Quantitative analysis of protein-ligand interactions by NMR

Cyclodextrin-based poly(pseudo)rotaxanes for transdermal delivery of carvedilol

This work aimed to design supramolecular gels combining Soluplus or Solutol and alfa- and hydroxypropyl-β-cyclodextrin (α-CD, HPβ-CD) for carvedilol (CAR) transdermal delivery. Poly(pseudo)rotaxane formation (appearance, SEM, ¹H NMR), drug solubilization, rheological properties and in vitro release were investigated. CAR-CD complexes were prepared in situ or by spray drying. For Solutol, poly(pseudo)rotaxanes were formed immediately after mixing with α-CD and did not influence CAR solubility. Differently, Soluplus poly(pseudo)rotaxanes took 24-48 h to be formed and CAR solubility decreased compared to Soluplus micelles. Soluplus 20% + α-CD (5-10%) showed higher G' and G'' but also faster CAR release than Solutol poly(pseudo)rotaxanes, which is explained by the different location of PEG chains in the two amphiphilic polymers. Faster drug release was achieved incorporating HPβ-CD or CAR-HPβ-CD spray-dried complexes. The results evidenced the versatility of the formulations in terms of rheological behavior and drug release patterns, which can be adjusted for CAR transdermal delivery.

Biliverdin Reductase B Dynamics Are Coupled to Coenzyme Binding

Biliverdin reductase B (BLVRB) is a newly identified cellular redox regulator that catalyzes the NADPH-dependent reduction of multiple substrates. Through mass spectrometry analysis, we identified high levels of BLVRB in mature red blood cells, highlighting the importance of BLVRB in redox regulation. The BLVRB conformational changes that occur during conezyme/substrate binding and the role of dynamics in BLVRB function, however, remain unknown. Through a combination of NMR, kinetics, and isothermal titration calorimetry studies, we determined that BLVRB binds its coenzyme 500-fold more tightly than its substrate. While the active site of apo BLVRB is highly dynamic on multiple timescales, active site dynamics are largely quenched within holo BLVRB, in which dynamics are redistributed to other regions of the enzyme. We show that a single point mutation of Arg78➔Ala leads to both an increase in active site micro-millisecond motions and an increase in the microscopic rate constants of coenzyme binding. This demonstrates that altering BLVRB active site dynamics can directly cause a change in functional characteristics. Our studies thus address the solution behavior of apo and holo BLVRB and identify a role of enzyme dynamics in coenzyme binding.

Investigating Protein-Ligand Interactions by Solution Nuclear Magnetic Resonance Spectroscopy

Protein-ligand interactions are of fundamental importance in almost all processes in living organisms. The ligands comprise small molecules, drugs or biological macromolecules and their interaction strength varies over several orders of magnitude. Solution NMR spectroscopy offers a large repertoire of techniques to study such complexes. Here, we give an overview of the different NMR approaches available. The information they provide ranges from the simple information about the presence of binding or epitope mapping to the complete 3 D structure of the complex. NMR spectroscopy is particularly useful for the study of weak interactions and for the screening of binding ligands with atomic resolution.

1H, 13C, and 15N resonance assignments of CW domain of the N-methyltransferase ASHH2 free and bound to the mono-, di- and tri-methylated histone H3 tail peptides

The ASHH2 CW domain is responsible for recognizing the methylation state at lysine 4 of histone 3 N-terminal tails and implicated in the recruitment of the ASHH2 methyltransferase enzyme correctly to the histones. The ASHH2 CW domain binds H3 lysine motifs that can be either mono-, di-, or tri-methylated [ARTK(meX)QTAR, where X denotes the number of methylations], but binds strongest to monomethylated instances (K_d values reported in the range of 1 µm to 500 nM). Hoppmann et al. published the uncomplexed NMR structure of an ASHH2 CW domain in 2011. Here we document the assignment of a shortened ASHH2 CW construct, CW42, with similar binding affinity and better expression yields than the one used to solve the uncomplexed structure. We also perform ¹H-¹⁵N HSQC-monitored titrations that document at which protein-peptide ratios the complex is saturated. Backbone resonance assignments are presented for this shortened ASHH2 CW domain alone and bound to an H3 histone tail mimicking peptide monomethylated on lysine 4 (ARTK(me1)QTAR). Likewise, the assignment was also performed for the protein in complex with the dimethylated (ARTK(me2)QTAR) and trimethylated (ARTK(me3)QTAR) peptide. Overall, these two latter situations displayed a similar perturbation of shifts as the mono-methylated instance. In the case of the monomethylated histone tail mimic, side-chain assignment of CW42 in this complex was performed and reported in addition to backbone assignment, in preparation of a future solution structure determination and dynamics characterization of the CW42-ARTK(me1)QTAR complex.

Current NMR Techniques for Structure-Based Drug Discovery

A variety of nuclear magnetic resonance (NMR) applications have been developed for structure-based drug discovery (SBDD). NMR provides many advantages over other methods, such as the ability to directly observe chemical compounds and target biomolecules, and to be used for ligand-based and protein-based approaches. NMR can also provide important information about the interactions in a protein-ligand complex, such as structure, dynamics, and affinity, even when the interaction is too weak to be detected by ELISA or fluorescence resonance energy transfer (FRET)-based high-throughput screening (HTS) or to be crystalized. In this study, we reviewed current NMR techniques. We focused on recent progress in NMR measurement and sample preparation techniques that have expanded the potential of NMR-based SBDD, such as fluorine NMR (¹⁹F-NMR) screening, structure modeling of weak complexes, and site-specific isotope labeling of challenging targets.

Interaction study between HCV NS5A-D2 and NS5B using 19F NMR

The non structural protein 5A (NS5A) regulates the replication of the hepatitis C viral RNA through a direct molecular interaction of its domain 2 (NS5A-D2) with the RNA dependent RNA polymerase NS5B. Because of conflicting data in the literature, we study here this molecular interaction using fluorinated versions of the NS5A-D2 protein derived from the JFH1 Hepatitis C Virus strain. Two methods to prepare fluorine-labelled NS5A-D2 involving the biosynthetic incorporation of a ¹⁹F-tryptophan using 5-fluoroindole and the posttranslational introduction of fluorine by chemical conjugation of 2-iodo-N-(trifluoromethyl)acetamide with the NS5A-D2 cysteine side chains are presented. The dissociation constants (K_D) between NS5A-D2 and NS5B obtained with these two methods are in good agreement, and yield values comparable to those derived previously from a surface plasmon resonance study. We compare benefits and limitations of both labeling methods to study the interaction between an intrinsically disordered protein and a large molecular target by ¹⁹F NMR.

Atomic structural details of a protein grafted onto gold nanoparticles

The development of a methodology for the structural characterization at atomic detail of proteins conjugated to nanoparticles would be a breakthrough in nanotechnology. Solution and solid-state NMR spectroscopies are currently used to investigate molecules and peptides grafted onto nanoparticles, but the strategies used so far fall short in the application to proteins, which represent a thrilling development in theranostics. We here demonstrate the feasibility of highly-resolved multidimensional heteronuclear spectra of a large protein assembly conjugated to PEGylated gold nanoparticles. The spectra have been obtained by direct proton detection under fast MAS and allow for both a fast fingerprinting for the assessment of the preservation of the native fold and for resonance assignment. We thus demonstrate that the structural characterization and the application of the structure-based methodologies to proteins bound to gold nanoparticles is feasible and potentially extensible to other hybrid protein-nanomaterials.

NMR studies of ligand binding

NMR spectroscopy is an established tool in drug discovery, but its strength is commonly regarded to be largely confined to the early stages of hit discovery and fragment based drug design, where NMR offers unique capabilities of characterizing the binding modes of ligand molecules that bind sufficiently weakly to be in rapid exchange between bound and free state. Here we, first, provide a meta-review of recent reviews on NMR studies of ligand binding and, second, review recent progress towards NMR characterization of the ligand binding mode in stable protein-ligand complexes, with particular emphasis on the global positioning system (GPS) approach enabled by paramagnetic lanthanide tags.

Characterization of Chemical Exchange Using Relaxation Dispersion of Hyperpolarized Nuclear Spins

Chemical exchange phenomena are ubiquitous in macromolecules, which undergo conformational change or ligand complexation. NMR relaxation dispersion (RD) spectroscopy based on a Carr-Purcell-Meiboom-Gill pulse sequence is widely applied to identify the exchange and measure the lifetime of intermediate states on the millisecond time scale. Advances in hyperpolarization methods improve the applicability of NMR spectroscopy when rapid acquisitions or low concentrations are required, through an increase in signal strength by several orders of magnitude. Here, we demonstrate the measurement of chemical exchange from a single aliquot of a ligand hyperpolarized by dissolution dynamic nuclear polarization (D-DNP). Transverse relaxation rates are measured simultaneously at different pulsing delays by dual-channel ¹⁹F NMR spectroscopy. This two-point measurement is shown to allow the determination of the exchange term in the relaxation rate expression. For the ligand 4-(trifluoromethyl)benzene-1-carboximidamide binding to the protein trypsin, the exchange term is found to be equal within error limits in neutral and acidic environments from D-DNP NMR spectroscopy, corresponding to a pre-equilibrium of trypsin deprotonation. This finding illustrates the capability for determination of binding mechanisms using D-DNP RD. Taking advantage of hyperpolarization, the ligand concentration in the exchange measurements can reach on the order of tens of μM and protein concentration can be below 1 μM, i.e., conditions typically accessible in drug discovery.

Identification of Small Molecule Translesion Synthesis Inhibitors That Target the Rev1-CT/RIR Protein-Protein Interaction

Translesion synthesis (TLS) is an important mechanism through which proliferating cells tolerate DNA damage during replication. The mutagenic Rev1/Polζ-dependent branch of TLS helps cancer cells survive first-line genotoxic chemotherapy and introduces mutations that can contribute to the acquired resistance so often observed with standard anticancer regimens. As such, inhibition of Rev1/Polζ-dependent TLS has recently emerged as a strategy to enhance the efficacy of first-line chemotherapy and reduce the acquisition of chemoresistance by decreasing tumor mutation rate. The TLS DNA polymerase Rev1 serves as an integral scaffolding protein that mediates the assembly of the active multiprotein TLS complexes. Protein-protein interactions (PPIs) between the C-terminal domain of Rev1 (Rev1-CT) and the Rev1-interacting region (RIR) of other TLS DNA polymerases play an essential role in regulating TLS activity. To probe whether disrupting the Rev1-CT/RIR PPI is a valid approach for developing a new class of targeted anticancer agents, we designed a fluorescence polarization-based assay that was utilized in a pilot screen for small molecule inhibitors of this PPI. Two small molecule scaffolds that disrupt this interaction were identified, and secondary validation assays confirmed that compound 5 binds to Rev1-CT at the RIR interface. Finally, survival and mutagenesis assays in mouse embryonic fibroblasts and human fibrosarcoma HT1080 cells treated with cisplatin and ultraviolet light indicate that these compounds inhibit mutagenic Rev1/Polζ-dependent TLS in cells, validating the Rev1-CT/RIR PPI for future anticancer drug discovery and identifying the first small molecule inhibitors of TLS that target Rev1-CT.

Multiple Ligand-Bound States of a Phosphohexomutase Revealed by Principal Component Analysis of NMR Peak Shifts

Enzymes sample multiple conformations during their catalytic cycles. Chemical shifts from Nuclear Magnetic Resonance (NMR) are hypersensitive to conformational changes and ensembles in solution. Phosphomannomutase/phosphoglucomutase (PMM/PGM) is a ubiquitous four-domain enzyme that catalyzes phosphoryl transfer across phosphohexose substrates. We compared states the enzyme visits during its catalytic cycle. Collective responses of Pseudomonas PMM/PGM to phosphosugar substrates and inhibitor were assessed using NMR-detected titrations. Affinities were estimated from binding isotherms obtained by principal component analysis (PCA). Relationships among phosphosugar-enzyme associations emerge from PCA comparisons of the titrations. COordiNated Chemical Shifts bEhavior (CONCISE) analysis provides novel discrimination of three ligand-bound states of PMM/PGM harboring a mutation that suppresses activity. Enzyme phosphorylation and phosphosugar binding appear to drive the open dephosphorylated enzyme to the free phosphorylated state, and on toward ligand-closed states. Domain 4 appears central to collective responses to substrate and inhibitor binding. Hydrogen exchange reveals that binding of a substrate analogue enhances folding stability of the domains to a uniform level, establishing a globally unified structure. CONCISE and PCA of NMR spectra have discovered novel states of a well-studied enzyme and appear ready to discriminate other enzyme and ligand binding states.

Practical considerations for investigation of protein conformational dynamics by 15N R 1ρ relaxation dispersion

It is becoming increasingly apparent that proteins are not static entities and that their function often critically depends on accurate sampling of multiple conformational states in aqueous solution. Accordingly, the development of methods to study conformational states in proteins beyond their ground-state structure ("excited states") has crucial biophysical importance. Here we investigate experimental schemes for optimally probing chemical exchange processes in proteins on the micro- to millisecond timescale by ¹⁵N R _1ρ relaxation dispersion. The schemes use selective Hartmann-Hahn cross-polarization (CP) transfer for excitation, and derive peak integrals from 1D NMR spectra (Korzhnev et al. in J Am Chem Soc 127:713-721, 2005; Hansen et al. in J Am Chem Soc 131:3818-3819, 2009). Simulation and experiment collectively show that in such CP-based schemes care has to be taken to achieve accurate suppression of undesired off-resonance coherences, when using weak spin-lock fields. This then (i) ensures that relaxation dispersion profiles in the absence of chemical exchange are flat, and (ii) facilitates extraction of relaxation dispersion profiles in crowded regions of the spectrum. Further improvement in the quality of the experimental data is achieved by recording the free-induction decays in an interleaved manner and including a heating-compensation element. The reported considerations will particularly benefit the use of CP-based R _1ρ relaxation dispersion to analyze conformational exchange processes in larger proteins, where resonance line overlap becomes the main limiting factor.

Triphosphate Reorientation of the Incoming Nucleotide as a Fidelity Checkpoint in Viral RNA-dependent RNA Polymerases

The nucleotide incorporation fidelity of the viral RNA-dependent RNA polymerase (RdRp) is important for maintaining functional genetic information but, at the same time, is also important for generating sufficient genetic diversity to escape the bottlenecks of the host's antiviral response. We have previously shown that the structural dynamics of the motif D loop are closely related to nucleotide discrimination. Previous studies have also suggested that there is a reorientation of the triphosphate of the incoming nucleotide, which is essential before nucleophilic attack from the primer RNA 3'-hydroxyl. Here, we have used ³¹P NMR with poliovirus RdRp to show that the binding environment of the triphosphate is different when correct versus incorrect nucleotide binds. We also show that amino acid substitutions at residues known to interact with the triphosphate can alter the binding orientation/environment of the nucleotide, sometimes lead to protein conformational changes, and lead to substantial changes in RdRp fidelity. The analyses of other fidelity variants also show that changes in the triphosphate binding environment are not always accompanied by changes in the structural dynamics of the motif D loop or other regions known to be important for RdRp fidelity, including motif B. Altogether, our studies suggest that the conformational changes in motifs B and D, and the nucleoside triphosphate reorientation represent separable, "tunable" fidelity checkpoints.

NMR in drug discovery: A practical guide to identification and validation of ligands interacting with biological macromolecules

Protein-ligand interactions are at the heart of drug discovery research. NMR spectroscopy is an excellent technology to identify and validate protein-ligand interactions. A plethora of NMR methods are available which are powerful, robust and information-rich, but also have pitfalls and limitations. In this review, we will focus on how to choose between different experiments, and assess their strengths and liabilities. We introduce the concept of the validation cross, which helps to categorize experiments according to their information content and to simplify the choice of the right experiment in order to address a specific question. Additionally, we will provide the framework for drawing correct conclusions from experimental results in order to accurately evaluate such interactions. Out of scope for this review are methods for subsequent characterization of the interaction such as quantitative K_D determination, binding mode analysis, or structure determination.

Binding Isotherms and Time Courses Readily from Magnetic Resonance

Evidence is presented that binding isotherms, simple or biphasic, can be extracted directly from noninterpreted, complex 2D NMR spectra using principal component analysis (PCA) to reveal the largest trend(s) across the series. This approach renders peak picking unnecessary for tracking population changes. In 1:1 binding, the first principal component captures the binding isotherm from NMR-detected titrations in fast, slow, and even intermediate and mixed exchange regimes, as illustrated for phospholigand associations with proteins. Although the sigmoidal shifts and line broadening of intermediate exchange distorts binding isotherms constructed conventionally, applying PCA directly to these spectra along with Pareto scaling overcomes the distortion. Applying PCA to time-domain NMR data also yields binding isotherms from titrations in fast or slow exchange. The algorithm readily extracts from magnetic resonance imaging movie time courses such as breathing and heart rate in chest imaging. Similarly, two-step binding processes detected by NMR are easily captured by principal components 1 and 2. PCA obviates the customary focus on specific peaks or regions of images. Applying it directly to a series of complex data will easily delineate binding isotherms, equilibrium shifts, and time courses of reactions or fluctuations.