Lysozyme's lectin-like characteristics facilitates its immune defense function.

Interactions between human lysozyme (HL) and the lipopolysaccharide (LPS) of Klebsiella pneumoniae O1, a causative agent of lung infection, were identified by surface plasmon resonance. To characterize the molecular mechanism of this interaction, HL binding to synthetic disaccharides and tetrasaccharides representing one and two repeating units, respectively, of the O-chain of this LPS were studied. pH-dependent structural rearrangements of HL after interaction with the disaccharide were observed through nuclear magnetic resonance. The crystal structure of the HL-tetrasaccharide complex revealed carbohydrate chain packing into the A, B, C, and D binding sites of HL, which primarily occurred through residue-specific, direct or water-mediated hydrogen bonds and hydrophobic contacts. Overall, these results support a crucial role of the Glu35/Asp53/Trp63/Asp102 residues in HL binding to the tetrasaccharide. These observations suggest an unknown glycan-guided mechanism that underlies recognition of the bacterial cell wall by lysozyme and may complement the HL immune defense function.

Complete protein assignment from sets of spectra recorded overnight.

A flexible and scalable approach for protein NMR is introduced that builds on rapid data collection via projection spectroscopy and analysis of the spectral input data via joint decomposition. Input data may originate from various types of spectra, depending on the ultimate goal: these may result from experiments based on triple-resonance pulse sequences, or on TOCSY or NOESY sequences, or mixtures thereof. Flexible refers to the free choice of spectra for the joint decompositions depending on the purpose: assignments, structure, dynamics, interactions. Scalable means that the approach is open to the addition of similar or different experiments, e.g. larger proteins may require a wider selection of triple-resonance based experiments. Central to the proposed approach is the mutual support among the different spectra during the spectral analysis: for example, sparser triple-resonance spectra may help decomposing (separating) spin systems in a TOCSY or identifying unique NOEs. In the example presented, backbone plus side chain assignments of ubiquitin were obtained from the combination of either two or three of the following projection experiments: a 4D HCCCONH, a 4D HNCACO and a 3D HNCACB. In all cases, TOCSY data (4D HCCCONH) proved crucial not only for the side chain assignments, but also for the sequential assignment. Even when total recording time was reduced to about 10 h, nearly complete assignments were obtained, with very few missing assignments and even fewer differences to a reference.

Solution structure and biophysical properties of MqsA, a Zn-containing antitoxin from Escherichia coli.

The gene ygiT (mqsA) of Escherichia coli encodes MqsA, the antitoxin of the motility quorum sensing regulator (MqsR). Both proteins are considered to form a DNA binding complex and to be involved in the formation of biofilms and persisters. We have determined the three-dimensional solution structure of MqsA by high-resolution NMR. The protein comprises a well-defined N-terminal domain with a Zn finger motif usually found in eukaryotes, and a defined C-terminal domain with a typical prokaryotic DNA binding helix-turn-helix motif. The two well-defined domains of MqsA have almost identical structure in solution and in the two published crystal structures of dimeric MqsA bound to either MqsR or DNA. However, the connection of the two domains with a flexible linker yields a large variety of possible conformations in solution, which is not reflected in the crystal structures. MqsA binds Zn with all four cysteines, a stoichiometry of 1:1 and a femtomolar affinity (K(a)≥ 10¹7M⁻¹ at 23°C, pH 7.0).

Initial DNA interactions of the binuclear threading intercalator Λ,Λ-[µ-bidppz(bipy)4Ru2]4+: an NMR study with [d(CGCGAATTCGCG)]2.

Binuclear polypyridine ruthenium compounds have been shown to slowly intercalate into DNA, following a fast initial binding on the DNA surface. For these compounds, intercalation requires threading of a bulky substituent, containing one Ru(II), through the DNA base-pair stack, and the accompanying DNA duplex distortions are much more severe than with intercalation of mononuclear compounds. Structural understanding of the process of intercalation may greatly gain from a characterisation of the initial interactions between binuclear Ru(II) compounds and DNA. We report a structural NMR study on the binuclear Ru(II) intercalator Λ,Λ-B (Λ,Λ-[µ-bidppz(bipy)4Ru2](4+); bidppz = 11,11'-bis(dipyrido[3,2-a:2',3'-c]phenazinyl, bipy = 2,2'-bipyridine) mixed with the palindromic DNA [d(CGCGAATTCGCG)]2. Threading of Λ,Λ-B depends on the presence and length of AT stretches in the DNA. Therefore, the latter was selected to promote initial binding, but due to the short stretch of AT base pairs, final intercalation is prevented. Structural calculations provide a model for the interaction: Λ,Λ-B is trapped in a well-defined surface-bound state consisting of an eccentric minor-groove binding. Most of the interaction enthalpy originates from electrostatic and van der Waals contacts, whereas intermolecular hydrogen bonds may help to define a unique position of Λ,Λ-B. Molecular dynamics simulations show that this minor-groove binding mode is stable on a nanosecond scale. To the best of our knowledge, this is the first structural study by NMR spectroscopy on a binuclear Ru compound bound to DNA. In the calculated structure, one of the positively charged Ru(2+) moieties is near the central AATT region; this is favourable in view of potential intercalation as observed by optical methods for DNA with longer AT stretches. Circular dichroism (CD) spectroscopy suggests that a similar binding geometry is formed in mixtures of Λ,Λ-B with natural calf thymus DNA. The present minor-groove binding mode is proposed to represent the initial surface interactions of binuclear Ru(II) compounds prior to intercalation into AT-rich DNA.

Automated protein backbone assignment using the projection-decomposition approach.

Spectral projection experiments by NMR in conjunction with decomposition analysis have been previously introduced for the backbone assignment of proteins; various pulse sequences as well as the behaviour with low signal-to-noise or chemical shift degeneracy have been illustrated. As a guide for routine applications of this combined tool, we provide here a systematic analysis on different types of proteins using welldefined run-time parameters. As a second result of this study, the backbone assignment module SHABBA was extensively rewritten and improved. Calculations on ubiquitin yielded again fully correct and nearly complete backbone and CHß assignments. For the 128 residue long azurin, missing assignments mostly affect Hα and Hß. Among the remaining backbone (plus Cß) nuclei 97.5 % could be assigned with 1.0 % differences to a reference. Finally, the new SHABBA algorithm was applied to projections recorded for a yeast histone protein domain at room temperature, where the protein is subject to partial unfolding: this leads to unobservable resonances (about a dozen missing signals in a normal 15N-HSQC) and extensive degeneracy among the resonances. From the clearly observable residues, 97.5 % of the backbone and CHßresonances could be assigned, of which only 0.8 % showed differences to published shifts. An additional study on the protein MMP20, which exhibits spectral difficulties to an even larger extent, explores the limitations of the approach.

TSAR: a program for automatic resonance assignment using 2D cross-sections of high dimensionality, high-resolution spectra.

While NMR studies of proteins typically aim at structure, dynamics or interactions, resonance assignments represent in almost all cases the initial step of the analysis. With increasing complexity of the NMR spectra, for example due to decreasing extent of ordered structure, this task often becomes both difficult and time-consuming, and the recording of high-dimensional data with high-resolution may be essential. Random sampling of the evolution time space, combined with sparse multidimensional Fourier transform (SMFT), allows for efficient recording of very high dimensional spectra (≥4 dimensions) while maintaining high resolution. However, the nature of this data demands for automation of the assignment process. Here we present the program TSAR (Tool for SMFT-based Assignment of Resonances), which exploits all advantages of SMFT input. Moreover, its flexibility allows to process data from any type of experiments that provide sequential connectivities. The algorithm was tested on several protein samples, including a disordered 81-residue fragment of the δ subunit of RNA polymerase from Bacillus subtilis containing various repetitive sequences. For our test examples, TSAR achieves a high percentage of assigned residues without any erroneous assignments.

NUScon: a community-driven platform for quantitative evaluation of nonuniform sampling in NMR.

Although the concepts of nonuniform sampling (NUS​​​​​​​) and non-Fourier spectral reconstruction in multidimensional NMR began to emerge 4 decades ago , it is only relatively recently that NUS has become more commonplace. Advantages of NUS include the ability to tailor experiments to reduce data collection time and to improve spectral quality, whether through detection of closely spaced peaks (i.e., "resolution") or peaks of weak intensity (i.e., "sensitivity"). Wider adoption of these methods is the result of improvements in computational performance, a growing abundance and flexibility of software, support from NMR spectrometer vendors, and the increased data sampling demands imposed by higher magnetic fields. However, the identification of best practices still remains a significant and unmet challenge. Unlike the discrete Fourier transform, non-Fourier methods used to reconstruct spectra from NUS data are nonlinear, depend on the complexity and nature of the signals, and lack quantitative or formal theory describing their performance. Seemingly subtle algorithmic differences may lead to significant variabilities in spectral qualities and artifacts. A community-based critical assessment of NUS challenge problems has been initiated, called the "Nonuniform Sampling Contest" (NUScon), with the objective of determining best practices for processing and analyzing NUS experiments. We address this objective by constructing challenges from NMR experiments that we inject with synthetic signals, and we process these challenges using workflows submitted by the community. In the initial rounds of NUScon our aim is to establish objective criteria for evaluating the quality of spectral reconstructions. We present here a software package for performing the quantitative analyses, and we present the results from the first two rounds of NUScon. We discuss the challenges that remain and present a roadmap for continued community-driven development with the ultimate aim of providing best practices in this rapidly evolving field. The NUScon software package and all data from evaluating the challenge problems are hosted on the NMRbox platform.

PRODECOMPv3: decompositions of NMR projections for protein backbone and side-chain assignments and structural studies.

UNLABELLED: PRODECOMP (projection decomposition) is an implementation of a multi-way decomposition algorithm for the analysis of two-dimensional projections of high-dimensional nuclear magnetic resonance spectra. The newest version, PRODECOMPv3, features a dramatic speedup, more reliable decompositions, a substantial reduction in memory demands, a new graphical user interface and integration into third-party software. These improvements extend the applicability of decompositions to novel types of NMR data on proteins, yielding backbone and side-chain assignments as well as structural information, and therewith enabling complete characterizations of proteins. AVAILABILITY: Program, short manual and an example calculation are freely available at www2.chem.gu.se/bcbp/nmr/prodecomp.html.

Nanomedical Relevance of the Intermolecular Interaction Dynamics-Examples from Lysozymes and Insulins.

Insulin and lysozyme share the common features of being prone to aggregate and having biomedical importance. Encapsulating lysozyme and insulin in micellar nanoparticles probably would prevent aggregation and facilitate oral drug delivery. Despite the vivid structural knowledge of lysozyme and insulin, the environment-dependent oligomerization (dimer, trimer, and multimer) and associated structural dynamics remain elusive. The knowledge of the intra- and intermolecular interaction profiles has cardinal importance for the design of encapsulation protocols. We have employed various biophysical methods such as NMR spectroscopy, X-ray crystallography, Thioflavin T fluorescence, and atomic force microscopy in conjugation with molecular modeling to improve the understanding of interaction dynamics during homo-oligomerization of lysozyme (human and hen egg) and insulin (porcine, human, and glargine). The results obtained depict the atomistic intra- and intermolecular interaction details of the homo-oligomerization and confirm the propensity to form fibrils. Taken together, the data accumulated and knowledge gained will further facilitate nanoparticle design and production with insulin or lysozyme-related protein encapsulation.

Propagation of dynamic changes in barnase upon binding of barstar: an NMR and computational study.

NMR spectroscopy and computer simulations were used to examine changes in chemical shifts and in dynamics of the ribonuclease barnase that result upon binding to its natural inhibitor barstar. Although the spatial structures of free and bound barnase are very similar, binding results in changes of the dynamics of both fast side-chains, as revealed by (2)H relaxation measurements, and NMR chemical shifts in an extended beta-sheet that is located far from the binding interface. Both side-chain dynamics and chemical shifts are sensitive to variations in the ensemble populations of the inter-converting molecular states, which can escape direct structural observation. Molecular dynamics simulations of free barnase and barnase in complex with barstar, as well as a normal mode analysis of barnase using a Gaussian network model, reveal relatively rigid domains that are separated by the extended beta-sheet mentioned above. The observed changes in NMR parameters upon ligation can thus be rationalized in terms of changes in inter-domain dynamics and in populations of exchanging states, without measurable structural changes. This provides an alternative model for the propagation of a molecular response to ligand binding across a protein that is based exclusively on changes in dynamics.

Structural characterization of an unusually stable cyclic peptide, kalata B2 from Oldenlandia affinis.

Kalata peptides are isolated from an African medicinal plant, Oldenlandia affinis, an aqueous decoction of which can be ingested to accelerate uterine contraction during childbirth. The closely packed disulfide core of kalata peptides confers unusual stability against thermal, chemical, and enzymatic degradation. The molecular arrangement may hamper NMR-assisted disulfide connectivity assignment. We have combined NMR with high-resolution mass spectrometry (MS) and MS/MS of native and chemically derivatized kalata B2 to determine its amino acid sequence and disulfide connectivity. Infrared multiphoton dissociation establishes the disulfide bond linkages in kalata B2 as I-IV, II-V and III-VI.

DNA adopts normal B-form upon incorporation of highly fluorescent DNA base analogue tC: NMR structure and UV-Vis spectroscopy characterization.

The influence of the highly fluorescent tricyclic cytosine base analogue (tC) on duplex DNA conformation is investigated. The duplex properties are characterized by absorbance and circular dichroism (CD) for all combinations of neighbouring bases to tC, and an NMR structure is determined for one tC-containing sequence. For the oligonucleotides with one tC incorporated instead of cytosine, the melting temperature is increased on average by 2.7 degrees C above that for the unmodified ones. CD spectra are practically identical for modified and unmodified sequences, indicating an unperturbed B-DNA conformation. The NMR structure determination of the self-complementary sequence 5'-CTC(tC)ACGTGGAG shows a DNA conformation consistent with B-form for the whole duplex. The root-mean-square distance for the nucleotides of the eight central base pairs between the 10 structures with lowest CYANA target functions and a mean structure is 0.45 +/- 0.17 A. The NMR data confirm correct base pairing for tC by the observation of both intrastrand and interstrand imino proton NOEs. Altogether, this suggests that tC works well as a cytosine analogue, i.e. it is situated in the base stack, forming hydrogen bonds with G in the complementary strand, without distorting the DNA backbone conformation. This first example of an artificial, highly fluorescent DNA base that does not perturb the DNA conformation could have valuable applications for the study of the structure and dynamics of nucleic acid systems.

DIADECOMP: A new approach to analyze decompositions from projection spectroscopy.

We demonstrate for the first time a complete small protein characterization with the projection-decomposition approach, including full assignments as well as determination of the 3D fold. In TOCSY- and NOESY-type 4D experiments, pairing of signals from hydrogens and from their respective heavy atoms in decompositions represents a new problem. An approach, referred to as "DIADECOMP" (diagonal decomposition), is introduced to solve this problem; it consists of two separate decompositions of the input projections, differing in a 45° rotation of the spectral axes. While DIADECOMP requires a somewhat complex formulation, in practice it results in observing signals in the rotated decompositions that correspond to sums or differences of frequencies. When applied to a small protein, human defensin ß6, the analysis of a HCC(CO)NH-TOCSY with DIADECOMP results in largely unambiguous assignments of the aliphatic side chain groups. Furthermore, DIADECOMP applied to a 15N-HSQC-NOESY-15N-HSQC provides all expected short distances between amide groups (defined as all HN-HN distances <3.5Å in a reference structure). It is worth noting that short HN-HN distances unambiguously define α-helices, the alignment of ß-strands in sheets, as well as the presence of ß-bulges. This approach of using a minimal amount of NMR data, namely four projection experiments recorded in ∼2.5days, resulted for the human defensin ß6 in complete assignments and a backbone fold with a RMSD of the non-flexible structure of 0.6Å. Uniqueness of decompositions specifically from TOCSY- and NOESY-type 4D experiments is discussed.

Molecular Basis of the Receptor Interactions of Polysialic Acid (polySia), polySia Mimetics, and Sulfated Polysaccharides.

Polysialic acid (polySia) and polySia glycomimetic molecules support nerve cell regeneration, differentiation, and neuronal plasticity. With a combination of biophysical and biochemical methods, as well as data mining and molecular modeling techniques, it is possible to correlate specific ligand-receptor interactions with biochemical processes and in vivo studies that focus on the potential therapeutic impact of polySia, polySia glycomimetics, and sulfated polysaccharides in neuronal diseases. With this strategy, the receptor interactions of polySia and polySia mimetics can be understood on a submolecular level. As the HNK-1 glycan also enhances neuronal functions, we tested whether similar sulfated oligo- and polysaccharides from seaweed could be suitable, in addition to polySia, for finding potential new routes into patient care focusing on an improved cure for various neuronal diseases. The knowledge obtained here on the structural interplay between polySia or sulfated polysaccharides and their receptors can be exploited to develop new drugs and application routes for the treatment of neurological diseases and dysfunctions.

Gated electron transfers and electron pathways in azurin: a NMR dynamic study at multiple fields and temperatures.

Dynamic properties of electron transfer pathways in a small blue copper cupredoxin are explored using an extensive 15N NMR relaxation study of reduced Pseudomonas aeruginosa azurin at four magnetic fields (500-900 MHz) and at two temperatures chosen well below the melting point of the protein. Following a careful model-free analysis, several protein regions with different dynamic regimes are identified. Nanosecond time-scale mobility characterizes various residues of the hydrophobic surface patch believed to mark the natural entry point for electrons, notably the surface-exposed copper-ligand His117. These findings are consistent with a gated electron transfer process according to the "dynamic docking" model. Residues 47-49 along intramolecular pathways of electrons show rigidity that is remarkably conserved when increasing the temperature. Three different conformational exchange processes were observed in the millisecond range, one near the only disulfide bridge in the molecule and two near the copper ion. The latter two processes are consistent with previous data such as crystal structures at various pH values and NMR relaxation dispersion experiments; they may indicate an additional gated electron transfer mechanism at slower time-scales.

Signal identification in NMR spectra with coupled evolution periods.

Novel multidimensional NMR experiments rely on modified time-domain sampling schemes to provide significant savings of experimental time. Several approaches are based on the coupling of evolution times resulting in a reduction of the dimensionality of the recorded spectra, and a concomitant saving of experimental time. We present a consistent and general tool, called EVOCOUP, for the analysis of these reduced dimensionality spectra. The approach is flexible in the sense that the input can consist of various forms of reduced dimensionality spectra, that any piece of information can be removed (provided enough information is left), e.g., signals undetectable due to poor signal-to-noise or covered by artifacts, and that it can be applied to spectra involving any number of nuclei. The use of a general optimization procedure and an appropriate target function provides for a robust approach with well-defined results and ensures optimal use of redundant information normally present in the input. Spectral overlap in the directly detected dimension is resolved in a fully automated manner, avoiding the assessment of signal quality and its use in combinatorial trials. The positions of all peaks in a corresponding full-dimensional spectrum are obtained without need for reconstruction of this spectrum. In a systematic analysis of a complete spectrum recorded for the 14 kDa protein azurin and involving five different nuclei, only four spin systems were missed and no false spins systems were detected.