Crystal structure of the dinuclear zinc aminopeptidase PepV from Lactobacillus delbrueckii unravels its preference for dipeptides

State of the Art in the Development of Human Serum Carnosinase Inhibitors

Human serum carnosinase is an enzyme that operates the preferential hydrolysis of dipeptides with a C-terminus histidine. Only higher primates excrete such an enzyme in serum and cerebrospinal fluid. In humans, the serum hydrolytic rate has high interindividual variability owing to gene polymorphism, although age, gender, diet, and also diseases and surgical interventions can modify serum activity. Human genetic diseases with altered carnosinase activity have been identified and associated with neurological disorders and age-related cognitive decline. On the contrary, low peripheral carnosinase activity has been associated with kidney protection, especially in diabetic nephropathy. Therefore, serum carnosinase is a druggable target for the development of selective inhibitors. However, only one molecule (i.e., carnostatine) has been discovered with the purpose of developing serum carnosinase inhibitors. Bestatin is the only inhibitor reported other than carnostatine, although its activity is not selective towards serum carnosinase. Herein, we present a review of the most critical findings on human serum carnosinase, including enzyme expression, localization and substrate selectivity, along with factors affecting the hydrolytic activity, its implication in human diseases and the properties of known inhibitors of the enzyme.

Structural and functional analysis of the key enzyme responsible for the degradation of ochratoxin A in the Alcaligenes genus

The potential to degrade ochratoxin A (OTA), a highly poisonous mycotoxin, was investigated in cultures from Alcaligenes-type strains. Genome sequence analyses from different Alcaligenes species have permitted us to demonstrate a direct, causal link between the gene coding a known N-acyl-L-amino acid amidohydrolase from A. faecalis (AfOTH) and the OTA-degrading activity of this bacterium. In agreement with this finding, we found the gene coding AfOTH in two additional species included in the Alcaligenes genus, namely, A. pakistanensis, and A. aquatilis, which also degraded OTA. Notably, A. faecalis subsp. faecalis DSM 30030T was able to transform OTα, the product of OTA hydrolysis. AfOTH from A. faecalis subsp. phenolicus DSM 16503T was recombinantly over-produced and enzymatically characterized. AfOTH is a Zn2+-containing metalloenzyme that possesses structural features and conserved residues identified in the M20D family of enzymes. AfOTH is a tetramer in solution that shows both aminoacylase and carboxypeptidase activities. Using diverse potential substrates, namely, N-acetyl-L-amino acids and carbobenzyloxy-L-amino acids, a marked preference towards C-terminal Phe and Tyr residues could be deduced. The structural basis for this specificity has been determined by in silico molecular docking analyses. The amidase activity of AfOTH on C-terminal Phe residues structurally supports its OTA and OTB degradation activity.

Utilization of glycoprotein-derived N-acetylglucosamine-L-asparagine during Enterococcus faecalis infection depends on catabolic and transport enzymes of the glycosylasparaginase locus

Enterococcus faecalis is a Gram-positive clinical pathogen causing severe infections. Its survival during infection depends on its ability to utilize host-derived metabolites, such as protein-deglycosylation products. We have identified in E. faecalis OG1RF a locus (ega) involved in the catabolism of the glycoamino acid N-acetylglucosamine-L-asparagine. This locus is separated into two transcription units, genes egaRP and egaGBCD1D2, respectively. RT-qPCR experiments revealed that the expression of the ega locus is regulated by the transcriptional repressor EgaR. Electromobility shift assays evidenced that N-acetylglucosamine-L-asparagine interacts directly with the EgaR protein, which leads to the transcription of the ega genes. Growth studies with egaG, egaB and egaC mutants confirmed that the encoded proteins are necessary for N-acetylglucosamine-L-asparagine catabolism. This glycoamino acid is transported and phosphorylated by a specific phosphotransferase system EIIABC components (OG1RF_10751, EgaB, EgaC) and subsequently hydrolyzed by the glycosylasparaginase EgaG, which generates aspartate and 6-P-N-acetyl-β-d-glucosaminylamine. The latter can be used as a fermentable carbon source by E. faecalis. Moreover, Galleria mellonella larvae had a significantly higher survival rate when infected with ega mutants compared to the wild-type strain, suggesting that the loss of N-acetylglucosamine-L-asparagine utilization affects enterococcal virulence.

Novel aminoacylases from Streptomyces griseus DSM 40236 and their recombinant production in Streptomyces lividans

Amino acid-based surfactants are valuable compounds for cosmetic formulations. The chemical synthesis of acyl amino acids is conventionally performed by the Schotten-Baumann reaction using fatty acyl chlorides, but aminoacylases have also been investigated for use in biocatalytic synthesis with free fatty acids. Aminoacylases and their properties are diverse; they belong to different peptidase families and show differences in substrate specificity and biocatalytic potential. Bacterial aminoacylases capable of synthesis have been isolated from Burkholderia, Mycolicibacterium, and Streptomyces. Although several proteases and peptidases from S. griseus have been described, no aminoacylases from this species have been identified yet. In this study, we investigated two novel enzymes produced by S. griseus DSM 40236T . We identified and cloned the respective genes and recombinantly expressed an α-aminoacylase (EC3.5.1.14), designated SgAA, and an ε-lysine acylase (EC3.5.1.17), designated SgELA, in S. lividans TK23. The purified aminoacylase SgAA was biochemically characterized, focusing on its hydrolytic activity to determine temperature- and pH optima and stabilities. The aminoacylase could hydrolyze various acetyl amino acids at the Nα -position with a broad specificity regarding the sidechain. Substrates with longer acyl chains, like lauroyl amino acids, were hydrolyzed to a lesser extent. Purified aminoacylase SgELA specific for the hydrolysis of Nε -acetyl-l-lysine was unstable and lost its enzymatic activity upon storage for a longer period but could initially be characterized. The pH optimum of SgELA was pH 8.0. While synthesis of acyl amino acids was not observed with SgELA, SgAA catalyzed the synthesis of lauroyl-methionine.

In vitro and in silico characterization of a novel glutamate carboxypeptidase from Cohnella sp. A01

Glutamate carboxypeptidase is a bacterial enzyme of metallopeptidase superfamily. This enzyme is an exo-peptidase that catalyzes the hydrolysis of glutamate residues at the C-terminus of folic acid. The rCP302 is a novel zinc ion-dependent recombinant glutamate carboxypeptidase derived from a thermophilic bacterium, Cohnella sp. A01 (PTCC No: 1921). By simulating the structure of rCP302, analyzing its activity in various environmental settings, and contrasting it with that of related enzymes, we wanted to evaluate the heterologous production, purification, and characterization of this enzyme. The bioinformatics study showed that rCP302 had maximum similarity to M20 family of metallopeptidases. The purified rCP302 molecular weight was about 41.6 kDa. The optimum temperature and pH for the catalytic activity of rCP302 were 50 °C and 7.2, respectively. Fluorescence spectroscopy data elucidated the secondary structure of rCP302 and determined conformational changes caused by alterations in ambient conditions. Using folate as a substrate, K_m and specific activity values were calculated as 0.108 μM and 687 μmol/min/mg, respectively. The enzyme activity was strongly inhibited when EDTA sequestered zinc ions. The half-life of this enzyme at 30 °C was 2012 min. Regarding the ability of rCP302 to degrade folic acid, and its long half-life at 37 °C, the normal temperature of many mammals, this enzyme can be introduced for further study for use in the pharmaceutical industry.

Phosphinic Peptides as Tool Compounds for the Study of Pharmacologically Relevant Zn-Metalloproteases

Phosphinic peptides constitute an important class of bioactive compounds that have found a wide range of applications in the field of biology and pharmacology of Zn-metalloproteases, the largest family of proteases in humans. They are designed to mimic the structure of natural substrates during their proteolysis, thus acting as mechanism-based, transition state analogue inhibitors. A combination of electrostatic interactions between the phosphinic acid group and the Zn cation as well as optimal noncovalent enzyme-ligand interactions can result in both high binding affinity for the desired target and selectivity against other proteases. Due to these unique properties, phosphinic peptides have been mainly employed as tool compounds for (a) the purposes of rational drug design by serving as ligands in X-ray crystal structures of target enzymes and allowing the identification of crucial interactions that govern optimal molecular recognition, and (b) the delineation of biological pathways where Zn-metalloproteases are key regulators. For the latter objective, inhibitors of the phosphinopeptidic type have been used either unmodified or after being transformed to probes of various types, thus expanding the arsenal of functional tools available to researchers. The aim of this review is to summarize all recent research achievements in which phosphinic peptides have played a central role as tool compounds in the understanding of the mechanism and biological functions of Zn-metalloproteases in both health and disease.

Effects of Different Nutritional Zinc Forms on the Proliferation of Beneficial Commensal Microorganisms

This study compared the minimal inhibition concentrations (MICs) and their effects on the growth kinetics of seven different types of zinc (Zn) compounds and Na2EDTA in the case of three typical commensal beneficial microorganisms (Bacillus subtilis, Lactococcus lactis, and Saccharomyces cerevisiae). The seven Zn compounds included ZnSO4, four Zn–amino acid chelates, and two Zn–EDTA complexes. Both MICs and growth kinetic parameters indicated that different microorganisms show different sensitivities; for example, B. subtilis, L. lactis, and S. cerevisiae were most sensitive to ZnSO4, Na2EDTA, and Zn(NH3)2(Gly)2, respectively. Both ZnEDTA and Zn(NH3)2(Lys)2 improved the growth rate of all beneficial commensal intestinal microorganisms at low concentrations (5–10 mg/L) and showed low toxicity towards all tested strains. At higher concentrations (100–500 mg/L), all compounds decreased the growth rate and increased the lag phase. In conclusion, both growth kinetic parameters and MICs tested effectively measured the inhibitory effects of the test materials; however, growth kinetics provides a more detailed picture of the concentration-dependent effects and those on the mechanisms of microbial growth inhibition.

Massively parallel, computationally guided design of a proenzyme

Proteins have shown promise as therapeutics and diagnostics, but their effectiveness is limited by our inability to spatially target their activity. To overcome this limitation, we developed a computationally guided method to design inactive proenzymes or zymogens, which are activated through cleavage by a protease. Since proteases are differentially expressed in various tissues and disease states, including cancer, these proenzymes could be targeted to the desired microenvironment. We tested our method on the therapeutically relevant protein carboxypeptidase G2 (CPG2). We designed Pro-CPG2s that are inhibited by 80 to 98% and are partially to fully reactivatable following protease treatment. The developed methodology, with further refinements, could pave the way for routinely designing protease-activated protein-based therapeutics and diagnostics that act in a spatially controlled manner.

Confining the activity of a designed protein to a specific microenvironment would have broad-ranging applications, such as enabling cell type-specific therapeutic action by enzymes while avoiding off-target effects. While many natural enzymes are synthesized as inactive zymogens that can be activated by proteolysis, it has been challenging to redesign any chosen enzyme to be similarly stimulus responsive. Here, we develop a massively parallel computational design, screening, and next-generation sequencing-based approach for proenzyme design. For a model system, we employ carboxypeptidase G2 (CPG2), a clinically approved enzyme that has applications in both the treatment of cancer and controlling drug toxicity. Detailed kinetic characterization of the most effectively designed variants shows that they are inhibited by ∼80% compared to the unmodified protein, and their activity is fully restored following incubation with site-specific proteases. Introducing disulfide bonds between the pro- and catalytic domains based on the design models increases the degree of inhibition to 98% but decreases the degree of restoration of activity by proteolysis. A selected disulfide-containing proenzyme exhibits significantly lower activity relative to the fully activated enzyme when evaluated in cell culture. Structural and thermodynamic characterization provides detailed insights into the prodomain binding and inhibition mechanisms. The described methodology is general and could enable the design of a variety of proproteins with precise spatial regulation.

Lysis of a Lactococcus lactis Dipeptidase Mutant and Rescue by Mutation in the Pleiotropic Regulator CodY

Lactococcus lactis subsp. cremoris MG1363 is a model for the lactic acid bacteria (LAB) used in the dairy industry. The proteolytic system, consisting of a proteinase, several peptide and amino acid uptake systems, and a host of intracellular peptidases, plays a vital role in nitrogen metabolism and is of eminent importance for flavor formation in dairy products. The dipeptidase PepV functions in the last stages of proteolysis. A link between nitrogen metabolism and peptidoglycan (PG) biosynthesis was underlined by the finding that deletion of the dipeptidase gene pepV (creating strain MGΔpepV) resulted in a prolonged lag phase when the mutant strain was grown with a high concentration of glycine. In addition, most MGΔpepV cells lyse and have serious defects in their shape. This phenotype is due to a shortage of alanine, since adding alanine can rescue the growth and shape defects. Strain MGΔpepV is more resistant to vancomycin, an antibiotic targeting peptidoglycan d-Ala-d-Ala ends, which confirmed that MGΔpepV has an abnormal PG composition. A mutant of MGΔpepV was obtained in which growth inhibition and cell shape defects were alleviated. Genome sequencing showed that this mutant has a single point mutation in the codY gene, resulting in an arginine residue at position 218 in the DNA-binding motif of CodY being replaced by a cysteine residue. Thus, this strain was named MGΔpepVcodY ^R218C Transcriptome sequencing (RNA-seq) data revealed a dramatic derepression in peptide uptake and amino acid utilization in MGΔpepVcodY ^R218C A model of the connections among PepV activity, CodY regulation, and PG synthesis of L. lactis is proposed.IMPORTANCE Precise control of peptidoglycan synthesis is essential in Gram-positive bacteria for maintaining cell shape and integrity as well as resisting stresses. Although neither the dipeptidase PepV nor alanine is essential for L. lactis MG1363, adequate availability of either ensures proper cell wall synthesis. We broaden the knowledge about the dipeptidase PepV, which acts as a linker between nitrogen metabolism and cell wall synthesis in L. lactis.

Influence of Zinc Sulphate on the Probiotic Properties of Lactobacillus plantarum CCM 7102

The effects of zinc sulphate on selected properties of L. plantarum CCM 7102 were tested in vitro. The resistance of lactobacilli to higher concentrations of ZnSO4 (up to 5000 mg Zn2+.l−1) in growth media was strain-dependent. Further studies were carried out on the most resistant strain of L. plantarum CCM 7102. While the addition of low concentrations of zinc sulphate into the growth media (< 100 mg Zn2+.l−1) did not influence the properties of L. plantarum CCM 7102, the concentrations of 100—500 mg Zn2+.l−1 stimulated: the growth rate, production of lactic acid, adhesion to porcine enterocytes and the inhibition of pathogens E. coli O8:K88+ent+, S. enterica and S. Typhimurium. Conversely, however, high concentrations > 500 mg Zn2+.l−1 inhibited these properties. The addition of zinc (250 mg Zn2+.l−1) did not affect the resistance to antimicrobials, low pH, and the resistance to bile salt was affected only weakly. Zinc-resistant probiotic Lactobacillus strains are suitable for use in feedstuffs with a higher content of zinc designed for the prevention of post weaning diarrhoea in pigs.

Synthesis and Structural/Functional Characterization of Selective M14 Metallocarboxypeptidase Inhibitors Based on Phosphinic Pseudopeptide Scaffold: Implications on the Design of Specific Optical Probes

Metallocarboxypeptidases (MCPs) of the M14 family are Zn²⁺-dependent exoproteases present in almost every tissue or fluid in mammals. These enzymes perform a large variety of physiological functions and are involved in several pathologies, such as pancreatic diseases, inflammation, fibrinolysis, and cancer. Here, we describe the synthesis and functional/structural characterization of a series of reversible tight-binding phosphinic pseudopeptide inhibitors that show high specificity and potency toward these proteases. Characterization of their inhibitory potential against a large variety of MCPs, combined with high-resolution crystal structures of three selected candidates in complex with human carboxypeptidase A (CPA)1, allowed to decipher the structural determinants governing selectivity for type-A of the M14A MCP family. Further, the phosphinic pseudopeptide framework was exploited to generate an optical probe selectively targeting human CPAs. The phosphinic pseudopeptides presented here constitute the first example of chemical probes useful to selectively report on type-A MCPs activity in complex media.

Characterisation of the Carboxypeptidase G2 Catalytic Site and Design of New Inhibitors for Cancer Therapy

The enzyme carboxypeptidase G2 (CPG2) is used in antibody-directed enzyme prodrug therapy (ADEPT) to catalyse the formation of an active drug from an inert prodrug. Free CPG2 in the bloodstream must be inhibited before administration of the prodrug in order to avoid a systemic reaction in the patient. Although a few small-molecule CPG2 inhibitors have been reported, none has been taken forward thus far. This lack of progress is due in part to a lack of structural understanding of the CPG2 active site as well as the absence of small molecules that can block the active site whilst targeting the complex for clearance. The work described here aimed to address both areas. We report the structural/functional impact of extensive point mutation across the putative CPG2 catalytic site and adjacent regions for the first time, revealing that residues outside the catalytic region (K208A, S210A and T357A) are crucial to enzyme activity. We also describe novel molecules that inhibit CPG2 whilst maintaining the accessibility of galactosylated moieties aimed at targeting the enzyme for clearance. This work acts as a platform for the future development of high-affinity CPG2 inhibitors that occupy new chemical space and will advance the safe application of ADEPT in cancer treatment.

Engineering carboxypeptidase G2 circular permutations for the design of an autoinhibited enzyme

Carboxypeptidase G2 (CPG2) is an Food and Drug Administration (FDA)-approved enzyme drug used to treat methotrexate (MTX) toxicity in cancer patients receiving MTX treatment. It has also been used in directed enzyme-prodrug chemotherapy, but this strategy has been hampered by off-site activation of the prodrug by the circulating enzyme. The development of a tumor protease activatable CPG2, which could be achieved using a circular permutation of CPG2 fused to an inactivating 'prodomain', would aid in these applications. We report the development of a protease accessibility-based screen to identify candidate sites for circular permutation in proximity of the CPG2 active site. The resulting six circular permutants showed similar expression, structure, thermal stability, and, in four cases, activity levels compared to the wild-type enzyme. We rationalize these results based on structural models of the permutants obtained using the Rosetta software. We developed a cell growth-based selection system, and demonstrated that when fused to periplasm-directing signal peptides, one of our circular permutants confers MTX resistance in Escherichia coli with equal efficiency as the wild-type enzyme. As the permutants have similar properties to wild-type CPG2, these enzymes are promising starting points for the development of autoinhibited, protease-activatable zymogen forms of CPG2 for use in therapeutic contexts.

Evidence for Cleavage of the Metalloprotease Vsm from Vibrio splendidus Strain JZ6 by an M20 Peptidase (PepT-like Protein) at Low Temperature

Metalloprotease Vsm is a major extracellular virulence factor of Vibrio splendidus. The toxicity of Vsm from V. splendidus strain JZ6 has been characterized, and production of this virulence factor proved to be temperature-regulated. The present study provides evidence that two forms (JZE1 and JZE2) of Vsm protein exist in extracellular products (ECPs) of strain JZ6, and a significant conversion of these two forms was detected by SDS-PAGE and immunoblotting analyses of samples obtained from cells grown at 4, 10, 16, 20, 24, and 28°C. Mass spectroscopy confirmed that JZE1 was composed only of the peptidase_M4 domain of Vsm, and JZE2 contained both the PepSY domain and the peptidase_M4 domain. An M20 peptidase T-like protein (PepTL) was screened from the transcriptome data of strain JZ6, which was considered as a crucial molecule to produce the active Vsm (JZE1) by cleavage of the propeptide. Similar to that of Vsm, PepTL mRNA accumulation was highest at 4°C (836.82-fold of that at 28°C), decreased with increasing of temperature and reached its lowest level at 28°C. Deletion of the gene encoding the PepTL resulted in a mutant strain that did not produce the JZE1 cleavage product. The peptidase activity of PepTL recombinant protein (rPepTL) was confirmed by cleaving the Vsm in ECPs with an in vitro degradation reaction. These results demonstrate that PepTL participates in activating Vsm in strain JZ6 by proteolytic cleavage at low temperature.

Biochemical characterization of a novel carboxypeptidase inhibitor from a variety of Andean potatoes

Natural protease inhibitors of metallocarboxypeptidases are rarely reported. In this work, the cloning, expression and characterization of a proteinaceous inhibitor of the A/B-type metallocarboxypeptidases, naturally occurring in tubers of Solanum tuberosum, subsp. andigenum cv. Imilla morada, are described. The obtained cDNA encoded a polypeptide of 80 residues, which displayed the features of metallocarboxypeptidase inhibitor precursors from the Potato Carboxypeptidase Inhibitor (PCI) family. The mature polypeptide (39 residues) was named imaPCI and in comparison with the prototype molecule of the family (PCI from S. tuberosum subsp. tuberosum), its sequence showed one difference at its N-terminus and another three located at the secondary binding site, a region described to contribute to the stabilization of the complex inhibitor-target enzyme. In order to gain insights into the relevance of the secondary binding site in nature, a recombinant form of imaPCI (rimaPCI) having only differences at the secondary binding site with respect to recombinant PCI (rPCI) was cloned and expressed in Escherichia coli. The rimaPCI exhibited a molecular mass of 4234.8Da by MALDI-TOF/MS. It displayed potent inhibitory activity towards A/B-type carboxypeptidases (with a Ki in the nanomolar range), albeit 2-4-fold lower inhibitory capacity compared to its counterpart rPCI. This result is in agreement with our bioinformatic analysis, which showed that the main interaction established between the secondary binding site of rPCI and the bovine carboxypeptidase A is likely lost in the case of rimaPCI. These observations reinforce the importance of the secondary binding site of PCI-family members on inhibitory effects towards A/B-type metallocarboxypeptidases. Furthermore, as a simple proof of concept of its applicability in biotechnology and biomedicine, the ability of rimaPCI to protect human epidermal growth factor from C-terminal cleavage and inactivation by carboxypeptidases A and B was demonstrated.

Evidence for an essential role of intradimer interaction in catalytic function of carnosine dipeptidase II using electrospray-ionization mass spectrometry

Carnosine dipeptidase II (CN2/CNDP2) is an M20 family metallopeptidase that hydrolyses various dipeptides including β-alanyl-L-histidine (carnosine). Crystallographic analysis showed that CN2 monomer is composed of one catalytic and one dimerization domains, and likely to form homodimer. In this crystal, H228 residue of the dimerization domain interacts with the substrate analogue bestatin on the active site of the dimer counterpart, indicating that H228 is involved in enzymatic reaction. In the present study, the role of intradimer interaction of CN2 in its catalytic activity was investigated using electrospray-ionization time-of-flight mass spectrometry (ESI-TOF MS). First, a dimer interface mutant I319K was prepared and shown to be present as a folded monomer in solution as examined by using ESI-TOF MS. Since the mutant was inactive, it was suggested that dimer formation is essential to its enzymatic activity. Next, we prepared H228A and D132A mutant proteins with different N-terminal extended sequences, which enabled us to monitor dimer exchange reaction by ESI-TOF MS. The D132A mutant is a metal ligand mutant and also inactive. But the activity was partially recovered time-dependently when H228A and D132A mutant proteins were incubated together. In parallel, H228A/D132A heterodimer was formed as detected by ESI-TOF MS, indicating that interaction of a catalytic center with H228 residue of the other subunit is essential to the enzymatic reaction. These results provide evidence showing that intradimer interaction of H228 with the reaction center of the dimer counterpart is essential to the enzymatic activity of CN2.

Molecular basis of peptide recognition in metallopeptidase Dug1p from Saccharomyces cerevisiae

Dug1p, a M20 family metallopeptidase and human orthologue of carnosinase, hydrolyzes Cys-Gly dipeptide, the last step of glutathione (GSH) degradation in Saccharomyces cerevisiae. Molecular bases of peptide recognition by Dug1p and other M20 family peptidases remain unclear in the absence of structural information about enzyme-peptide complexes. We report the crystal structure of Dug1p at 2.55 Å resolution in complex with a Gly-Cys dipeptide and two Zn(2+) ions. The dipeptide is trapped in the tunnel-like active site; its C-terminus is held by residues at the S1' binding pocket, whereas the S1 pocket coordinates Zn(2+) ions and the N-terminus of the peptide. Superposition with the carnosinase structure shows that peptide mimics the inhibitor bestatin, but active site features are altered upon peptide binding. The space occupied by the N-terminus of bestatin is left unoccupied in the Dug1p structure, suggesting that tripeptides could bind. Modeling of tripeptides into the Dug1p active site showed tripeptides fit well. Guided by the structure and modeling, we examined the ability of Dug1p to hydrolyze tripeptides, and results show that Dug1p hydrolyzes tripeptides selectively. Point mutations of catalytic residues do not abolish the peptide binding but abolish the hydrolytic activity, suggesting a noncooperative mode in peptide recognition. In summary, results reveal that peptides are recognized primarily through their amino and carboxyl termini, but hydrolysis depends on the properties of peptide substrates, dictated by their respective sequences. Structural similarity between the Dug1p-peptide complex and the bestatin-bound complex of CN2 suggests that the Dug1p-peptide structure can be used as a template for designing natural peptide inhibitors.

Carnosinases, their substrates and diseases

Carnosinases are Xaa-His dipeptidases that play diverse functions throughout all kingdoms of life. Human isoforms of carnosinase (CN1 and CN2) under appropriate conditions catalyze the hydrolysis of the dipeptides carnosine (β-alanyl-l-histidine) and homocarnosine (γ-aminobutyryl-l-histidine). Alterations of serum carnosinase (CN1) activity has been associated with several pathological conditions, such as neurological disorders, chronic diseases and cancer. For this reason the use of carnosinase levels as a biomarker in cerebrospinal fluid (CSF) has been questioned. The hydrolysis of imidazole-related dipeptides in prokaryotes and eukaryotes is also catalyzed by aminoacyl-histidine dipeptidases like PepD (EC 3.4.13.3), PepV (EC 3.4.13.19) and anserinase (EC 3.4.13.5). The review deals with the structure and function of this class of enzymes in physiological and pathological conditions. The main substrates of these enzymes, i.e., carnosine, homocarnosine and anserine (β-alanyl-3-methyl-l-histidine) will also be described.

Entamoeba histolytica: identification of thioredoxin-targeted proteins and analysis of serine acetyltransferase-1 as a prototype example

Entamoeba histolytica, the causative agent of amoebiasis, possesses the dithiol-containing redox proteins Trx (thioredoxin) and TrxR (Trx reductase). Both proteins were found to be covalently modified and inactivated by metronidazole, a 5-nitroimidazole drug that is commonly used to treat infections with microaerophilic protozoan parasites in humans. Currently, very little is known about enzymes and other proteins participating in the Trx-dependent redox network of the parasite that could be indirectly affected by metronidazole treatment. On the basis of the disulfide/dithiol-exchange mechanism we constructed an active-site mutant of Trx, capable of binding interacting proteins as a stable mixed disulfide intermediate to screen the target proteome of Trx in E. histolytica. By applying Trx affinity chromatography, two-dimensional gel electrophoresis and MS, peroxiredoxin and 15 further potentially redox-regulated proteins were identified. Among them, EhSat1 (E. histolytica serine acetyltransferase-1), an enzyme involved in the L-cysteine biosynthetic pathway, was selected for detailed analysis. Binding of Trx to EhSat1 was verified by Far-Western blot analysis. Trx was able to restore the activity of the oxidatively damaged EhSat1 suggesting that the TrxR/Trx system protects sensitive proteins against oxidative stress in E. histolytica. Furthermore, the activity of peroxiredoxin, which is dependent on a functioning TrxR/Trx system, was strongly reduced in metronidazole-treated parasites.

Structure of an amidohydrolase, SACOL0085, from methicillin-resistant Staphylococcus aureus COL

Staphylococcus aureus is an opportunistic pathogen that rapidly acquires resistance to frontline antibiotics. The characterization of novel protein targets from this bacterium is thus an important step towards future therapeutic strategies. Here, the crystal structure of an amidohydrolase, SACOL0085, from S. aureus COL is described. SACOL0085 is a member of the M20D family of peptidases. Unlike other M20D peptidases, which are either monomers or dimers, SACOL0085 adopts a butterfly-shaped homotetrameric arrangement with extensive intersubunit interactions. Each subunit of SACOL0085 contains two Mn(2+) ions at the active site. A conserved cysteine residue at the active site distinguishes M20D peptidases from other M20 family members. This cysteine, Cys103, serves as bidentate ligand coordinating both Mn(2+) ions in SACOL0085.

Synthesis and modifications of phosphinic dipeptide analogues

Pseudopeptides containing the phosphinate moiety (-P(O)(OH)CH₂-) have been studied extensively, mainly as transition state analogue inhibitors of metalloproteases. The key synthetic aspect of their chemistry is construction of phosphinic dipeptide derivatives bearing appropriate side-chain substituents. Typically, this synthesis involves a multistep preparation of two individual building blocks, which are combined in the final step. As this methodology does not allow simple variation of the side-chain structure, many efforts have been dedicated to the development of alternative approaches. Recent achievements in this field are summarized in this review. Improved methods for the formation of the phosphinic peptide backbone, including stereoselective and multicomponent reactions, are presented. Parallel modifications leading to the structurally diversified substituents are also described. Finally, selected examples of the biomedical applications of the title compounds are given.

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.

Structure of human aspartyl aminopeptidase complexed with substrate analogue: insight into catalytic mechanism, substrate specificity and M18 peptidase family

Backround

Aspartyl aminopeptidase (DNPEP), with specificity towards an acidic amino acid at the N-terminus, is the only mammalian member among the poorly understood M18 peptidases. DNPEP has implicated roles in protein and peptide metabolism, as well as the renin-angiotensin system in blood pressure regulation. Despite previous enzyme and substrate characterization, structural details of DNPEP regarding ligand recognition and catalytic mechanism remain to be delineated.

Results

The crystal structure of human DNPEP complexed with zinc and a substrate analogue aspartate-β-hydroxamate reveals a dodecameric machinery built by domain-swapped dimers, in agreement with electron microscopy data. A structural comparison with bacterial homologues identifies unifying catalytic features among the poorly understood M18 enzymes. The bound ligands in the active site also reveal the coordination mode of the binuclear zinc centre and a substrate specificity pocket for acidic amino acids.

Conclusions

The DNPEP structure provides a molecular framework to understand its catalysis that is mediated by active site loop swapping, a mechanism likely adopted in other M18 and M42 metallopeptidases that form dodecameric complexes as a self-compartmentalization strategy. Small differences in the substrate binding pocket such as shape and positive charges, the latter conferred by a basic lysine residue, further provide the key to distinguishing substrate preference. Together, the structural knowledge will aid in the development of enzyme-/family-specific aminopeptidase inhibitors.

Purification and identification of two carnosine-cleaving enzymes, carnosine dipeptidase I and Xaa-methyl-His dipeptidase, from Japanese eel (Anguilla japonica)

Three enzymes, carnosine dipeptidase I (EC 3.4.13.20, CNDP1), carnosine dipeptidase II (EC 3.4.13.18, CNDP2), and Xaa-methyl-His dipeptidase (or anserinase: EC 3.4.13.5, ANSN), are known to be capable of catalyzing the hydrolysis of carnosine (β-alanyl-l-histidine), in vertebrates. Here we report the purification and identification of two unidentified carnosine-cleaving enzymes from Japanese eel (Anguilla japonica). Two different dipeptidases were successfully purified to homogeneity from the skeletal muscle; one exhibited a broad substrate specificity, while the other a narrow specificity. N-terminal amino-acid sequencing, deglycosylation analysis, and genetic analysis clearly revealed that the former is a homodimer of glycosylated subunits, encoded by ANSN, and the latter is another homodimer of glycosylated subunits, encoded by CNDP1; that is, Xaa-methyl-His dipeptidase, and carnosine dipeptidase I respectively. This is the first report on the identification of carnosine dipeptidase I from a non-mammal. Database search revealed presence of a CNDP1 ortholog only from salmonid fishes, including Atlantic salmon and rainbow trout, but not from other ray-finned fish species, such as zebrafish, fugu, and medaka whose genomes have been completely sequenced. The mRNAs of CNDP1 and ANSN are strongly expressed in the liver of Japanese eel, compared with other tissues, while that of CNDP2 is widely distributed in all tissues tested.

Transition metal ions induce carnosinase activity in PepD-homologous protein from Porphyromonas gingivalis

Aminoacylhistidine dipeptidase (EC 3.4.13.3; also Xaa-His dipeptidase, carnosinase, or PepD) catalyzes the cleavage and release of an N-terminal amino acid, which is usually a neutral or hydrophobic residue, from an Xaa-His dipeptide or degraded peptide fragment. PepD enzyme is found extensively in prokaryotes and eukaryotes, and belongs to the metallopeptidase family M20, a part of the metallopeptidase H (MH) clan. Carnosine is a naturally occurring dipeptide (β-alanyl-l-histidine) present in mammalian tissues that has protective functions in addition to anti-oxidant and free-radical scavenging roles. During bacterial infections, degradation of l-carnosine via carnosinase or PepD-like enzymes may enhance the destructive potential of bacteria, resulting in a pathological impact. This process has been proposed to act in an anti-oxidant manner in vivo. In the present study, the recombinant PepD protein encoded by Porphyromonas gingivalis TDC60 pepD was generated and biochemically characterized. In addition, a recombinant dipeptidase enzyme was found to function not only as an alanine-aminopeptidase, but also as a carnosinase. Furthermore, when carnosine was used as substrate for PepD, the transition metals, Mn(2+), Fe(2+), Co(2+), and Ni(2+) stimulated the hydrolyzing activity of rPepD with β-alanine and l-histidine. Based on its metal ion specificity, we propose that this enzyme should not only be termed l-aminopeptidase, but also a carnosinase.

Structural and functional analyses reveal that Staphylococcus aureus antibiotic resistance factor HmrA is a zinc-dependent endopeptidase

HmrA is an antibiotic resistance factor of methicillin-resistant Staphylococcus aureus. Molecular analysis of this protein revealed that it is not a muramidase or β-lactamase but a nonspecific double-zinc endopeptidase consisting of a catalytic domain and an inserted oligomerization domain, which probably undergo a relative interdomain hinge rotation upon substrate binding. The active-site cleft is located at the domain interface. Four HmrA protomers assemble to a large ∼170-kDa homotetrameric complex of 125 Å. All four active sites are fully accessible and ∼50-70 Å apart, far enough apart to act on a large meshwork substrate independently but simultaneously. In vivo studies with four S. aureus strains of variable resistance levels revealed that the extracellular addition of HmrA protects against loss of viability in the presence of oxacillin and that this protection depends on proteolytic activity. All of these results indicate that HmrA is a peptidase that participates in resistance mechanisms in vivo in the presence of β-lactams. Furthermore, our results have implications for most S. aureus strains of known genomic sequences and several other cocci and bacilli, which harbor close orthologs. This suggests that HmrA may be a new widespread antibiotic resistance factor in bacteria.

Molecular and biochemical characterization of the 5-nitroanthranilic acid degradation pathway in Bradyrhizobium sp. strain JS329

Biodegradation pathways of synthetic nitroaromatic compounds and anilines are well documented, but little is known about those of nitroanilines. We previously reported that the initial step in 5-nitroanthranilic acid (5NAA) degradation by Bradyrhizobium sp. strain JS329 is a hydrolytic deamination to form 5-nitrosalicylic acid (5NSA), followed by ring fission catalyzed by 5NSA dioxygenase. The mechanism of release of the nitro group was unknown. In this study, we subcloned, sequenced, and expressed the genes encoding 5NAA deaminase (5NAA aminohydrolase, NaaA), 5NSA dioxygenase (NaaB) and lactonase (NaaC), the key genes responsible for 5NAA degradation. Sequence analysis and enzyme characterization revealed that NaaA is a hydrolytic metalloenzyme with a narrow substrate range. The nitro group is spontaneously eliminated as nitrite concomitant with the formation of a lactone from the ring fission product of 5NSA dioxygenation. The elimination of the nitro group during lactone formation is a previously unreported mechanism for denitration of nitro aliphatic compounds.

Crystal structure of Staphylococcus aureus metallopeptidase (Sapep) reveals large domain motions between the manganese-bound and apo-states

Proteases belonging to the M20 family are characterized by diverse substrate specificity and participate in several metabolic pathways. The Staphylococcus aureus metallopeptidase, Sapep, is a member of the aminoacylase-I/M20 protein family. This protein is a Mn(2+)-dependent dipeptidase. The crystal structure of this protein in the Mn(2+)-bound form and in the open, metal-free state suggests that large interdomain movements could potentially regulate the activity of this enzyme. We note that the extended inactive conformation is stabilized by a disulfide bond in the vicinity of the active site. Although these cysteines, Cys(155) and Cys(178), are not active site residues, the reduced form of this enzyme is substantially more active as a dipeptidase. These findings acquire further relevance given a recent observation that this enzyme is only active in methicillin-resistant S. aureus. The structural and biochemical features of this enzyme provide a template for the design of novel methicillin-resistant S. aureus-specific therapeutics.

Crystal structure and mutational analysis of aminoacylhistidine dipeptidase from Vibrio alginolyticus reveal a new architecture of M20 metallopeptidases

Aminoacylhistidine dipeptidases (PepD, EC 3.4.13.3) belong to the family of M20 metallopeptidases from the metallopeptidase H clan that catalyze a broad range of dipeptide and tripeptide substrates, including L-carnosine and L-homocarnosine. Homocarnosine has been suggested as a precursor for the neurotransmitter γ-aminobutyric acid (GABA) and may mediate the antiseizure effects of GABAergic therapies. Here, we report the crystal structure of PepD from Vibrio alginolyticus and the results of mutational analysis of substrate-binding residues in the C-terminal as well as substrate specificity of the PepD catalytic domain-alone truncated protein PepD(CAT). The structure of PepD was found to exist as a homodimer, in which each monomer comprises a catalytic domain containing two zinc ions at the active site center for its hydrolytic function and a lid domain utilizing hydrogen bonds between helices to form the dimer interface. Although the PepD is structurally similar to PepV, which exists as a monomer, putative substrate-binding residues reside in different topological regions of the polypeptide chain. In addition, the lid domain of the PepD contains an "extra" domain not observed in related M20 family metallopeptidases with a dimeric structure. Mutational assays confirmed both the putative di-zinc allocations and the architecture of substrate recognition. In addition, the catalytic domain-alone truncated PepD(CAT) exhibited substrate specificity to l-homocarnosine compared with that of the wild-type PepD, indicating a potential value in applications of PepD(CAT) for GABAergic therapies or neuroprotection.

Investigation of atomic level patterns in protein--small ligand interactions

BACKGROUND

Shape complementarity and non-covalent interactions are believed to drive protein-ligand interaction. To date protein-protein, protein-DNA, and protein-RNA interactions were systematically investigated, which is in contrast to interactions with small ligands. We investigate the role of covalent and non-covalent bonds in protein-small ligand interactions using a comprehensive dataset of 2,320 complexes.

METHODOLOGY AND PRINCIPAL FINDINGS

We show that protein-ligand interactions are governed by different forces for different ligand types, i.e., protein-organic compound interactions are governed by hydrogen bonds, van der Waals contacts, and covalent bonds; protein-metal ion interactions are dominated by electrostatic force and coordination bonds; protein-anion interactions are established with electrostatic force, hydrogen bonds, and van der Waals contacts; and protein-inorganic cluster interactions are driven by coordination bonds. We extracted several frequently occurring atomic-level patterns concerning these interactions. For instance, 73% of investigated covalent bonds were summarized with just three patterns in which bonds are formed between thiol of Cys and carbon or sulfur atoms of ligands, and nitrogen of Lys and carbon of ligands. Similar patterns were found for the coordination bonds. Hydrogen bonds occur in 67% of protein-organic compound complexes and 66% of them are formed between NH- group of protein residues and oxygen atom of ligands. We quantify relative abundance of specific interaction types and discuss their characteristic features. The extracted protein-organic compound patterns are shown to complement and improve a geometric approach for prediction of binding sites.

CONCLUSIONS AND SIGNIFICANCE

We show that for a given type (group) of ligands and type of the interaction force, majority of protein-ligand interactions are repetitive and could be summarized with several simple atomic-level patterns. We summarize and analyze 10 frequently occurring interaction patterns that cover 56% of all considered complexes and we show a practical application for the patterns that concerns interactions with organic compounds.

Purification, crystallization and preliminary X-ray analysis of an aminoacylhistidine dipeptidase (PepD) from Vibrio alginolyticus

The aminoacylhistidine dipeptidase (PepD) protein encoded by Vibrio alginolyticus pepD was successfully overexpressed and characterized and the putative active-site residues responsible for metal binding and catalysis were identified. The purified enzyme contained two zinc ions per monomer. The recombinant dipeptidase enzyme, which was identified as a homodimer in solution, exhibited broad substrate specificity for Xaa-His dipeptides, with highest activity towards the His-His dipeptide. The purified protein was crystallized using the hanging-drop vapour-diffusion method. Preliminary crystallographic analysis showed that the crystal belonged to space group P6(1) or P6(5), with unit-cell parameters a = b = 80.42, c = 303.11 A. The crystal contained two molecules per asymmetric unit and the predicted solvent content was 53.4%.

Expression and characterization of the biofilm-related and carnosine-hydrolyzing aminoacylhistidine dipeptidase from Vibrio alginolyticus

The biofilm-related and carnosine-hydrolyzing aminoacylhistidine dipeptidase (pepD) gene from Vibrio alginolyticus was cloned and sequenced. The recombinant PepD protein was produced and biochemically characterized and the putative active-site residues responsible for metal binding and catalysis were identified. The recombinant enzyme, which was identified as a homodimeric dipeptidase in solution, exhibited broad substrate specificity for Xaa-His and His-Xaa dipeptides, with the highest activity for the His-His dipeptide. Sequence and structural homologies suggest that the enzyme is a member of the metal-dependent metallopeptidase family. Indeed, the purified enzyme contains two zinc ions per monomer. Reconstitution of His.Tag-cleaved native apo-PepD with various metal ions indicated that enzymatic activity could be optimally restored when Zn2+ was replaced with other divalent metal ions, including Mn2+, Co2+, Ni2+, Cu2+ and Cd2+, and partially restored when Zn2+ was replaced with Mg2+. Structural homology modeling of PepD also revealed a 'catalytic domain' and a 'lid domain' similar to those of the Lactobacillus delbrueckii PepV protein. Mutational analysis of the putative active-site residues supported the involvement of His80, Asp119, Glu150, Asp173 and His461 in metal binding and Asp82 and Glu149 in catalysis. In addition, individual substitution of Glu149 and Glu150 with aspartic acid resulted in the partial retention of enzymatic activity, indicating a functional role for these residues on the catalysis and zinc ions, respectively. These effects may be necessary either for the activation of the catalytic water molecule or for the stabilization of the substrate-enzyme tetrahedral intermediate. Taken together, these results may facilitate the design of PepD inhibitors for application in antimicrobial treatment and antibody-directed enzyme prodrug therapy.

Bacterioferritin from Mycobacterium smegmatis contains zinc in its di-nuclear site

Bacterioferritins, also known as cytochrome b (1), are oligomeric iron-storage proteins consisting of 24 identical amino acid chains, which form spherical particles consisting of 24 subunits and exhibiting 432 point-group symmetry. They contain one haem b molecule at the interface between two subunits and a di-nuclear metal binding center. The X-ray structure of bacterioferritin from Mycobacterium smegmatis (Ms-Bfr) was determined to a resolution of 2.7 A in the monoclinic space group C2. The asymmetric unit of the crystals contains 12 protein molecules: five dimers and two half-dimers located along the crystallographic twofold axis. Unexpectedly, the di-nuclear metal binding center contains zinc ions instead of the typically observed iron ions in other bacterioferritins.

Crystal structures of yeast beta-alanine synthase complexes reveal the mode of substrate binding and large scale domain closure movements

Beta-alanine synthase is the final enzyme of the reductive pyrimidine catabolic pathway, which is responsible for the breakdown of uracil and thymine in higher organisms. The fold of the homodimeric enzyme from the yeast Saccharomyces kluyveri identifies it as a member of the AcyI/M20 family of metallopeptidases. Its subunit consists of a catalytic domain harboring a di-zinc center and a smaller dimerization domain. The present site-directed mutagenesis studies identify Glu(159) and Arg(322) as crucial for catalysis and His(262) and His(397) as functionally important but not essential. We determined the crystal structures of wild-type beta-alanine synthase in complex with the reaction product beta-alanine, and of the mutant E159A with the substrate N-carbamyl-beta-alanine, revealing the closed state of a dimeric AcyI/M20 metallopeptidase-like enzyme. Subunit closure is achieved by a approximately 30 degrees rigid body domain rotation, which completes the active site by integration of substrate binding residues that belong to the dimerization domain of the same or the partner subunit. Substrate binding is achieved via a salt bridge, a number of hydrogen bonds, and coordination to one of the zinc ions of the di-metal center.

Bacterial β-peptidyl aminopeptidases: on the hydrolytic degradation of β-peptides

The special chemical and biological features of beta-peptides have been investigated intensively during recent years. Many studies emphasize the restricted biodegradability and the high metabolic stability of this class of compounds. beta-Peptidyl aminopeptidases form the first family of enzymes that hydrolyze a variety of short beta-peptides and beta-amino-acid-containing peptides. All representatives of this family were isolated from Gram-negative bacteria. The substrate specificities of the peptidases vary greatly, but the enzymes have common structural properties, and a similar reaction mechanism can be expected. This review gives an overview on the beta-peptidyl aminopeptidases with emphasis on their biochemical and structural properties. Their possible physiological function is discussed. Functionally and structurally related enzymes are compared to the beta-peptidyl aminopeptidases.

The alternative pathway of glutathione degradation is mediated by a novel protein complex involving three new genes in Saccharomyces cerevisiae

Glutathione (GSH), L-gamma-glutamyl-L-cysteinyl-glycine, is the major low-molecular-weight thiol compound present in almost all eukaryotic cells. GSH degradation proceeds through the gamma-glutamyl cycle that is initiated, in all organisms, by the action of gamma-glutamyl transpeptidase. A novel pathway for the degradation of GSH that requires the participation of three previously uncharacterized genes is described in the yeast Saccharomyces cerevisiae. These genes have been named DUG1 (YFR044c), DUG2 (YBR281c), and DUG3 (YNL191w) (defective in utilization of glutathione). Although dipeptides and tripeptides with a normal peptide bond such as cys-gly or glu-cys-gly required the presence of only a functional DUG1 gene that encoded a protein belonging to the M20A metallohydrolase family, the presence of an unusual peptide bond such as in the dipeptide, gamma-glu-cys, or in GSH, required the participation of the DUG2 and DUG3 gene products as well. The DUG2 gene encodes a protein with a peptidase domain and a large WD40 repeat region, while the DUG3 gene encoded a protein with a glutamine amidotransferase domain. The Dug1p, Dug2p, and Dug3p proteins were found to form a degradosomal complex through Dug1p-Dug2p and Dug2p-Dug3p interactions. A model is proposed for the functioning of the Dug1p/Dug2p/Dug3p proteins as a specific GSH degradosomal complex.

Characterization and kinetic analysis of enzyme-substrate recognition by three recombinant lactococcal PepVs

The dipeptidases (PepVs) from three typical lactococcal strains, Lactococcus lactis subsp. lactis (L9), L. lactis subsp. cremoris (L6) and L. lactis subsp. hordniae (hT) were cloned and characterized. The metal-binding, catalytic, and substrate-binding sites are highly conserved among of them. A computer-generated three-dimensional model suggested that the amino acid differences between these PepVs were mostly located away from the active center. L9 PepV does not hydrolyze dipeptides bearing Pro or D-amino acid at the C-terminal amino acid. Unlike PepV from Lactobacillus delbrueckii, L9 PepV does not cleave beta-Asp-His, and has little ability to cleave dipeptides containing a beta-alanine. In addition, L9 PepV has a much higher kcat for dipeptides with an N-terminal Ala but a significantly higher Km when the N-terminal amino acid is Gly. The substrate recognition profile of PepV is further discussed on the basis of the kinetic analysis and the structural model.

Crystallization and preliminary crystallographic study of carnosinase CN2 from mice

Mammalian tissues contain several histidine-containing dipeptides, of which L-carnosine is the best characterized and is found in various tissues including the brain and skeletal muscles. However, the mechanism for its biosynthesis and degradation have not yet been fully elucidated. Crystallographic study of carnosinase CN2 from mouse has been undertaken in order to understand its enzymatic mechanism from a structural viewpoint. CN2 was crystallized by the hanging-drop vapour-diffusion technique using PEG 3350 as a precipitant. Crystals were obtained in complex with either Mn(2+) or Zn(2+). Both crystals of CN2 belong to the monoclinic space group P2(1) and have almost identical unit-cell parameters (a = 54.41, b = 199.77, c = 55.49 A, beta = 118.52 degrees for the Zn(2+) complex crystals). Diffraction data were collected to 1.7 and 2.3 A for Zn(2+) and Mn(2+) complex crystals, respectively, using synchrotron radiation. Structure determination is ongoing using the multiple-wavelength anomalous diffraction (MAD) method.