Toward flexibility-activity relationships by NMR spectroscopy: dynamics of Pin1 ligands

Design, synthesis and evaluation of bioactivity of peptidomimetics based on chloroalkene dipeptide isosteres

Ample biologically active peptides have been found, identified and modified for use in drug discovery to date. However, several factors, such as low metabolic stability due to proteolysis and non-specific interactions with multiple off-target molecules, might limit the therapeutic use of peptides. To enhance the stability and/or bioactivity of peptides, the development of "peptidomimetics," which mimick peptide molecules, is considered to be idealistic. Hence, chloroalkene dipeptide isosteres (CADIs) was designed, and their synthetic methods have been developed by us. Briefly, in a CADI an amide bond in peptides is replaced with a chloroalkene structure. CADIs might be superior mimetics of amide bonds because the Van der Waals radii (VDR) and the electronegativity value of a chlorine atom are close to those of the replaced oxygen atom. By a developed method of the "liner synthesis", N-tert-butylsulfonyl protected CADIs can be synthesized via a key reaction involving diastereoselective allylic alkylation using organocopper reagents. On the other hand, by a developed method of the "convergent synthesis", N-fluorenylmethoxycarbonyl (Fmoc)-protected carboxylic acids can be also constructed based on N- and C-terminal analogues from corresponding amino acid starting materials via an Evans syn aldol reaction and the Ichikawa allylcyanate rearrangement reaction involving a [3.3] sigmatropic rearrangement. Notably, CADIs can also be applied for Fmoc-based solid-phase peptide synthesis and therefore introduced into bioactive peptides including as the Arg-Gly-Asp (RGD) peptide and the amyloid β fragment Lys-Leu-Val-Phe-Phe (KLVFF) peptide, which are correlated with cell attachment and Alzheimer's disease (AD), respectively. These CADI-containing peptidomimetics stabilized the conformation and enhanced the potency of the cyclic RGD peptide and the cyclic KLVFF peptide.

Ligand-specific conformational change drives interdomain allostery in Pin1

Pin1 is a two-domain cell regulator that isomerizes peptidyl-prolines. The catalytic domain (PPIase) and the other ligand-binding domain (WW) sample extended and compact conformations. Ligand binding changes the equilibrium of the interdomain conformations, but the conformational changes that lead to the altered domain sampling were unknown. Prior evidence has supported an interdomain allosteric mechanism. We recently introduced a magnetic resonance-based protocol that allowed us to determine the coupling of intra- and interdomain structural sampling in apo Pin1. Here, we describe ligand-specific conformational changes that occur upon binding of pCDC25c and FFpSPR. pCDC25c binding doubles the population of the extended states compared to the virtually identical populations of the apo and FFpSPR-bound forms. pCDC25c binding to the WW domain triggers conformational changes to propagate via the interdomain interface to the catalytic site, while FFpSPR binding displaces a helix in the PPIase that leads to repositioning of the PPIase catalytic loop.

The role of NMR in leveraging dynamics and entropy in drug design

Nuclear magnetic resonance (NMR) spectroscopy has contributed to structure-based drug development (SBDD) in a unique way compared to the other biophysical methods. The potency of a ligand binding to a protein is dictated by the binding free energy, which is an intricate interplay between entropy and enthalpy. In addition to providing the atomic resolution structural information, NMR can help to identify protein-ligand interactions that potentially contribute to the enthalpic component of the free energy. NMR can also illuminate dynamic aspects of the interaction, which correspond to the entropic term of the free energy. The ability of NMR to access both terms in the free energy equation stems from the suite of experiments developed to shed light on various aspects that contribute to both entropy and enthalpy, deepening our understanding of the biological function of macromolecules and assisting to target them in physiological conditions. Here we provide a brief account of the contribution of NMR to SBDD, highlighting hallmark examples and discussing the challenges that demand further method development. In the era of integrated biology, the unique ability of NMR to directly ascertain structural and dynamical aspects of macromolecule and monitor changes in these properties upon engaging a ligand can be combined with computational and other structural and biophysical methods to provide a more complete picture of the energetics of drug engagement with the target. Such efforts can be used to engineer better drugs.

NMR Relaxation Dispersion Reveals Macrocycle Breathing Dynamics in a Cyclodextrin-based Rotaxane

A distinctive feature of mechanically interlocked molecules (MIMs) is the relative motion between the mechanically bonded components, and often it is the functional basis for artificial molecular machines and new functional materials. Optimization of machine or materials performance requires knowledge of the underlying atomic-level mechanisms that control the motion. The field of biomolecular NMR spectroscopy has developed a diverse set of pulse schemes that can characterize molecular dynamics over a broad time scale, but these techniques have not yet been used to characterize the motion within MIMs. This study reports the first observation of NMR relaxation dispersion related to MIM motion. The rotary (pirouette) motion of α-cyclodextrin (αCD) wheels was characterized in a complementary pair of rotaxanes with pirouetting switched ON or OFF. ¹³C and ¹H NMR relaxation dispersion measurements reveal previously unknown exchange dynamics for the αCD wheels in the pirouette-ON rotaxane with a rate constant of 2200 s^-1 at 298 K and an activation barrier of ΔF^‡ = 43 ± 3 kJ/mol. The exchange dynamics disappear in the pirouette-OFF rotaxane, demonstrating their switchable nature. The ¹³C and ¹H sites exhibiting relaxation dispersion suggest that the exchange involves "macrocycle breathing", in which the αCD wheel fluctuates between a contracted or expanded state, the latter enabling diffusive rotary motion about the axle. The substantial insight from these NMR relaxation dispersion methods suggests similar dynamic NMR methods can illuminate the fast time scale (microsecond to millisecond) mechanisms of intercomponent motion in a wide range of MIMs.

Spotlight on the Ballet of Proteins: The Structural Dynamic Properties of Proteins Illuminated by Solution NMR

Solution NMR spectroscopy is a unique and powerful technique that has the ability to directly connect the structural dynamics of proteins in physiological conditions to their activity and function. Here, we summarize recent studies in which solution NMR contributed to the discovery of relationships between key dynamic properties of proteins and functional mechanisms in important biological systems. The capacity of NMR to quantify the dynamics of proteins over a range of time scales and to detect lowly populated protein conformations plays a critical role in its power to unveil functional protein dynamics. This analysis of dynamics is not only important for the understanding of biological function, but also in the design of specific ligands for pharmacologically important proteins. Thus, the dynamic view of structure provided by NMR is of importance in both basic and applied biology.

Activity and Affinity of Pin1 Variants

Pin1 is a peptidyl-prolyl isomerase responsible for isomerizing phosphorylated S/T-P motifs. Pin1 has two domains that each have a distinct ligand binding site, but only its PPIase domain has catalytic activity. Vast evidence supports interdomain allostery of Pin1, with binding of a ligand to its regulatory WW domain impacting activity in the PPIase domain. Many diverse studies have made mutations in Pin1 in order to elucidate interactions that are responsible for ligand binding, isomerase activity, and interdomain allostery. Here, we summarize these mutations and their impact on Pin1′s structure and function.

Enhanced dynamics of conformationally heterogeneous T7 bacteriophage lysozyme native state attenuates its stability and activity

Proteins are dynamic in nature and exist in a set of equilibrium conformations on various timescale motions. The flexibility of proteins governs various biological functions, and therefore elucidation of such functional dynamics is essential. In this context, we have studied the structure–dynamics–stability–activity relationship of bacteriophage T7 lysozyme/endolysin (T7L) native-state ensemble in the pH range of 6–8. Our studies established that T7L native state is conformationally heterogeneous, as several residues of its C-terminal half are present in two conformations (major and minor) in the slow exchange time scale of nuclear magnetic resonance (NMR). Structural and dynamic studies suggested that the residues belonging to minor conformations do exhibit native-like structural and dynamic features. Furthermore, the NMR relaxation experiments unraveled that the native state is highly dynamic and the dynamic behavior is regulated by the pH, as the pH 6 conformation exhibited enhanced dynamics compared with pH 7 and 8. The stability measurements and cell-based activity studies on T7L indicated that the native protein at pH 6 is ∼2 kcal less stable and is ∼50% less active than those of pH 7 and 8. A comprehensive analysis of the T7L active site, unfolding initiation sites and the residues with altered dynamics outlined that the attenuation of stability and activity is a resultant of its enhanced dynamic properties, which, in turn, can be attributed to the protonation/deprotonation of its partially buried His residues. Our study on T7L structure–dynamics–activity paradigm could assist in engineering novel amidase-based endolysins with enhanced activity and stability over a broad pH range.

Molecular Mechanism of the Pin1-Histone H1 Interaction

Pin1 is an essential peptidyl-prolyl isomerase (PPIase) that catalyzes cis-trans prolyl isomerization in proteins containing pSer/Thr-Pro motifs. It has an N-terminal WW domain that targets these motifs and a C-terminal PPIase domain that catalyzes isomerization. Recently, Pin1 was shown to modify the conformation of phosphorylated histone H1 and stabilize the chromatin-H1 interaction by increasing its residence time. This Pin1-histone H1 interaction plays a key role in pathogen response, in infection, and in cell cycle control; therefore, anti-Pin1 therapeutics are an important focus for treating infections as well as cancer. Each of the H1 histones (H1.0-H1.5) contains several potential Pin1 recognition pSer/pThr-Pro motifs. To understand the Pin1-histone H1 interaction fully, we investigated how both the isolated WW domain and full-length Pin1 interact with three H1 histone substrate peptide sequences that were previously identified as important binding partners (H1.1, H1.4, and H1.5). NMR spectroscopy was used to measure the binding affinities and the interdomain dynamics upon binding to these sequences. We observed different K_D values depending on the histone binding site, suggesting that energetics play a role in guiding the Pin1-histone interaction. While interdomain interactions vary between the peptides, we find no evidence for allosteric activation for the histone H1 substrates.

Synthesis of rigidified shikimic acid derivatives by ring-closing metathesis to imprint inhibitor efficacy against shikimate kinase enzyme

The rigidification of the high-energy conformation of shikimic acid was used to enhance inhibitor efficacy against shikimate kinase enzyme, an attractive target for antibiotic drug discovery.

Computational conformational analysis of α-thrombin inhibitors possessing distinct scaffolds in aqueous solution and on Ala-sheet

Recent computational simulations on protein-ligand binding/unbinding have precisely been uncovering the ligand-binding process at the atomic level. In the process, the non-specific binding of ligands to the target site is suggested to occur before binding to the target. We in this study analyzed the conformations of ligands under the non-specific binding on a protein surface to figure out the differences in the conformational characteristics in aqueous solution using the 55-ns molecular dynamic simulation. As for the protein surface, we constructed an artificial β-sheet, composed of poly-alanine residues (Ala-sheet). For the ligands, the four α-thrombin inhibitors possessing two scaffolds with distinct hydrophobicity profiles were used. During the simulation, all the inhibitors kept interaction with Ala-sheet and had the limited conformational fluctuations compared with in aqueous solution. The representative conformations obtained from the cluster analysis showed that two of hydrophobic inhibitors adopted the extended conformations in aqueous solution and also on Ala-sheet. For the other two hydrophilic inhibitors, the conformations in aqueous solution adopted the bent conformation with two terminal hydrophobic rings closely packed. On Ala-sheet, contrarily, the two hydrophobic rings were open and took the extended conformations, which were placed on the sheet as a foothold. The charged moieties in the hydrophilic inhibitors were protruded into aqueous environment with the extended conformation. The conformational characteristics of the inhibitors in aqueous solution and Ala-sheet varied likely by chemical features or structures of the inhibitors, but each was considered to be physicochemically reasonable.

Measurement and Characterization of Hydrogen-Deuterium Exchange Chemistry Using Relaxation Dispersion NMR Spectroscopy

One-dimensional heteronuclear relaxation dispersion NMR spectroscopy at ¹³C natural abundance successfully characterized the dynamics of the hydrogen-deuterium exchange reaction occurring at the N^ε position in l-arginine by monitoring C^δ in varying amounts of D₂O. A small equilibrium isotope effect was observed and quantified, corresponding to ΔG = -0.14 kcal mol^-1. A bimolecular rate constant of k_D = 5.1 × 10⁹ s^-1 M^-1 was determined from the pH*-dependence of k_ex (where pH* is the direct electrode reading of pH in 10% D₂O and k_ex is the nuclear spin exchange rate constant), consistent with diffusion-controlled kinetics. The measurement of ΔG serves to bridge the millisecond time scale lifetimes of the detectable positively charged arginine species with the nanosecond time scale lifetime of the nonobservable low-populated neutral arginine intermediate species, thus allowing for characterization of the equilibrium lifetimes of the various arginine species in solution as a function of fractional solvent deuterium content. Despite the system being in fast exchange on the chemical shift time scale, the magnitude of the secondary isotope shift due to the exchange reaction at N^ε was accurately measured to be 0.12 ppm directly from curve-fitting D₂O-dependent dispersion data collected at a single static field strength. These results indicate that relaxation dispersion NMR spectroscopy is a robust and general method for studying base-catalyzed hydrogen-deuterium exchange chemistry at equilibrium.

Label-free NMR-based dissociation kinetics determination

Understanding the dissociation of molecules is the basis to modulate interactions of biomedical interest. Optimizing drugs for dissociation rates is found to be important for their efficacy, selectivity, and safety. Here, we show an application of the high-power relaxation dispersion (RD) method to the determination of the dissociation rates of weak binding ligands from receptors. The experiment probes proton RD on the ligand and, therefore, avoids the need for any isotopic labeling. The large ligand excess eases the detection significantly. Importantly, the use of large spin-lock fields allows the detection of faster dissociation rates than other relaxation approaches. Moreover, this experimental approach allows to access directly the off-rate of the binding process without the need for analyzing a series of samples with increasing ligand saturation. The validity of the method is shown with small molecule interactions using two macromolecules, bovine serum albumin and tubulin heterodimers.

Towards understanding the unbound state of drug compounds: Implications for the intramolecular reorganization energy upon binding

There has been an explosion of structural information for pharmaceutical compounds bound to biological targets, but the conformations and dynamics of compounds free in solution are poorly characterized, if at all. Yet, knowledge of the unbound state is essential to understand the fundamentals of molecular recognition, including the much debated conformational intramolecular reorganization energy of a compound upon binding (ΔEReorg). Also, dependable observation of the unbound compounds is important for ligand-based drug discovery, e.g. with pharmacophore modelling. Here, these questions are addressed with long (⩾0.5μs) state-of-the-art molecular dynamics (MD) simulations of 26 compounds (including 7 approved drugs) unbound in explicit solvent. These compounds were selected to be chemically diverse, with a range of flexibility, and good quality bioactive X-ray structures. The MD-simulated free compounds are compared to their bioactive structure and conformers generated with ad hoc sampling in vacuo or with implicit generalized Born (GB) aqueous solvation models. The GB conformational models clearly depart from those obtained in explicit solvent, and suffer from conformational collapse almost as severe as in vacuo. Thus, the global energy minima in vacuo or with GB are not suitable representations of the unbound state, which can instead be extensively sampled by MD simulations. Many, but not all, MD-simulated compounds displayed some structural similarity to their bioactive structure, supporting the notion of conformational pre-organization for binding. The ligand-protein complexes were also simulated in explicit solvent, to estimate ΔEReorg as an enthalpic difference ΔHReorg between the intramolecular energies in the bound and unbound states. This fresh approach yielded ΔHReorg values⩽6kcal/mol for 18 out of 26 compounds. For three particularly polar compounds 15⩽ΔHReorg⩽20kcal/mol, supporting the notion that ΔHReorg can be substantial. Those large ΔHReorg values correspond to a redistribution of electrostatic interactions upon binding. Overall, the study illustrates how MD simulations offer a promising avenue to characterize the unbound state of medicinal compounds.

Enzyme Selectivity Fine-Tuned through Dynamic Control of a Loop

Allostery has been revealed as an essential property of all proteins. For enzymes, shifting of the structural equilibrium distribution at one site can have substantial impacts on protein dynamics and selectivity. Promising sites of remotely shifting such a distribution by changing the dynamics would be at flexible loops because relatively large changes may be achieved with minimal modification of the protein. A ligand-selective change of binding affinity to the active site of cyclophilin is presented involving tuning of the dynamics of a highly flexible loop. Binding affinity is increased upon substitution of double Gly to Ala at the hinge regions of the loop. Quenching of the motional amplitudes of the loop slightly rearranges the active site. In particular, key residues for binding Phe60 and His126 adopt a more fixed orientation in the bound protein. Our system may serve as a model system for studying the effects of various time scales of loop motion on protein function tuned by mutations.

Allosteric Communication across STAT3 Domains Associated with STAT3 Function and Disease-Causing Mutation

STAT3 is a member of STAT (signal transducer and activator of transcription) transcription activators. Aberration in STAT3 activity due to constitutive activation or mutations leads to diseases such as cancer and hyper-immunoglobulin E syndrome (HIES). STAT3 contains several structured domains including the Src homology 2 domain (SH2), linker domain (LD), DNA-binding domain (DBD) and the coiled-coil domain. Here we report the discovery of inter-domain allosteric communications in STAT3 from studies using nuclear magnetic resonance (NMR) and other methods. We found that pTyr-peptide interactions with SH2 cause structural and dynamics changes in LD and DBD. The inter-domain allosteric effect is likely mediated by the flexibility in the hydrophobic core. In addition, a mutation in LD found in HIES (I568F) induces NMR chemical shift perturbation in SH2, DBD and the coiled-coil domain, suggesting conformational changes in these domains. Consistent with conformational changes in SH2, the I568F mutant reduces SH2's binding affinity to a pTyr-containing peptide. This study provides an example of dynamics-dependent allosteric effects, and due to the structural conservation of the STAT family of proteins, the inter-domain allosteric communication observed in STAT3 likely occurs in other STATs.

Chemical Tools To Decipher Regulation of Phosphatases by Proline Isomerization on Eukaryotic RNA Polymerase II

Proline isomerization greatly impacts biological signaling but is subtle and difficult to detect in proteins. We characterize this poorly understood regulatory mechanism for RNA polymerase II carboxyl terminal domain (CTD) phosphorylation state using novel, direct, and quantitative chemical tools. We determine the proline isomeric preference of three CTD phosphatases: Ssu72 as cis-proline specific, Scp1 and Fcp1 as strongly trans-preferred. Due to this inherent characteristic, these phosphatases respond differently to enzymes that catalyze the isomerization of proline, like Ess1/Pin1. We demonstrate that this selective regulation of RNA polymerase II phosphorylation state exists within human cells, consistent with in vitro assays. These results support a model in which, instead of a global enhancement of downstream enzymatic activities, proline isomerases selectively boost the activity of a subset of CTD regulatory factors specific for cis-proline. This leads to diversified phosphorylation states of CTD in vitro and in cells. We provide the chemical tools to investigate proline isomerization and its ability to selectively enhance signaling in transcription and other biological contexts.

Stereochemical Control in the Still-Wittig Rearrangement Synthesis of Cyclohexyl (Z)-Alkene Inhibitors of Pin1

Three stereoisomeric inhibitors of Pin1: (2R,5S)-, (2S,5R)- and (2S,5S)-Ac–pSer–Ψ[(Z)CH = C]–pipecolyl(Pip)–2-(2-naphthyl)ethylamine 1, that mimic L-pSer–D-Pro, D-pSer–L-Pro, and D-pSer–D-Pro amides respectively, were synthesized by a 13-step route. The newly formed stereogenic centers in the pipecolyl ring were introduced by Luche reduction, followed by stereospecific [2,3]-Still-Wittig rearrangement. The (Z)- to (E)-alkene ratio in the rearrangements were consistently 5.5 to 1. The stereochemistry at the original Ser α-carbon controlled the stereochemistry of the Luche reduction, but it did not affect the stereochemical outcome of the rearrangement, which consistently gave the (Z)-alkene. The epimerized by-product, (2S,5S)-10, resulting from the work-up after Na/NH₃ debenzylation of (2S,5R)-9, was carried on to the (2S,5S)-1 isomer. Compound (2S,5S)-10 was resynthesized from the Luche reduction by-product, (2R,3R)-3, and the stereochemistry was confirmed by comparison of the optical rotations. The IC₅₀ values for (2R,5S)-1, (2S,5R)-1 and (2S,5S)-1 Pin1 inhibition were: 52, 85, and 140 μM, respectively.

Investigating Dynamic Interdomain Allostery in Pin1

Signaling proteins often sequester complementary functional sites in separate domains. How do the different domains communicate with one another? An attractive system to address this question is the mitotic regulator, human Pin1 (Lu et al. 1996). Pin-1 consists of two tethered domains: a WW domain for substrate binding, and a catalytic domain for peptidyl-prolyl isomerase (PPIase) activity. Pin1 accelerates the cis-trans isomerization of phospho-Ser/Thr-Pro (pS/T-P) motifs within proteins regulating the cell cycle and neuronal development. The early x-ray (Ranganathan et al. 1997; Verdecia et al. 2000) and solution NMR studies (Bayer et al. 2003; Jacobs et al. 2003) of Pin1 indicated inter- and intradomain motion. We became interested in exploring how such motions might affect interdomain communication, using NMR. Our accumulated results indicate substrate binding to Pin1 WW domain changes the intra/inter domain mobility, thereby altering substrate activity in the distal PPIase domain catalytic site. Thus, Pin1 shows evidence of dynamic allostery, in the sense of Cooper and Dryden (Cooper and Dryden 1984). We highlight our results supporting this conclusion, and summarize them via a simple speculative model of conformational selection.

Stereospecific phosphorylation by the central mitotic kinase Cdk1-cyclin B

The cis vs trans conformation, or shape, of phosphoserine-proline (pSer-Pro), a prevalent motif in cell cycle proteins, may play a significant role in regulating mitosis. We demonstrate that Cdk1-cyclin B, the central mitotic kinase, is specific for the trans conformation, not cis, of synthetic, locked Ser-Pro 11-residue peptide substrates, using LC-MSMS detection and sequencing of phosphorylated products. This substrate stereospecificity may contribute an additional level of mitotic regulation.

Enzyme dynamics from NMR spectroscopy

Biological activities of enzymes, including regulation or coordination of mechanistic stages preceding or following the chemical step, may depend upon kinetic or equilibrium changes in protein conformations. Exchange of more open or flexible conformational states with more closed or constrained states can influence inhibition, allosteric regulation, substrate recognition, formation of the Michaelis complex, side reactions, and product release. NMR spectroscopy has long been applied to the study of conformational dynamic processes in enzymes because these phenomena can be characterized over multiple time scales with atomic site resolution. Laboratory-frame spin-relaxation measurements, sensitive to reorientational motions on picosecond–nanosecond time scales, and rotating-frame relaxation-dispersion measurements, sensitive to chemical exchange processes on microsecond–millisecond time scales, provide information on both conformational distributions and kinetics. This Account reviews NMR spin relaxation studies of the enzymes ribonuclease HI from mesophilic (Escherichia coli) and thermophilic (Thermus thermophilus) bacteria, E. coli AlkB, and Saccharomyces cerevisiae triosephosphate isomerase to illustrate the contributions of conformational flexibility and dynamics to diverse steps in enzyme mechanism.

Spin relaxation measurements and molecular dynamics (MD) simulations of the bacterial ribonuclease H enzymes show that the handle region, one of three loop regions that interact with substrates, interconverts between two conformations. Comparison of these conformations with the structure of the complex between Homo sapiens ribonuclease H and a DNA:RNA substrate suggests that the more closed state is inhibitory to binding. The large population of the closed conformation in T. thermophilus ribonuclease H contributes to the increased Michaelis constant compared with the E. coli enzyme. NMR spin relaxation and fluorescence spectroscopy have characterized a conformational transition in AlkB between an open state, in which the side chains of methionine residues in the active site are disordered, and a closed state, in which these residues are ordered. The open state is highly populated in the AlkB/Zn(II) complex, and the closed state is highly populated in the AlkB/Zn(II)/2OG/substrate complex, in which 2OG is the 2-oxoglutarate cosubstrate and the substrate is a methylated DNA oligonucleotide. The equilibrium is shifted to approximately equal populations of the two conformations in the AlkB/Zn(II)/2OG complex. The conformational shift induced by 2OG ensures that 2OG binds to AlkB/Zn(II) prior to the substrate. In addition, the opening rate of the closed conformation limits premature release of substrate, preventing generation of toxic side products by reaction with water. Closure of active site loop 6 in triosephosphate isomerase is critical for forming the Michaelis complex, but reopening of the loop after the reaction is (partially) rate limiting. NMR spin relaxation and MD simulations of triosephosphate isomerase in complex with glycerol 3-phosphate demonstrate that closure of loop 6 is a highly correlated rigid-body motion. The MD simulations also indicate that motions of Gly173 in the most flexible region of loop 6 contribute to opening of the active site loop for product release. Considered together, these three enzyme systems illustrate the power of NMR spin relaxation investigations in providing global insights into the role of conformational dynamic processes in the mechanisms of enzymes from initial activation to final product release.

Ligand-detected relaxation dispersion NMR spectroscopy: dynamics of preQ1-RNA binding

An NMR-based approach to characterizing the binding kinetics of ligand molecules to biomolecules, like RNA or proteins, by ligand-detected Carr-Purcell-Meiboom-Gill (CPMG) relaxation dispersion experiments is described. A (15)N-modified preQ1 ligand is used to acquire relaxation dispersion experiments in the presence of low amounts of the Fsu class I preQ1 aptamer RNA, and increasing ligand concentrations to probe the RNA small molecule interaction. Our experimental data strongly support the conformational selection mechanism postulated. The approach gives direct access to two parameters of a ligand-receptor interaction: the off rate and the population of the small molecule-receptor complex. A detailed description of the kinetics underlying the ligand binding process is of crucial importance to fully understanding a riboswitch's function and to evaluate potential new antibiotics candidates targeting the noncoding RNA species. Ligand-detected NMR relaxation dispersion experiments represent a valuable diagnostic tool for the characterization of binding mechanisms.

Isostere-based design of 8-azacoumarin-type photolabile protecting groups: a hydrophilicity-increasing strategy for coumarin-4-ylmethyls

Described is the development of 8-azacoumarin-4-ylmethyl groups as aqueous photolabile protecting groups. A key feature of the strategy is the isosteric replacement of the C7-C8 enol double bond of the Bhc derivative with an amide bond, resulting in conversion of the chromophore from coumarin to 8-azacoumarin. This strategy makes dramatically enhanced water solubility and facile photocleavage possible.

A prolyl-isomerase mediates dopamine-dependent plasticity and cocaine motor sensitization

Synaptic plasticity induced by cocaine and other drugs underlies addiction. Here we elucidate molecular events at synapses that cause this plasticity and the resulting behavioral response to cocaine in mice. In response to D1-dopamine-receptor signaling that is induced by drug administration, the glutamate-receptor protein metabotropic glutamate receptor 5 (mGluR5) is phosphorylated by microtubule-associated protein kinase (MAPK), which we show potentiates Pin1-mediated prolyl-isomerization of mGluR5 in instances where the product of an activity-dependent gene, Homer1a, is present to enable Pin1-mGluR5 interaction. These biochemical events potentiate N-methyl-D-aspartate receptor (NMDAR)-mediated currents that underlie synaptic plasticity and cocaine-evoked motor sensitization as tested in mice with relevant mutations. The findings elucidate how a coincidence of signals from the nucleus and the synapse can render mGluR5 accessible to activation with consequences for drug-induced dopamine responses and point to depotentiation at corticostriatal synapses as a possible therapeutic target for treating addiction.

Interdomain interactions support interdomain communication in human Pin1

Pin1 is an essential mitotic regulator consisting of a peptidyl–prolyl isomerase (PPIase) domain flexibly tethered to a smaller Trp–Trp (WW) binding domain. Communication between these domains is important for Pin1 in vivo activity; however, the atomic basis for this communication has remained elusive. Our previous nuclear magnetic resonance (NMR) studies of Pin1 functional dynamics suggested that weak interdomain contacts within Pin1 enable allosteric communication between the domain interface and the distal active site of the PPIase domain.1,2 A necessary condition for this hypothesis is that the intrinsic properties of the PPIase domain should be sensitive to interdomain contact. Here, we test this sensitivity by generating a Pin1 mutant, I28A, which weakens the wild-type interdomain contact while maintaining the overall folds of the two domains. Using NMR, we show that I28A leads to altered substrate binding affinity and isomerase activity. Moreover, I28A causes long-range perturbations to conformational flexibility in both domains, for both the apo and substrate-complexed states of the protein. These results show that the distribution of conformations sampled by the PPIase domain is sensitive to interdomain contact and strengthen the hypothesis that such contact supports interdomain allosteric communication in Pin1. Other modular systems may exploit interdomain interactions in a similar manner.

Structural and kinetic analysis of prolyl-isomerization/phosphorylation cross-talk in the CTD code

The C-terminal domain (CTD) of eukaryotic RNA polymerase II is an essential regulator for RNA polymerase II-mediated transcription. It is composed of multiple repeats of a consensus sequence Tyr(1)Ser(2)Pro(3)Thr(4)Ser(5)Pro(6)Ser(7). CTD regulation of transcription is mediated by both phosphorylation of the serines and prolyl isomerization of the two prolines. Interestingly, the phosphorylation sites are typically close to prolines, and thus the conformation of the adjacent proline could impact the specificity of the corresponding kinases and phosphatases. Experimental evidence of cross-talk between these two regulatory mechanisms has been elusive. Pin1 is a highly conserved phosphorylation-specific peptidyl-prolyl isomerase (PPIase) that recognizes the phospho-Ser/Thr (pSer/Thr)-Pro motif with CTD as one of its primary substrates in vivo. In the present study, we provide structural snapshots and kinetic evidence that support the concept of cross-talk between prolyl isomerization and phosphorylation. We determined the structures of Pin1 bound with two substrate isosteres that mimic peptides containing pSer/Thr-Pro motifs in cis or trans conformations. The results unequivocally demonstrate the utility of both cis- and trans-locked alkene isosteres as close geometric mimics of peptides bound to a protein target. Building on this result, we identified a specific case in which Pin1 differentially affects the rate of dephosphorylation catalyzed by two phosphatases (Scp1 and Ssu72) that target the same serine residue in the CTD heptad repeat but have different preferences for the isomerization state of the adjacent proline residue. These data exemplify for the first time how modulation of proline isomerization can kinetically impact signal transduction in transcription regulation.

Exposing the Moving Parts of Proteins with NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful tool for investigating the dynamics of biomolecules since it provides a description of motion that is comprehensive, site-specific, and relatively non-invasive. In particular, the study of protein dynamics has benefited from sustained methodological advances in NMR that have expanded the scope and time scales of accessible motion. Yet, many of these advances may not be well known to the more general physical chemistry community. Accordingly, this Perspective provides a glimpse of some of the more powerful methods in liquid state NMR that are helping reshape our understanding of functional motions of proteins.

Evidence for dynamics in proteins as a mechanism for ligand dissociation

Signal transduction, regulatory processes, and pharmaceutical responses are highly dependent upon ligand residence times. Gaining insight into how physical factors influence residence times, or k_off, should enhance our ability to manipulate biological interactions. We report experiments that yield structural insight into k_off for a series of eight 2,4-diaminopyrimidine inhibitors of dihydrofolate reductase that vary by six orders of magnitude in binding affinity. NMR relaxation dispersion experiments revealed a common set of residues near the binding site that undergo a concerted, millisecond-timescale switching event to a previously unidentified conformation. The rate of switching from ground to excited conformations correlates exponentially with K_i and k_off, suggesting that protein dynamics serves as a mechanical initiator of ligand dissociation within this series and potentially for other macromolecule-ligand systems. Although k_conf,forward is faster than k_off, use of the ligand series allowed for connections to be drawn between kinetic events on different timescales.

GUARDD: user-friendly MATLAB software for rigorous analysis of CPMG RD NMR data

Molecular dynamics are essential for life, and nuclear magnetic resonance (NMR) spectroscopy has been used extensively to characterize these phenomena since the 1950s. For the past 15 years, the Carr-Purcell Meiboom-Gill relaxation dispersion (CPMG RD) NMR experiment has afforded advanced NMR labs access to kinetic, thermodynamic, and structural details of protein and RNA dynamics in the crucial μs-ms time window. However, analysis of RD data is challenging because datasets are often large and require many non-linear fitting parameters, thereby confounding assessment of accuracy. Moreover, novice CPMG experimentalists face an additional barrier because current software options lack an intuitive user interface and extensive documentation. Hence, we present the open-source software package GUARDD (Graphical User-friendly Analysis of Relaxation Dispersion Data), which is designed to organize, automate, and enhance the analytical procedures which operate on CPMG RD data ( http://code.google.com/p/guardd/). This MATLAB-based program includes a graphical user interface, permits global fitting to multi-field, multi-temperature, multi-coherence data, and implements χ (2)-mapping procedures, via grid-search and Monte Carlo methods, to enhance and assess fitting accuracy. The presentation features allow users to seamlessly traverse the large amount of results, and the RD Simulator feature can help design future experiments as well as serve as a teaching tool for those unfamiliar with RD phenomena. Based on these innovative features, we expect that GUARDD will fill a well-defined gap in service of the RD NMR community.

A reduced-amide inhibitor of Pin1 binds in a conformation resembling a twisted-amide transition state

The mechanism of the cell cycle regulatory peptidyl prolyl isomerase (PPIase), Pin1, was investigated using reduced-amide inhibitors designed to mimic the twisted-amide transition state. Inhibitors, R-pSer-Ψ[CH(2)N]-Pro-2-(indol-3-yl)ethylamine, 1 [R = fluorenylmethoxycarbonyl (Fmoc)] and 2 (R = Ac), of Pin1 were synthesized and bioassayed. Inhibitor 1 had an IC(50) value of 6.3 μM, which is 4.5-fold better for Pin1 than our comparable ground-state analogue, a cis-amide alkene isostere-containing inhibitor. The change of Fmoc to Ac in 2 improved aqueous solubility for structural determination and resulted in an IC(50) value of 12 μM. The X-ray structure of the complex of 2 bound to Pin1 was determined to 1.76 Å resolution. The structure revealed that the reduced amide adopted a conformation similar to the proposed twisted-amide transition state of Pin1, with a trans-pyrrolidine conformation of the prolyl ring. A similar conformation of substrate would be destabilized relative to the planar amide conformation. Three additional reduced amides, with Thr replacing Ser and l- or d-pipecolate (Pip) replacing Pro, were slightly weaker inhibitors of Pin1.

Stereospecific gating of functional motions in Pin1

Pin1 is a modular enzyme that accelerates the cis-trans isomerization of phosphorylated-Ser/Thr-Pro (pS/T-P) motifs found in numerous signaling proteins regulating cell growth and neuronal survival. We have used NMR to investigate the interaction of Pin1 with three related ligands that include a pS-P substrate peptide, and two pS-P substrate analogue inhibitors locked in the cis and trans conformations. Specifically, we compared the ligand binding modes and binding-induced changes in Pin1 side-chain flexibility. The cis and trans binding modes differ, and produce different mobility in Pin1. The cis-locked inhibitor and substrate produced a loss of side-chain flexibility along an internal conduit of conserved hydrophobic residues, connecting the domain interface with the isomerase active site. The trans-locked inhibitor produces a weaker conduit response. Thus, the conduit response is stereoselective. We further show interactions between the peptidyl-prolyl isomerase and Trp-Trp (WW) domains amplify the conduit response, and alter binding properties at the remote peptidyl-prolyl isomerase active site. These results suggest that specific input conformations can gate dynamic changes that support intraprotein communication. Such gating may help control the propagation of chemical signals by Pin1, and other modular signaling proteins.

Crystallographic and nuclear magnetic resonance evaluation of the impact of peptide binding to the second PDZ domain of protein tyrosine phosphatase 1E

PDZ (PSD95/Discs large/ZO-1) domains are ubiquitous protein interaction motifs found in scaffolding proteins involved in signal transduction. Despite the fact that many PDZ domains show a limited tendency to undergo structural change, the PDZ family has been associated with long-range communication and allostery. One of the PDZ domains studied most in terms of structure and biophysical properties is the second PDZ ("PDZ2") domain from protein tyrosine phosphatase 1E (PTP1E, also known as PTPL1). Previously, we showed through NMR relaxation studies that binding of the RA-GEF2 C-terminal peptide substrate results in long-range propagation of side-chain dynamic changes in human PDZ2 [Fuentes, E. J., et al. (2004) J. Mol. Biol. 335, 1105-1115]. Here, we present the first X-ray crystal structures of PDZ2 in the absence and presence of RA-GEF2 ligand, determined to resolutions of 1.65 and 1.3 Å, respectively. These structures deviate somewhat from previously determined NMR structures and indicate that very minor structural changes in PDZ2 accompany peptide binding. NMR residual dipolar couplings confirm the crystal structures to be accurate models of the time-averaged atomic coordinates of PDZ2. The impact on side-chain dynamics was further tested with a C-terminal peptide from APC, which showed results nearly identical to those of RA-GEF2. Thus, allosteric transmission in PDZ2 induced by peptide binding is conveyed purely and robustly by dynamics. (15)N relaxation dispersion measurements did not detect appreciable populations of a kinetic structural intermediate. Collectively, for ligand binding to PDZ2, these data support a lock-and-key binding model from a structural perspective and an allosteric model from a dynamical perspective, which together suggest a complex energy landscape for functional transitions within the ensemble.

Probing slow protein dynamics by adiabatic R(1rho) and R(2rho) NMR experiments

Slow micros/ms dynamics involved in protein folding, binding, catalysis, and allostery are currently detected using NMR dispersion experiments such as CPMG (Carr-Purcell-Meiboom-Gill) or spin-lock R(1rho). In these methods, protein dynamics are obtained by analyzing relaxation dispersion curves obtained from either changing the time spacing between 180 degree pulses or by changing the effective spin-locking field strength. In this Communication, we introduce a new method to induce a dispersion of relaxation rates. Our approach relies on altering the shape of the adiabatic full passage pulse and is conceptually different from existing approaches. By changing the nature of the adiabatic radiofrequency irradiation, we are able to obtain rotating frame R(1rho) and R(2rho) dispersion curves that are sensitive to slow micros/ms protein dynamics (demonstrated with ubiquitin). The strengths of this method are to (a) extend the dynamic range of the relaxation dispersion analysis, (b) avoid the need for multiple magnetic field strengths to extract dynamic parameters, (c) measure accurate relaxation rates that are independent of frequency offset, and (d) reduce the stress to NMR hardware (e.g., cryoprobes).

An introduction to NMR-based approaches for measuring protein dynamics

Proteins are inherently flexible at ambient temperature. At equilibrium, they are characterized by a set of conformations that undergo continuous exchange within a hierarchy of spatial and temporal scales ranging from nanometers to micrometers and femtoseconds to hours. Dynamic properties of proteins are essential for describing the structural bases of their biological functions including catalysis, binding, regulation and cellular structure. Nuclear magnetic resonance (NMR) spectroscopy represents a powerful technique for measuring these essential features of proteins. Here we provide an introduction to NMR-based approaches for studying protein dynamics, highlighting eight distinct methods with recent examples, contextualized within a common experimental and analytical framework. The selected methods are (1) Real-time NMR, (2) Exchange spectroscopy, (3) Lineshape analysis, (4) CPMG relaxation dispersion, (5) Rotating frame relaxation dispersion, (6) Nuclear spin relaxation, (7) Residual dipolar coupling, (8) Paramagnetic relaxation enhancement. This article is part of a Special Issue entitled: Protein Dynamics: Experimental and Computational Approaches.