PRE-driven protein NMR structures: an alternative approach in highly paramagnetic systems.

Metalloproteins play key roles across biology, and knowledge of their structure is essential to understand their physiological role. For those metalloproteins containing paramagnetic states, the enhanced relaxation caused by the unpaired electrons often makes signal detection unfeasible near the metal center, precluding adequate structural characterization right where it is more biochemically relevant. Here, we report a protein structure determination by NMR where two different sets of restraints, one containing Nuclear Overhauser Enhancements (NOEs) and another containing Paramagnetic Relaxation Enhancements (PREs), are used separately and eventually together. The protein PioC from Rhodopseudomonas palustris TIE-1 is a High Potential Iron-Sulfur Protein (HiPIP) where the [4Fe-4S] cluster is paramagnetic in both oxidation states at room temperature providing the source of PREs used as alternative distance restraints. Comparison of the family of structures obtained using NOEs only, PREs only, and the combination of both reveals that the pairwise root-mean-square deviation (RMSD) between them is similar and comparable with the precision within each family. This demonstrates that, under favorable conditions in terms of protein size and paramagnetic effects, PREs can efficiently complement and eventually replace NOEs for the structural characterization of small paramagnetic metalloproteins and de novo-designed metalloproteins by NMR. DATABASES: The 20 conformers with the lowest target function constituting the final family obtained using the full set of NMR restraints were deposited to the Protein Data Bank (PDB ID: 6XYV). The 20 conformers with the lowest target function obtained using NOEs only (PDB ID: 7A58) and PREs only (PDB ID: 7A4L) were also deposited to the Protein Data Bank. The chemical shift assignments were deposited to the BMRB (code 34487).

Evolution of a highly active and enantiospecific metalloenzyme from short peptides.

Primordial sequence signatures in modern proteins imply ancestral origins tracing back to simple peptides. Although short peptides seldom adopt unique folds, metal ions might have templated their assembly into higher-order structures in early evolution and imparted useful chemical reactivity. Recapitulating such a biogenetic scenario, we have combined design and laboratory evolution to transform a zinc-binding peptide into a globular enzyme capable of accelerating ester cleavage with exacting enantiospecificity and high catalytic efficiency (k cat/K M ~ 106 M-1 s-1). The simultaneous optimization of structure and function in a naïve peptide scaffold not only illustrates a plausible enzyme evolutionary pathway from the distant past to the present but also proffers exciting future opportunities for enzyme design and engineering.

Structure and electrochemistry of proteins harboring iron-sulfur clusters of different nuclearities. Part III. [4Fe-4S], [3Fe-4S] and [2Fe-2S] iron-sulfur proteins.

A systematic rationalization of the hundreds of proteins harboring iron-sulfur clusters and able to exhibit the most diverse biological functions is missing. In this picture we have already reviewed structure/electrochemistry of metalloproteins expressing single types of iron-sulfur centres [namely, {Fe(Cys)4}, {[Fe2S2](Cys)4}, {[Fe2S2](Cys)3(X)} (X = Asp, Arg, His), {[Fe2S2](Cys)2(His)2}, {[Fe3S4](Cys)3}, {[Fe4S4](Cys)4} and {[Fe4S4](SγCys)3(nonthiolate ligand)}] and their synthetic analogs. Recently we are focussing on structure/electrochemistry of metalloproteins containing iron-sulfur centres of different nuclearities. Having started such a subject with proteins harboring [4Fe-4S] and [2Fe-2S] (Zanello, 2017c) as well as [4Fe-4S] and [3Fe-4S] (Zanello, in press) clusters, we now provide the state of art of proteins harboring [4Fe-4S], [3Fe-4S] and [2Fe-2S] clusters, a subject that resulted strictly limited to enzymes active in the respiratory Complex II.

Structure and electrochemistry of proteins harboring iron-sulfur clusters of different nuclearities. Part II. [4Fe-4S] and [3Fe-4S] iron-sulfur proteins.

In the context of the plethora of proteins harboring iron-sulfur clusters we have already reviewed structure/electrochemistry of metalloproteins expressing single types of iron-sulfur clusters (namely: {Fe(Cys)4}, {[Fe2S2](Cys)4}, {[Fe2S2](Cys)3(X)} (X = Asp, Arg, His), {[Fe2S2](Cys)2(His)2}, {[Fe3S4](Cys)3}, {[Fe4S4](Cys)4} and {[Fe4S4](SγCys)3(nonthiolate ligand)} cores) and their synthetic analogs. More recently we are focussing on structure/electrochemistry of metalloproteins harboring iron-sulfur centres of different nuclearities. Having started such a subject with proteins harboring [4Fe-4S] and [2Fe-2S] clusters, we now depict the state of art of proteins containing [4Fe-4S] and [3Fe-4S] clusters.

Human FAD synthase is a bi-functional enzyme with a FAD hydrolase activity in the molybdopterin binding domain.

FAD synthase (FMN:ATP adenylyl transferase, FMNAT or FADS, EC 2.7.7.2) is involved in the biochemical pathway for converting riboflavin into FAD. Human FADS exists in different isoforms. Two of these have been characterized and are localized in different subcellular compartments. hFADS2 containing 490 amino acids shows a two domain organization: the 3'-phosphoadenosine-5'-phosphosulfate (PAPS) reductase domain, that is the FAD-forming catalytic domain, and a resembling molybdopterin-binding (MPTb) domain. By a multialignment of hFADS2 with other MPTb containing proteins of various organisms from bacteria to plants, the critical residues for hydrolytic function were identified. A homology model of the MPTb domain of hFADS2 was built, using as template the solved structure of a T. acidophilum enzyme. The capacity of hFADS2 to catalyse FAD hydrolysis was revealed. The recombinant hFADS2 was able to hydrolyse added FAD in a Co(2+) and mersalyl dependent reaction. The recombinant PAPS reductase domain is not able to perform the same function. The mutant C440A catalyses the same hydrolytic function of WT with no essential requirement for mersalyl, thus indicating the involvement of C440 in the control of hydrolysis switch. The enzyme C440A is also able to catalyse hydrolysis of FAD bound to the PAPS reductase domain, which is quantitatively converted into FMN.

Mutagenesis study to disrupt electrostatic interactions on the twofold symmetry interface of Escherichia coli bacterioferritin.

Ferritins and other cage proteins have been utilized as models to understand the fundamentals of protein folding and self-assembly. The bacterioferritin (BFR) from Escherichia coli, a maxi-ferritin made up of 24 subunits, was chosen as the basis for a mutagenesis study to investigate the role of electrostatic intermolecular interactions mediated through charged amino acids. Through structural and computational analyses, three charged amino acids R30, D56 and E60 which involved in an electrostatic interaction network were mutated to the opposite charge. Four mutants, R30D, D56R, E60H and D56R-E60H, were expressed, purified and characterized. All of the mutants fold into α-helical structures. Consistent with the computational prediction, they all show a lowered thermostability; double mutant D56R-E60H was found to be 16°C less stable than the wild type. Except for the mutant E60H, all the other mutations completely shut down the formation of protein cages to favour the dimer state in solution. The mutants, however, retain their ability to form cage-like nanostructures in the dried, surface immobilized conditions of transmission electron microscopy. Our findings confirm that even a single charge-inversion mutation at the 2-fold interface of BFR can affect the quaternary structure of its dimers and their ability to self-assemble into cage structures.

Function-based assessment of structural similarity measurements using metal co-factor orientation.

Structure comparison is widely used to quantify protein relationships. Although there are several approaches to calculate structural similarity, specifying significance thresholds for similarity metrics is difficult due to the inherent likeness of common secondary structure elements. In this study, metal co-factor location is used to assess the biological relevance of structural alignments. The distance between the centroids of bound co-factors adds a chemical and function-relevant constraint to the structural superimposition of two proteins. This additional dimension can be used to define cut-off values for discriminating valid and spurious alignments in large alignment sets. The hypothesis underlying our approach is that metal coordination sites constrain structural evolution, thus revealing functional relationships between distantly related proteins. A comparison of three related nitrogenases shows the sequence and fold constraints imposed on the protein structures up to 18 Å away from the centers of their bound metal clusters.

Stabilization of a protein nanocage through the plugging of a protein-protein interfacial water pocket.

The unique structural properties of the ferritin protein cages have provided impetus to focus on the methodical study of these self-assembling nanosystems. Among these proteins, Escherichia coli bacterioferritin (EcBfr), although architecturally very similar to other members of the family, shows structural instability and an incomplete self-assembly behavior by populating two oligomerization states. Through computational analysis and comparison to its homologues, we have found that this protein has a smaller than average dimeric interface on its 2-fold symmetry axis mainly because of the existence of an interfacial water pocket centered around two water-bridged asparagine residues. To investigate the possibility of engineering EcBfr for modified structural stability, we have used a semiempirical computational method to virtually explore the energy differences of the 480 possible mutants at the dimeric interface relative to that of wild-type EcBfr. This computational study also converged on the water-bridged asparagines. Replacing these two asparagines with hydrophobic amino acids resulted in proteins that folded into α-helical monomers and assembled into cages as evidenced by circular dichroism and transmission electron microscopy. Both thermal and chemical denaturation confirmed that, in all cases, these proteins, in agreement with the calculations, possessed increased stability. One of the three mutations shifts the population in favor of the higher-order oligomerization state in solution as evidenced by both size exclusion chromatography and native gel electrophoresis. These results taken together suggest that our low-level design was successful and that it may be possible to apply the strategy of targeting water pockets at protein--protein interfaces to other protein cage and self-assembling systems. More generally, this study further demonstrates the power of jointly employing in silico and in vitro techniques to understand and enhance biostructural energetics.

Nanoscale spectroscopy and imaging of hemoglobin.

Sub diffraction limited infrared absorption imaging of hemoglobin was performed by coupling IR optics with an atomic force microscope. Comparisons between the AFM topography and IR absorption images of micron sized hemoglobin features are presented, along with nanoscale IR spectroscopic analysis of the metalloprotein.

Can soaked-in scavengers protect metalloprotein active sites from reduction during data collection?

One of the first events taking place when a crystal of a metalloprotein is exposed to X-ray radiation is photoreduction of the metal centres. The oxidation state of a metal cannot always be determined from routine X-ray diffraction experiments alone, but it may have a crucial impact on the metal's environment and on the analysis of the structural data when considering the functional mechanism of a metalloenzyme. Here, UV-Vis microspectrophotometry is used to test the efficacy of selected scavengers in reducing the undesirable photoreduction of the iron and copper centres in myoglobin and azurin, respectively, and X-ray crystallography to assess their capacity of mitigating global and specific radiation damage effects. UV-Vis absorption spectra of native crystals, as well as those soaked in 18 different radioprotectants, show dramatic metal reduction occurring in the first 60 s of irradiation with an X-ray beam from a third-generation synchrotron source. Among the tested radioprotectants only potassium hexacyanoferrate(III) seems to be capable of partially mitigating the rate of metal photoreduction at the concentrations used, but not to a sufficient extent that would allow a complete data set to be recorded from a fully oxidized crystal. On the other hand, analysis of the X-ray crystallographic data confirms ascorbate as an efficient protecting agent against radiation damage, other than metal centre reduction, and suggests further testing of HEPES and 2,3-dichloro-1,4-naphtoquinone as potential scavengers.

Metal binding affinity and selectivity in metalloproteins: insights from computational studies.

This review highlights insights gained from computational studies on protein-metal recognition. We systematically dissect the various factors governing metal binding affinity and selectivity in proteins starting from (a) the intrinsic properties of the metal and neighboring metal cations (if present), to (b) the primary coordination sphere, (c) the second coordination shell, (d) the protein matrix, (e) the bulk solvent, and (f) competing non-protein ligands from the surrounding biological environment. The results herein reveal the fundamental principles and the molecular bases underlying protein-metal recognition, which serve as a guide to engineer novel metalloproteins with programmed properties.

Predicting zinc binding at the proteome level.

BACKGROUND: Metalloproteins are proteins capable of binding one or more metal ions, which may be required for their biological function, for regulation of their activities or for structural purposes. Metal-binding properties remain difficult to predict as well as to investigate experimentally at the whole-proteome level. Consequently, the current knowledge about metalloproteins is only partial. RESULTS: The present work reports on the development of a machine learning method for the prediction of the zinc-binding state of pairs of nearby amino-acids, using predictors based on support vector machines. The predictor was trained using chains containing zinc-binding sites and non-metalloproteins in order to provide positive and negative examples. Results based on strong non-redundancy tests prove that (1) zinc-binding residues can be predicted and (2) modelling the correlation between the binding state of nearby residues significantly improves performance. The trained predictor was then applied to the human proteome. The present results were in good agreement with the outcomes of previous, highly manually curated, efforts for the identification of human zinc-binding proteins. Some unprecedented zinc-binding sites could be identified, and were further validated through structural modelling. The software implementing the predictor is freely available at: http://zincfinder.dsi.unifi.it CONCLUSION: The proposed approach constitutes a highly automated tool for the identification of metalloproteins, which provides results of comparable quality with respect to highly manually refined predictions. The ability to model correlations between pairwise residues allows it to obtain a significant improvement over standard 1D based approaches. In addition, the method permits the identification of unprecedented metal sites, providing important hints for the work of experimentalists.

NMR structures of paramagnetic metalloproteins.

Metalloproteins represent a large share of the proteome and many of them contain paramagnetic metal ions. The knowledge, at atomic resolution, of their structure in solution is important to understand processes in which they are involved, such as electron transfer mechanisms, enzymatic reactions, metal homeostasis and metal trafficking, as well as interactions with their partners. Formerly considered as unfeasible, the first structure in solution by nuclear magnetic resonance (NMR) of a paramagnetic protein was obtained in 1994. Methodological and instrumental advancements pursued over the last decade are such that NMR structure of paramagnetic proteins may be now routinely obtained. We focus here on approaches and problems related to the structure determination of paramagnetic proteins in solution through NMR spectroscopy. After a survey of the background theory, we show how the effects produced by the presence of a paramagnetic metal ion on the NMR parameters, which are in many cases deleterious for the detection of NMR spectra, can be overcome and turned into an additional source of structural restraints. We also briefly address features and perspectives given by the use of 13C-detected protonless NMR spectroscopy for proteins in solution. The structural information obtained through the exploitation of a paramagnetic center are discussed for some Cu2+ -binding proteins and for Ca2+ -binding proteins, where the replacement of a diamagnetic metal ion with suitable paramagnetic metal ions suggests novel approaches to the structural characterization of proteins containing diamagnetic and NMR-silent metal ions.

An approach to long-range electron transfer mechanisms in metalloproteins: in situ scanning tunneling microscopy with submolecular resolution.

In situ scanning tunneling microscopy (STM) of redox molecules, in aqueous solution, shows interesting analogies and differences compared with interfacial electrochemical electron transfer (ET) and ET in homogeneous solution. This is because the redox level represents a deep indentation in the tunnel barrier, with possible temporary electronic population. Particular perspectives are that both the bias voltage and the overvoltage relative to a reference electrode can be controlled, reflected in spectroscopic features when the potential variation brings the redox level to cross the Fermi levels of the substrate and tip. The blue copper protein azurin adsorbs on gold(111) via a surface disulfide group. Well resolved in situ STM images show arrays of molecules on the triangular gold(111) terraces. This points to the feasibility of in situ STM of redox metalloproteins directly in their natural aqueous medium. Each structure also shows a central brighter contrast in the constant current mode, indicative of 2- to 4-fold current enhancement compared with the peripheral parts. This supports the notion of tunneling via the redox level of the copper atom and of in situ STM as a new approach to long-range electron tunneling in metalloproteins.

Structures of murine carbonic anhydrase IV and human carbonic anhydrase II complexed with brinzolamide: molecular basis of isozyme-drug discrimination.

Carbonic anhydrase IV (CAIV) is a membrane-associated enzyme anchored to plasma membrane surfaces by a phosphatidylinositol glycan linkage. We have determined the 2.8-angstroms resolution crystal structure of a truncated, soluble form of recombinant murine CAIV. We have also determined the structure of its complex with a drug used for glaucoma therapy, the sulfonamide inhibitor brinzolamide (Azopt). The overall structure of murine CAIV is generally similar to that of human CAIV; however, some local structural differences are found in the active site resulting from amino acid sequence differences in the "130's segment" and the residue-63 loop (these may affect the nearby catalytic proton shuttle, His-64). Similar to human CAIV, the C-terminus of murine CAIV is surrounded by a substantial electropositive surface potential that may stabilize the interaction with the phospholipid membrane. Binding interactions observed for brinzolamide rationalize the generally weaker affinity of inhibitors used in glaucoma therapy toward CAIV compared with CAII.

The crystal structure of hepatitis C virus NS3 proteinase reveals a trypsin-like fold and a structural zinc binding site.

During replication of hepatitis C virus (HCV), the final steps of polyprotein processing are performed by a viral proteinase located in the N-terminal one-third of nonstructural protein 3. The structure of NS3 proteinase from HCV BK strain was determined by X-ray crystallography at 2.4 angstrom resolution. NS3P folds as a trypsin-like proteinase with two beta barrels and a catalytic triad of His-57, Asp-81, Ser-139. The structure has a substrate-binding site consistent with the cleavage specificity of the enzyme. Novel features include a structural zinc-binding site and a long N-terminus that interacts with neighboring molecules by binding to a hydrophobic surface patch.

Elongation factor TFIIS contains three structural domains: solution structure of domain II.

Transcription elongation by RNA polymerase II is regulated by the general elongation factor TFIIS. This factor stimulates RNA polymerase II to transcribe through regions of DNA that promote the formation of stalled ternary complexes. Limited proteolytic digestion showed that yeast TFIIS is composed of three structural domains, termed I, II, and III. The two C-terminal domains (II and III) are required for transcription activity. The structure of domain III has been solved previously by using NMR spectroscopy. Here, we report the NMR-derived structure of domain II: a three-helix bundle built around a hydrophobic core composed largely of three tyrosines protruding from one face of the C-terminal helix. The arrangement of known inactivating mutations of TFIIS suggests that two surfaces of domain II are critical for transcription activity.

The structure of a phytocyanin, the basic blue protein from cucumber, refined at 1.8 A resolution.

The crystal structure of the cucumber basic protein (CBP), a type 1 or blue copper protein, has been refined at 1.8 A resolution. The molecule resembles other blue copper proteins in having a Greek key beta-barrel structure, except that the barrel is open on one side and is better described as a "beta-sandwich" or "beta-taco". The Cu atom has the normal blue copper NNSS' co-ordination with bond lengths Cu-N(His39) = 1.93 A, Cu-S(Cys79) = 2.16 A, Cu-N(His84) = 1.95 A, Cu-S(Met89) = 2.61 A. The Cu-S(Met) bond is the shortest so far observed in a blue copper protein. A disulphide link, (Cys52)-S-S-(Cys85), appears to play an important role in stabilising the molecular structure. It is suggested that the polypeptide fold is typical of a sub-family of blue copper proteins (phytocyanins) as well as a non-metalloprotein, ragweed allergen Ra3, with which CBP has a high degree of sequence identify. The proteins currently identifiable as phytocyanins are CBP, stellacyanin, mavicyanin, umecyanin, a cucumber peeling cupredoxin, a putative blue copper protein in pea pods, and a blue copper protein from Arabidopsis thaliana. In all except CBP and the pea-pod protein, the axial methionine ligand normally found at blue copper sites is replaced by glutamine. The structure of CBP was originally solved by the multiple wavelength anomalous scattering method, using data recorded at four wavelengths. All these data were included in the restrained least squares refinement. The final model comprises 96 amino acid residues, 122 solvent molecules and a copper atom. Several residues are modelled as having more than one conformation. The residual R is 0.141 for 41,910 observations (including Bijvoet-related observations) of 8.142 unique reflections in the resolution range 7 to 1.8 A.

Crystal structure of tRNA-guanine transglycosylase: RNA modification by base exchange.

tRNA-guanine transglycosylases (TGT) are enzymes involved in the modification of the anticodon of tRNAs specific for Asn, Asp, His and Tyr, leading to the replacement of guanine-34 at the wobble position by the hypermodified base queuine. In prokaryotes TGT catalyzes the exchange of guanine-34 with the queuine (.)precursor 7-aminomethyl-7-deazaguanine (preQ1). The crystal structure of TGT from Zymomonas mobilis was solved by multiple isomorphous replacement and refined to a crystallographic R-factor of 19% at 1.85 angstrom resolution. The structure consists of an irregular (beta/alpha)8-barrel with a tightly attached C-terminal zinc-containing subdomain. The packing of the subdomain against the barrel is mediated by an alpha-helix, located close to the C-terminus, which displaces the eighth helix of the barrel. The structure of TGT in complex with preQ1 suggests a binding mode for tRNA where the phosphate backbone interacts with the zinc subdomain and the U33G34U35 sequence is recognized by the barrel. This model for tRNA binding is consistent with a base exchange mechanism involving a covalent tRNA-enzyme intermediate. This structure is the first example of a (beta/alpha)-barrel protein interacting specifically with a nucleic acid.

Characterization of crystalline formate dehydrogenase H from Escherichia coli. Stabilization, EPR spectroscopy, and preliminary crystallographic analysis.

The selenocysteine-containing formate dehydrogenase H (FDH) is an 80-kDa component of the Escherichia coli formate-hydrogen lyase complex. The molybdenum-coordinated selenocysteine is essential for catalytic activity of the native enzyme. FDH in dilute solutions (30 microg/ml) was rapidly inactivated at basic pH or in the presence of formate under anaerobic conditions, but at higher enzyme concentrations (>/=3 mg/ml) the enzyme was relatively stable. The formate-reduced enzyme was extremely sensitive to air inactivation under all conditions examined. Active formate-reduced FDH was crystallized under anaerobic conditions in the presence of ammonium sulfate and PEG 400. The crystals diffract to 2.6 A resolution and belong to a space group of P4(1)2(1)2 or P4(3)2(1)2 with unit cell dimensions a = b = 146.1 A and c = 82.7 A. There is one monomer of FDH per crystallographic asymmetric unit. Similar diffraction quality crystals of oxidized FDH could be obtained by oxidation of crystals of formate-reduced enzyme with benzyl viologen. By EPR spectroscopy, a signal of a single reduced FeS cluster was found in a crystal of reduced FDH, but not in a crystal of oxidized enzyme, whereas Mo(V) signal was not detected in either form of crystalline FDH. This suggests that Mo(IV)- and the reduced FeS cluster-containing form of the enzyme was crystallized and this could be converted into Mo(VI)- and oxidized FeS cluster form upon oxidation. A procedure that combines anaerobic and cryocrystallography has been developed that is generally applicable to crystallographic studies of oxygen-sensitive enzymes. These data provide the first example of crystallization of a substrate-reduced form of a Se- and Mo-containing enzyme.