complex network. In this model, SUN-KASH 6:6 complexes act as nodes for force distribution and integration between adjacent SUN and KASH molecules, enabling the coordinated transduction of large forces across the nuclear envelope.

Characterization of metal binding of bifunctional kinase/phosphatase AceK and implication in activity modulation

A unique bifunctional enzyme, isocitrate dehydrogenase kinase/phosphatase (AceK) regulates isocitrate dehydrogenase (IDH) by phosphorylation and dephosphorylation in response to nutrient availability. Herein we report the crystal structure of AceK in complex with ADP and Mn²⁺ ions. Although the overall structure is similar to the previously reported structures which contain only one Mg²⁺ ion, surprisingly, two Mn²⁺ ions are found in the catalytic center of the AceK-Mn²⁺ structure. Our enzymatic assays demonstrate that AceK-Mn²⁺ showed higher phosphatase activity than AceK-Mg²⁺, whereas the kinase activity was relatively unaffected. We created mutants of AceK for all metal-coordinating residues. The phosphatase activities of these mutants were significantly impaired, suggesting the pivotal role of the binuclear (M1-M2) core in AceK phosphatase catalysis. Moreover, we have studied the interactions of Mn²⁺ and Mg²⁺ with wild-type and mutant AceK and found that the number of metal ions bound to AceK is in full agreement with the crystal structures. Combined with the enzymatic results, we demonstrate that AceK exhibits phosphatase activity in the presence of two, but not one, Mn²⁺ ions, similar to PPM phosphatases. Taken together, we suggest that metal ions help AceK to balance and fine tune its kinase and phosphatase activities.

Comparison of the Response of Bacterial IscU and SufU to Zn2+ and Select Transition-Metal Ions

IscU, the central scaffold protein in the bacterial ISC iron-sulfur (Fe-S) cluster biosynthesis system, has long been recognized to bind a Zn²⁺ ion at its active site. While initially regarded as an artifact, Zn²⁺ binding has been shown to induce stabilization of the IscU structure that may mimic a state biologically relevant to IscU's role in Fe-S cluster biosynthesis. More recent studies have revealed that SufU, a homologous protein involved in Fe-S cluster biosynthesis in Gram-positive bacteria, also binds a Zn²⁺ ion with structural implications. Given the widespread occurrence of the "IscU-like" protein fold, particularly among Fe-S cluster biosynthesis systems, an interesting question arises as to whether Zn²⁺ ion binding and the resulting structural alterations are common properties in IscU-like proteins. Interactions between IscU and specific metal ions are investigated and compared side-by-side with those of SufU from a representative Gram-positive bacterium in the phylum Firmicutes. These studies were extended with additional transition metal ions chosen to investigate the influence of coordination geometry on selectivity for binding at the active sites of IscU and SufU. Monitoring and comparing the conformational behavior and stabilization afforded by different transition metal ions upon IscU and SufU revealed similarities between the two proteins and suggest that metal-dependent conformational transitions may be characteristic of U-type proteins involved in Fe-S cluster biosynthesis.

Zinc Binding to S100B Affords Regulation of Trace Metal Homeostasis and Excitotoxicity in the Brain

Neuronal metal ions such as zinc are essential for brain function. In particular synaptic processes are tightly related to metal and protein homeostasis, for example through extracellular metal-binding proteins. One such protein is neuronal S100B, a calcium and zinc binding damage-associated molecular pattern (DAMP), whose chronic upregulation is associated with aging, Alzheimer’s disease (AD), motor neuron disease and traumatic brain injury (TBI). Despite gained insights on the structure of S100B, it remains unclear how its calcium and zinc binding properties regulate its function on cellular level. Here we report a novel role of S100B in trace metal homeostasis, in particular the regulation of zinc levels in the brain. Our results show that S100B at increased extracellular levels is not toxic, persists at high levels, and is taken up into neurons, as shown by cell culture and biochemical analysis. Combining protein bioimaging and zinc quantitation, along with a zinc-binding impaired S100B variant, we conclude that S100B effectively scavenges zinc ions through specific binding, resulting in a redistribution of the intracellular zinc pool. Our results indicate that scavenging of zinc by increased levels of S100B affects calcium levels in vitro. Thereby S100B is able to mediate the cross talk between calcium and zinc homeostasis. Further, we investigated a possible new neuro-protective role of S100B in excitotoxicity via its effects on calcium and zinc homeostasis. Exposure of cells to zinc-S100B but not the zinc-binding impaired S100B results in an inhibition of excitotoxicity. We conclude that in addition to its known functions, S100B acts as sensor and regulator of elevated zinc levels in the brain and this metal-buffering activity is tied to a neuroprotective role.

Production of small cysteine-rich effector proteins in Escherichia coli for structural and functional studies

Although the lifestyles and infection strategies of plant pathogens are diverse, a prevailing feature is the use of an arsenal of secreted proteins, known as effectors, which aid in microbial infection. In the case of eukaryotic filamentous pathogens, such as fungi and oomycetes, effector proteins are typically dissimilar, at the protein sequence level, to known protein families and functional domains. Consequently, we currently have a limited understanding of how fungal and oomycete effectors promote disease. Protein biochemistry and structural biology are two methods that can contribute greatly to the understanding of protein function. Both techniques are dependent on obtaining proteins that are pure and functional, and generally require the use of heterologous recombinant protein expression systems. Here, we present a general scheme and methodology for the production and characterization of small cysteine-rich (SCR) effectors utilizing Escherichia coli expression systems. Using this approach, we successfully produced cysteine-rich effectors derived from the biotrophic fungal pathogen Melampsora lini and the necrotrophic fungal pathogen Parastagonospora nodorum. Access to functional recombinant proteins facilitated crystallization and functional experiments. These results are discussed in the context of a general workflow that may serve as a template for others interested in understanding the function of SCR effector(s) from their plant pathogen(s) of interest.

An Interplay of S-Nitrosylation and Metal Ion Binding for Astrocytic S100B Protein

Mammalian S100B protein plays multiple important roles in cellular brain processes. The protein is a clinically used marker for several pathologies including brain injury, neurodegeneration and cancer. High levels of S100B released by astrocytes in Down syndrome patients are responsible for reduced neurogenesis of neural progenitor cells and induction of cell death in neurons. Despite increasing understanding of S100B biology, there are still many questions concerning the detailed molecular mechanisms that determine specific activities of S100B. Elevated overexpression of S100B protein is often synchronized with increased nitric oxide-related activity. In this work we show S100B is a target of exogenous S-nitrosylation in rat brain protein lysate and identify endogenous S-nitrosylation of S100B in a cellular model of astrocytes. Biochemical studies are presented indicating S-nitrosylation tunes the conformation of S100B and modulates its Ca²⁺ and Zn²⁺ binding properties. Our in vitro results suggest that the possibility of endogenous S-nitrosylation should be taken into account in the further studies of in vivo S100B protein activity, especially under conditions of increased NO-related activity.

4-(2-Pyridylazo)-resorcinol Functionalized Thermosensitive Ionic Microgels for Optical Detection of Heavy Metal Ions at Nanomolar Level

4-(2-Pyridylazo)-resorcinol (PAR) functionalized thermosensitive ionic microgels (PAR-MG) were synthesized by a one-pot quaternization method. The PAR-MG microgels were spherical in shape with radius of ca. 166.0 nm and narrow size distribution and exhibited thermo-sensitivity in aqueous solution. The PAR-MG microgels could optically detect trace heavy metal ions, such as Cu(2+), Mn(2+), Pb(2+), Zn(2+), and Ni(2+), in aqueous solutions with high selectivity and sensitivity. The PAR-MG microgel suspensions exhibited characteristic color with the presence of various trace heavy metal ions, which could be visually distinguished by naked eyes. The limit of colorimetric detection (DL) was determined to be 38 nM for Cu(2+) at pH 3, 12 nM for Cu(2+) at pH 7, and 14, 79, 20, and 21 nM for Mn(2+), Pb(2+), Zn(2+), and Ni(2+), respectively, at pH 11, which was lower than (or close to) the United States Environmental Protection Agency standard for the safety limit of these heavy metal ions in drinking water. The mechanism of detection was attributed to the chelation between the nitrogen atoms and o-hydroxyl groups of PAR within the microgels and heavy metal ions.

Molar absorption coefficients and stability constants of metal complexes of 4-(2-pyridylazo)resorcinol (PAR): Revisiting common chelating probe for the study of metalloproteins

4-(2-Pyridylazo)resorcinol (PAR) is one of the most popular chromogenic chelator used in the determination of the concentrations of various metal ions from the d, p and f blocks and their affinities for metal ion-binding biomolecules. The most important characteristics of such a sensor are the molar absorption coefficient and the metal-ligand complex dissociation constant. However, it must be remembered that these values are dependent on the specific experimental conditions (e.g. pH, solvent components, and reactant ratios). If one uses these values to process data obtained in different conditions, the final result can be under- or overestimated. We aimed to establish the spectral properties and the stability of PAR and its complexes accurately with Zn(2+), Cd(2+), Hg(2+), Co(2+), Ni(2+), Cu(2+), Mn(2+) and Pb(2+) at a multiple pH values. The obtained results account for the presence of different species of metal-PAR complexes in the physiological pH range of 5 to 8 and have been frequently neglected in previous studies. The effective molar absorption coefficient at 492 nm for the ZnHx(PAR)2 complex at pH7.4 in buffered water solution is 71,500 M(-1) cm(-1), and the dissociation constant of the complex in these conditions is 7.08×10(-13) M(2). To confirm these values and estimate the range of the dissociation constants of zinc-binding biomolecules that can be measured using PAR, we performed several titrations of zinc finger peptides and zinc chelators. Taken together, our results provide the updated parameters that are applicable to any experiment conducted using inexpensive and commercially available PAR.

Influence of chemical denaturants on the activity, fold and zinc status of anthrax lethal factor

Anthrax lethal factor (LF) is a zinc-dependent endopeptidase which, through a process facilitated by protective antigen, translocates to the host cell cytosol in a partially unfolded state. In the current report, the influence of urea and guanidine hydrochloride (GdnHCl) on LF׳s catalytic function, fold and metal binding was assessed at neutral pH. Both urea and GdnHCl were found to inhibit LF prior to the onset of unfolding, with the inhibition by the latter denaturant being a consequence of its ionic strength. With the exception of demetallated LF (apoLF) in urea, unfolding, as monitored by tryptophan fluorescence spectroscopy, was found to follow a two-state (native to unfolded) mechanism. Analysis of the metal status of LF with 4-(2-pyridylazoresorcinol) (PAR) following urea or GdnHCl exposure suggests the enzyme to be capable of maintaining its metal ion passed the observed unfolding transition in a chelator-inaccessible form. Although an increase in the concentration of the denaturants eventually allowed the chelator access to the protein׳s zinc ion, such process is not correlated with the release of the metal ion. Indeed, significant dissociation of the zinc ion from LF was not observed even at 6 M urea, and only high concentrations of GdnHCl (>3 M) were capable of inducing the release of the metal ion from the protein. Hence, the current study demonstrates not only the propensity of LF to tightly bind its zinc ion beyond the spectroscopically determined unfolding transition, but also the utility of PAR as a structural probe.

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Lethal factor (LF) is strongly inhibited by guanidine hydrochloride.

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Except of apoLF in urea, unfolding follows a two-state mechanism.

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LF shields and retains its zinc ion in an unfolded state.

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Pyridylazoresorcinol is a useful probe to assess metal accessibility and release.

CtIP tetramer assembly is required for DNA-end resection and repair

Mammalian CtIP protein has major roles in DNA double-strand break (DSB) repair. Although it is well established that CtIP promotes DNA-end resection in preparation for homology-dependent DSB repair, the molecular basis for this function has remained unknown. Here we show by biophysical and X-ray crystallographic analyses that the N-terminal domain of human CtIP exists as a stable homotetramer. Tetramerization results from interlocking interactions between the N-terminal extensions of CtIP's coiled-coil region, which lead to a 'dimer-of-dimers' architecture. Through interrogation of the CtIP structure, we identify a point mutation that abolishes tetramerization of the N-terminal domain while preserving dimerization in vitro. Notably, we establish that this mutation abrogates CtIP oligomer assembly in cells, thus leading to strong defects in DNA-end resection and gene conversion. These findings indicate that the CtIP tetramer architecture described here is essential for effective DSB repair by homologous recombination.

Determining the role of metal binding in protein cage assembly

Assembly of protein cages may require structural metal ions to nucleate or stabilize association of protein subunits. We describe here an approach to establishing the role of metal ions in protein cage assembly and stability, focusing on detecting the presence of structural metal ions as well as establishing oligomeric state. A colorimetric assay for detection of metal is described, along with a gel filtration assay to assess protein cage assembly and a fluorescence-based method for determining protein stability.

The mononuclear metal center of type-I dihydroorotase from Aquifex aeolicus

BACKGROUND

Dihydroorotase (DHO) is a zinc metalloenzyme, although the number of active site zinc ions has been controversial. E. coli DHO was initially thought to have a mononuclear metal center, but the subsequent X-ray structure clearly showed two zinc ions, α and β, at the catalytic site. Aquifex aeolicus DHO, is a dodecamer comprised of six DHO and six aspartate transcarbamoylase (ATC) subunits. The isolated DHO monomer, which lacks catalytic activity, has an intact α-site and conserved β-site ligands, but the geometry of the second metal binding site is completely disrupted. However, the putative β-site is restored when the complex with ATC is formed and DHO activity is regained. Nevertheless, the X-ray structure of the complex revealed a single zinc ion at the active site. The structure of DHO from the pathogenic organism, S. aureus showed that it also has a single active site metal ion.

RESULTS

Zinc analysis showed that the enzyme has one zinc/DHO subunit and the addition of excess metal ion did not stimulate catalytic activity, nor alter the kinetic parameters. The metal free apoenzyme was inactive, but the full activity was restored upon the addition of one equivalent of Zn2+ or Co2+. Moreover, deletion of the β-site by replacing the His180 and His232 with alanine had no effect on catalysis in the presence or absence of excess zinc. The 2.2 Å structure of the double mutant confirmed that the β-site was eliminated but that the active site remained otherwise intact.

CONCLUSIONS

Thus, kinetically competent A. aeolicus DHO has a mononuclear metal center. In contrast, elimination of the putative second metal binding site in amidohydrolyases with a binuclear metal center, resulted in the abolition of catalytic activity. The number of active site metal ions may be a consideration in the design of inhibitors that selectively target either the mononuclear or binuclear enzymes.

Caffeic acid phenethyl ester (CAPE), an active component of propolis, inhibits Helicobacter pylori peptide deformylase activity

Helicobacter pylori (H. pylori) is a major causative factor for gastrointestinal illnesses, H. pylori peptide deformylase (HpPDF) catalyzes the removal of formyl group from the N-terminus of nascent polypeptide chains, which is essential for H. pylori survival and is considered as a promising drug target for anti-H. pylori therapy. Propolis, a natural antibiotic from honeybees, is reported to have an inhibitory effect on the growth of H. pylori in vitro. In addition, previous studies suggest that the main active constituents in the propolis are phenolic compounds. Therefore, we evaluated a collection of phenolic compounds derived from propolis for enzyme inhibition against HpPDF. Our study results show that Caffeic acid phenethyl ester (CAPE), one of the main medicinal components of propolis, is a competitive inhibitor against HpPDF, with an IC50 value of 4.02 μM. Furthermore, absorption spectra and crystal structural characterization revealed that different from most well known PDF inhibitors, CAPE block the substrate entrance, preventing substrate from approaching the active site, but CAPE does not have chelate interaction with HpPDF and does not disrupt the metal-dependent catalysis. Our study provides valuable information for understanding the potential anti-H. pylori mechanism of propolis, and CAPE could be served as a lead compound for further anti-H. pylori drug discovery.

Mutagenesis of zinc ligand residue Cys221 reveals plasticity in the IMP-1 metallo-β-lactamase active site

Metallo-β-lactamases catalyze the hydrolysis of a broad range of β-lactam antibiotics and are a concern for the spread of drug resistance. To analyze the determinants of enzyme structure and function, the sequence requirements for the subclass B1 IMP-1 β-lactamase zinc binding residue Cys221 were tested by saturation mutagenesis and evaluated for protein expression, as well as hydrolysis of β-lactam substrates. The results indicated that most substitutions at position 221 destabilized the enzyme. Only the enzymes containing C221D and C221G substitutions were expressed well in Escherichia coli and exhibited catalytic activity toward β-lactam antibiotics. Despite the lack of a metal-chelating group at position 221, the C221G enzyme exhibited high levels of catalytic activity in the presence of exogenous zinc. Molecular modeling suggests the glycine substitution is unique among substitutions in that the complete removal of the cysteine side chain allows space for a water molecule to replace the thiol and coordinate zinc at the Zn2 zinc binding site to restore function. Multiple methods were used to estimate the C221G Zn2 binding constant to be 17 to 43 μM. Studies of enzyme function in vivo in E. coli grown on minimal medium showed that both IMP-1 and the C221G mutant exhibited compromised activity when zinc availability was low. Finally, substitutions at residue 121, which is the IMP-1 equivalent of the subclass B3 zinc-chelating position, failed to rescue C221G function, suggesting the coordination schemes of subclasses B1 and B3 are not interchangeable.

Mutational and structural analysis of L-N-carbamoylase reveals new insights into a peptidase M20/M25/M40 family member

N-Carbamoyl-L-amino acid amidohydrolases (L-carbamoylases) are important industrial enzymes used in kinetic resolution of racemic mixtures of N-carbamoyl-amino acids due to their strict enantiospecificity. In this work, we report the first L-carbamoylase structure belonging to Geobacillus stearothermophilus CECT43 (BsLcar), at a resolution of 2.7 Å. Structural analysis of BsLcar and several members of the peptidase M20/M25/M40 family confirmed the expected conserved residues at the active site in this family, and site-directed mutagenesis revealed their relevance to substrate binding. We also found an unexpectedly conserved arginine residue (Arg(234) in BsLcar), proven to be critical for dimerization of the enzyme. The mutation of this sole residue resulted in a total loss of activity and prevented the formation of the dimer in BsLcar. Comparative studies revealed that the dimerization domain of the peptidase M20/M25/M40 family is a "small-molecule binding domain," allowing further evolutionary considerations for this enzyme family.

An inorganic-organic hybrid optical sensor for heavy metal ion detection based on immobilizing 4-(2-pyridylazo)-resorcinol on functionalized HMS

A novel and low-cost optical sensor for heavy metal ion detection has been prepared by immobilizing 4-(2-pyridylazo)-resorcinol (PAR) on the functionalized hexagonal mesoporous silica (HMS) via N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TMAC). The successful fabrication of this optical sensor is confirmed by extensive characterizations using FT-IR, low angle X-ray diffraction (XRD), UV-vis spectroscopy and N(2) sorption, meanwhile its colorimetric properties, selectivity, sensitivity and reversibility are also investigated. The optical sensor responds selectively to heavy metal ions, such as Fe(3+), Cd(2+), Ni(2+), Zn(2+), Pb(2+), Co(2+), Hg(2+) and Cu(2+) with a color change from yellow to orange or purple in alkaline solutions, while it shows a color change only for Cu(2+) under strongly acidic conditions. At pH 12.0, this optical sensor has a high sensitivity that makes it possible to detect Cu(2+) in aqueous solution with the detection limit as low as 40ppb by naked-eye. This optical sensor also shows excellent reversibility and regeneration by treatment with a solution of EDTA.

A spectrophotometric method for the determination of zinc, copper, and cobalt ions in metalloproteins using Zincon

Zincon (2-carboxy-2'-hydroxy-5'-sulfoformazylbenzene) has long been known as an excellent colorimetric reagent for the detection of zinc and copper ions in aqueous solution. To extend the chelator's versatility to the quantification of metal ions in metalloproteins, the spectral properties of Zincon and its complexes with Zn(2+), Cu(2+), and Co(2+) were investigated in the presence of guanidine hydrochloride and urea, two common denaturants used to labilize metal ions in proteins. These studies revealed the detection of metals to be generally more sensitive with urea. In addition, pH profiles recorded for these metals indicated the optimal pH for complex formation and stability to be 9.0. As a consequence, an optimized method that allows the facile determination of Zn(2+), Cu(2+), and Co(2+) with detection limits in the high nanomolar range is presented. Furthermore, a simple two-step procedure for the quantification of both Zn(2+) and Cu(2+) within the same sample is described. Using the prototypical Cu(2+)/Zn(2+)-protein superoxide dismutase as an example, the effectiveness of this method of dual metal quantification in metalloproteins is demonstrated. Thus, the spectrophotometric determination of metal ions with Zincon can be exploited as a rapid and inexpensive means of assessing the metal contents of zinc-, copper-, cobalt-, and zinc/copper-containing proteins.