A lipid-anchored neurokinin 1 receptor antagonist prolongs pain relief by a three-pronged mechanism of action targeting the receptor at the plasma membrane and in endosomes.

G-protein-coupled receptors (GPCRs) are traditionally known for signaling at the plasma membrane, but they can also signal from endosomes after internalization to control important pathophysiological processes. In spinal neurons, sustained endosomal signaling of the neurokinin 1 receptor (NK1R) mediates nociception, as demonstrated in models of acute and neuropathic pain. An NK1R antagonist, Spantide I (Span), conjugated to cholestanol (Span-Chol), accumulates in endosomes, inhibits endosomal NK1R signaling, and causes prolonged antinociception. However, the extent to which the Chol-anchor influences long-term location and activity is poorly understood. Herein, we used fluorescent correlation spectroscopy and targeted biosensors to characterize Span-Chol over time. The Chol-anchor increased local concentration of probe at the plasma membrane. Over time we observed an increase in NK1R-binding affinity and more potent inhibition of NK1R-mediated calcium signaling. Span-Chol, but not Span, caused a persistent decrease in NK1R recruitment of ß-arrestin and receptor internalization to early endosomes. Using targeted biosensors, we mapped the relative inhibition of NK1R signaling as the receptor moved into the cell. Span selectively inhibited cell surface signaling, whereas Span-Chol partitioned into endosomal membranes and blocked endosomal signaling. In a preclinical model of pain, Span-Chol caused prolonged antinociception (>9 h), which is attributable to a three-pronged mechanism of action: increased local concentration at membranes, a prolonged decrease in NK1R endocytosis, and persistent inhibition of signaling from endosomes. Identifying the mechanisms that contribute to the increased preclinical efficacy of lipid-anchored NK1R antagonists is an important step toward understanding how we can effectively target intracellular GPCRs in disease.

Interaction of Nucleotides with a Trinuclear Terbium(III)-Dizinc(II) Complex: Efficient Sensitization of Terbium Luminescence by Guanosine Monophosphate and Application to Real-Time Monitoring of Phosphodiesterase Activity.

An in-depth study of the interaction of a trinuclear terbium(III)-dizinc(II) complex with an array of nucleotides differing in the type of nucleobase and number of phosphate groups, as well as cyclic versus acyclic variants, is presented. The study examined the nature of the interaction and the efficiency at which guanine was able to sensitize terbium(III) luminescence. Competitive binding and titration studies were performed to help establish the nature/mode of the interactions. These established that (1) interaction occurs by the coordination of phosphate groups to zinc(II) (in addition to uridine in the case of uridine monophosphate), (2) acyclic nucleotides bind more strongly than cyclic counterparts because of their higher negative charge, (3) guanine-containing nucleotides are able to sensitize terbium(III) luminescence with the efficiency of sensitization following the order guanosine monophosphate (GMP) > guanosine diphosphate > guanosine triphosphate because of the mode of binding, and (4) nucleoside monophosphates bind to a single zinc(II) ion, whereas di- and triphosphates appear to bind in a bridging mode between two host molecules. Furthermore, it has been shown that guanine is a sensitizer of terbium(III) luminescence. On the basis of the ability of GMP to effectively sensitize terbium(III)-based luminescence while cyclic GMP (cGMP) does not, the complex has been utilized to monitor the catalytic conversion of cGMP to GMP by a phosphodiesterase enzyme in real time using time-gated luminescence on a benchtop fluorimeter. The complex has the potential to find broad application in monitoring the activity of enzymes that process nucleotides (co)substrates, including high-throughput drug-screening programs.

Genetically encoded amino acids with tert-butyl and trimethylsilyl groups for site-selective studies of proteins by NMR spectroscopy.

The amino acids 4-(tert-butyl)phenylalanine (Tbf) and 4-(trimethylsilyl)phenylalanine (TMSf), as well as a partially deuterated version of Tbf (dTbf), were chemically synthesized and site-specifically incorporated into different proteins, using an amber stop codon, suppressor tRNA and the broadband aminoacyl-tRNA synthetase originally evolved for the incorporation of p-cyano-phenylalanine. The 1H-NMR signals of the tert-butyl and TMS groups were compared to the 1H-NMR signal of tert-butyltyrosine (Tby) in protein systems with molecular weights ranging from 8 to 54 kDa. The 1H-NMR resonance of the TMS group appeared near 0 ppm in a spectral region with few protein resonances, facilitating the observation of signal changes in response to ligand binding. In all proteins, the R 2 relaxation rate of the tert-butyl group of Tbf was only little greater than that of Tby (less than two-fold). Deuteration of the phenyl ring of Tbf made only a relatively small difference. The effective T 2 relaxation time of the TMS signal was longer than 140 ms even in the 54 kDa system.

Trimethylsilyl tag for probing protein-ligand interactions by NMR.

Protein-ligand titrations can readily be monitored with a trimethylsilyl (TMS) tag. Owing to the intensity, narrow line shape and unique chemical shift of a TMS group, dissociation constants can be determined from straightforward 1D 1H-NMR spectra not only in the fast but also in the slow exchange limit. The tag is easily attached to cysteine residues and a sensitive reporter of ligand binding also at sites where it does not interfere with ligand binding or catalytic efficiency of the target protein. Its utility is demonstrated for the Zika virus NS2B-NS3 protease and the human prolyl isomerase FK506 binding protein.

Site-Specific Incorporation of Selenocysteine by Genetic Encoding as a Photocaged Unnatural Amino Acid.

Selenocysteine (Sec) is a naturally occurring amino acid that is also referred to as the 21st amino acid. Site-specific incorporation of Sec into proteins is attractive, because the reactivity of a selenol group exceeds that of a thiol group and thus allows site-specific protein modifications. It is incorporated into proteins by an unusual enzymatic mechanism which, in E. coli and other organisms, involves the recognition of a selenocysteine insertion sequence (SECIS) in the mRNA of the target protein. Reengineering of the natural machinery for Sec incorporation at arbitrary sites independent of SECIS elements, however, is challenging. Here we demonstrate an alternative route, whereby a photocaged selenocysteine (PSc) is incorporated as an unnatural amino acid in response to an amber stop codon, using a mutant Methanosarcina mazei pyrrolysyl-tRNA synthetase, Mm PCC2RS, and its cognate tRNACUA. Following decaging by UV irradiation, proteins synthesized with PSc are readily tagged, e.g., with NMR probes to study ligand binding by NMR spectroscopy. The approach provides a facile route for genetically encoded Sec incorporation. It allows the production of pure selenoproteins and the Sec residue enables site-specific covalent protein modification with reagents that would usually react first with naturally occurring cysteine residues. The much greater reactivity of Sec residues allows their selective alkylation in the presence of highly solvent-exposed cysteine residues.

Spectroscopic Studies on Photoinduced Reactions of the Anticancer Prodrug, trans,trans,trans-[Pt(N3 )2 (OH)2 (py)2 ].

The photodecomposition mechanism of trans,trans,trans-[Pt(N3 )2 (OH)2 (py)2 ] (1, py=pyridine), an anticancer prodrug candidate, was probed using complementary Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR), transient electronic absorption, and UV/Vis spectroscopy. Data fitting using Principal Component Analysis (PCA) and Multi-Curve Resolution Alternating Least Squares, suggests the formation of a trans-[Pt(N3 )(py)2 (OH/H2 O)] intermediate and trans-[Pt(py)2 (OH/H2 O)2 ] as the final product upon 420 nm irradiation of 1 in water. Rapid disappearance of the hydroxido ligand stretching vibration upon irradiation is correlated with a -10 cm-1 shift to the antisymmetric azido vibration, suggesting a possible second intermediate. Experimental proof of subsequent dissociation of azido ligands from platinum is presented, in which at least one hydroxyl radical is formed in the reduction of PtIV to PtII . Additionally, the photoinduced reaction of 1 with the nucleotide 5'-guanosine monophosphate (5'-GMP) was comprehensively studied, and the identity of key photoproducts was assigned with the help of ATR-FTIR spectroscopy, mass spectrometry, and density functional theory calculations. The identification of marker bands for some of these photoproducts (e.g., trans-[Pt(N3 )(py)2 (5'-GMP)] and trans-[Pt(py)2 (5'-GMP)2 ]) will aid elucidation of the chemical and biological mechanism of anticancer action of 1. In general, these studies demonstrate the potential of vibrational spectroscopic techniques as promising tools for studying such metal complexes.

8-Mercaptoguanine Derivatives as Inhibitors of Dihydropteroate Synthase.

Dihydropteroate synthase (DHPS) is an enzyme of the folate biosynthesis pathway, which catalyzes the formation of 7,8-dihydropteroate (DHPt) from 6-hydroxymethyl-7,8-dihydropterin pyrophosphate (DHPPP) and para-aminobenzoic acid (pABA). DHPS is the long-standing target of the sulfonamide class of antibiotics that compete with pABA. In the wake of sulfa drug resistance, targeting the structurally rigid (and more conserved) pterin site has been proposed as an alternate strategy to inhibit DHPS in wild-type and sulfa drug resistant strains. Following the work on developing pterin-site inhibitors of the adjacent enzyme 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK), we now present derivatives of 8-mercaptoguanine, a fragment that binds weakly within both enzymes, and quantify sub-µm binding using surface plasmon resonance (SPR) to Escherichia coli DHPS (EcDHPS). Eleven ligand-bound EcDHPS crystal structures delineate the structure-activity relationship observed providing a structural framework for the rational development of novel, substrate-envelope-compliant DHPS inhibitors.

Small Gd(III) Tags for Gd(III)-Gd(III) Distance Measurements in Proteins by EPR Spectroscopy.

The C7-Gd and C8-Gd tags are compact hydrophilic cyclen-based lanthanide tags for conjugation to cysteine residues in proteins. The tags are enantiomers, which differ in the configuration of the 2-hydroxylpropyl pendant arms coordinating the lanthanide ion. Here, we report the electron paramagnetic resonance (EPR) performance of the C7-Gd ( S configuration) and C8-Gd ( R configuration) tags loaded with Gd(III) on two mutants of the homodimeric ERp29 protein. The W-band EPR spectra were found to differ between the tags in the free state and after conjugation to the protein. In addition, the spectra were sensitive to the labeling position, which may originate from an environment-dependent charge density on the Gd(III)-coordinating oxygens. This is in agreement with previous NMR experiments with different lanthanide ions, which suggested sensitivity to H-bonding. W-band 1H-ENDOR (electron-electron double resonance) experiments detected effects from orientation selection in the central transition, due to a relatively narrow distribution in the ZFS parameters as indicated by simulations. In contrast, the distance distributions derived from DEER (double electron-electron resonance) measurements were insensitive to the R or S configuration of the tags and did not exhibit any orientation selection effects. The DEER measurements faithfully reflected the different widths of the distance distributions at the different protein sites in agreement with previous DEER measurements using other Gd(III) tags. Due to their small size, short tether to the protein, and a broad central EPR transition, the C7-Gd and C8-Gd tags are attractive Gd(III) tags for measurements of relatively short (<4 nm) distances by EPR spectroscopy.

Small neutral Gd(iii) tags for distance measurements in proteins by double electron-electron resonance experiments.

Spin labels containing a Gd(iii) ion have become important for measuring nanometer distances in proteins by double electron-electron resonance (DEER) experiments at high EPR frequencies. The distance resolution and sensitivity of these measurements strongly depend on the Gd(iii) tag used. Here we report the performance of two Gd(iii) tags, propargyl-DO3A and C11 in DEER experiments carried out at W-band (95 GHz). Both tags are small, uncharged and devoid of bulky hydrophobic pendants. The propargyl-DO3A tag is designed for conjugation to the azide-group of an unnatural amino acid. The C11 tag is a new tag designed for attachment to a single cysteine residue. The tags delivered narrower distance distributions in the E. coli aspartate/glutamate binding protein and the Zika virus NS2B-NS3 protease than previously established Gd(iii) tags. The improved performance is consistent with the absence of specific hydrophobic or charge-charge interactions with the protein. In the case of the Zika virus NS2B-NS3 protease, unexpectedly broad Gd(iii)-Gd(iii) distance distributions observed with the previously published charged C9 tag, but not the C11 tag, illustrate the potential of tags to perturb a labile protein structure and the importance of different tags. The results obtained with the C11 tag demonstrate the closed conformation in the commonly used linked construct of the Zika virus NS2B-NS3 protease, both in the presence and absence of an inhibitor.

Towards Utilising Photocrosslinking of Polydiacetylenes for the Preparation of "Stealth" Upconverting Nanoparticles.

We demonstrate a novel strategy for preparing hydrophilic upconverting nanoparticles (UCNPs) by harnessing the photocrosslinking ability of diacetylenes. Replacement of the hydrophobic oleate coating on the UCNPs with 10,12-pentacosadiynoic acid, followed by overcoating with diacetylene phospholipid and subsequent photocrosslinking under 254 nm irradiation produces water-dispersible polydiacetylene-coated UCNPs. These UCNPs resist the formation of a biomolecular corona and show great colloidal stability. Furthermore, amine groups on the diacetylene phospholipid allow for functionalisation of the UCNPs with, for example, radiolabels or targeting moieties. These results demonstrate that this new surface-coating method has great potential for use in the preparation of UCNPs with improved biocompatibility.

Structure restraints from heteronuclear pseudocontact shifts generated by lanthanide tags at two different sites.

Pseudocontact shifts (PCS) encode long-range information on 3D structures of protein backbones and side-chains. The level of structural detail that can be obtained increases with the number of different sites tagged with a paramagnetic metal ion to generate PCSs. Here we show that PCSs from two different sites can suffice to determine the structure of polypeptide chains and their location and orientation relative to the magnetic susceptibility tensor χ, provided that PCSs are available for 1H as well as heteronuclear spins. In addition, PCSs from two different sites are shown to provide detailed structural information on the conformation of methyl group-bearing amino-acid side-chains. A previously published ensemble structure of ubiquitin is shown to explain the magnetic susceptibility and alignment tensors slightly better than structures that try to explain the experimental data by a single conformation, illustrating the potential of PCSs as a tool to investigate small conformational changes.

New Lanthanide Tag for the Generation of Pseudocontact Shifts in DNA by Site-Specific Ligation to a Phosphorothioate Group.

Pseudocontact shifts (PCS) generated by paramagnetic lanthanides provide a rich source of long-range structural restraints that can readily be measured by nuclear magnetic resonance (NMR) spectroscopy. Many different lanthanide-binding tags have been designed for site-specific tagging of proteins, but established routes for tagging DNA with a single metal ion rely on difficult chemical synthesis. Here we present a simple and practical strategy for site-specific tagging of inexpensive phosphorothioate (PT) oligonucleotides. Commercially available PT oligonucleotides are diastereomers with S and R stereoconfiguration at the backbone PT site. The respective SP and RP diastereomers can readily be separated by HPLC. A new alkylating lanthanide-binding tag, C10, was synthesized that delivered quantitative tagging yields with both diastereomers. PCSs were observed following ligation with the complementary DNA strand to form double-stranded DNA duplexes. The PCSs were larger for the SP than the RP oligonucleotide and good correlation between back-calculated and experimental PCSs was observed. The C10 tag can also be attached to cysteine residues in proteins, where it generates a stable thioether bond. Ligated to the A28C mutant of ubiquitin, the tag produced excellent fits of magnetic susceptibility anisotropy (Δχ) tensors, with larger tensors than for the tagged PT oligonucleotides, indicating that the tag is not completely immobilized after ligation with a PT group.

Chemical Tagging with tert-Butyl and Trimethylsilyl Groups for Measuring Intermolecular Nuclear Overhauser Effects in a Large Protein-Ligand Complex.

Intermolecular 1 H-1 H nuclear Overhauser effects (NOE) present a powerful tool to assess contacts between proteins and binding partners, but are difficult to identify for complexes of high molecular weight. This report shows that intermolecular NOEs can readily be observed following chemical labeling with tert-butyl or trimethylsilyl (TMS) groups. Proteins can be furnished with tert-butyl or TMS groups site-specifically using genetically encoded unnatural amino acids or by chemical modification of single cysteine residues. No isotope labeling is required. The approach is demonstrated with the 95 kDa complex between tetrameric E. coli single-stranded DNA binding protein (SSB) and single-stranded DNA.

Double-Arm Lanthanide Tags Deliver Narrow Gd3+ -Gd3+ Distance Distributions in Double Electron-Electron Resonance (DEER) Measurements.

Double-arm cyclen-based Gd3+ tags are shown to produce accurate nanometer scale Gd3+ -Gd3+ distance measurements in double electron-electron resonance (DEER) experiments by confining the space accessible to the metal ion. The results show excellent agreement with predictions both for the maximum and width of the measured distance distributions. For distance measurements in proteins, the tags can be attached to two cysteine residues located in positions i and i+4, or i and i+8, of an α-helix. In the latter case, an additional mutation introducing an aspartic acid at position i+4 achieves particularly narrow distribution widths. The concept is demonstrated with cysteine mutants of T4 lysozyme and maltose binding protein. We report the narrowest Gd3+ -Gd3+ distance distributions observed to date for a protein. By limiting the contribution of tag mobility to the distances measured, double-arm Gd3+ tags open new opportunities to study the conformational landscape of proteins in solution with high sensitivity.

Investigation of Donor-Acceptor Stenhouse Adducts as New Visible Wavelength-Responsive Switching Elements for Lipid-Based Liquid Crystalline Systems.

The ability of donor-acceptor Stenhouse adducts (DASAs) to function as a green light responsive switch for lipid-based liquid crystalline drug delivery systems was investigated. The host matrix comprising phytantriol cubic phase was selected due to its high sensitivity toward changes in lipid packing. Small-angle X-ray scattering demonstrated that the matrix undergoes rapid and fully reversible order-order phase transitions upon irradiation with 532 nm light, converting between the bicontinuous cubic phase and reversed hexagonal phases. This approach shows promise for use as an actuator for the development of visible wavelength light-activated, "on-demand" drug delivery systems.

Fluorescent diphenylphosphonate-based probes for detection of serine protease activity during inflammation.

Activity-based probes are small molecules that covalently bind to the active site of a protease in an activity-dependent manner. We synthesized and characterized two fluorescent activity-based probes that target serine proteases with trypsin-like or elastase-like activity. We assessed the selectivity and potency of these probes against recombinant enzymes and demonstrated that while they are efficacious at labeling active proteases in complex protein mixtures in vitro, they are less valuable for in vivo studies. We used these probes to evaluate serine protease activity in two mouse models of acute inflammation, including pancreatitis and colitis. As anticipated, the activity of trypsin-like proteases was increased during pancreatitis. Levels of elastase-like proteases were low in pancreatic lysates and colonic luminal fluids, whether healthy or inflamed. Exogenously added recombinant neutrophil elastase was inhibited upon incubation with these samples, an effect that was augmented in inflamed samples compared to controls. These data suggest that endogenous inhibitors and elastase-degrading proteases are upregulated during inflammation.

Studies of Carbon Monoxide Release from Ruthenium(II) Bipyridine Carbonyl Complexes upon UV-Light Exposure.

The UV-light-induced CO release characteristics of a series of ruthenium(II) carbonyl complexes of the form trans-Cl[RuLCl2(CO)2] (L = 4,4'-dimethyl-2,2'-bipyridine, 4'-methyl-2,2'-bipyridine-4-carboxylic acid, or 2,2'-bipyridine-4,4'-dicarboxylic acid) have been elucidated using a combination of UV-vis absorbance and Fourier transform infrared spectroscopies, multivariate curve resolution alternating least-squares analysis, and density functional theory calculations. In acetonitrile, photolysis appears to proceed via a serial three-step mechanism involving the sequential formation of [RuL(CO)(CH3CN)Cl2], [RuL(CH3CN)2Cl2], and [RuL(CH3CN)3Cl]+. Release of the first CO molecule occurs quickly (k1 ≫ 3 min-1), while release of the second CO molecule proceeds at a much more modest rate (k2 = 0.099-0.17 min-1) and is slowed by the presence of electron-withdrawing carboxyl substituents on the bipyridine ligand. In aqueous media (1% dimethyl sulfoxide in H2O), the two photodecarbonylation steps proceed much more slowly (k1 = 0.46-1.3 min-1 and k2 = 0.026-0.035 min-1, respectively) and the influence of the carboxyl groups is less pronounced. These results have implications for the design of new light-responsive CO-releasing molecules ("photoCORMs") intended for future medical use.

Extending the Excitation Wavelength of Potential Photosensitizers via Appendage of a Kinetically Stable Terbium(III) Macrocyclic Complex for Applications in Photodynamic Therapy.

The development of viable photodynamic therapy protocols is often hindered by photosensitizers that require high-energy UV irradiation that has limited potential for clinical use due to its low tissue penetration. Herein, we report a strategy for extending the excitation wavelength of potential photosensitizers via the covalent attachment of a terbium(III)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetate complex (DO3A-Tb). The method was systematically demonstrated with a series of polycyclic aromatic hydrocarbons (naphthalene, phenanthrene, anthracene, pyrene, and fluoranthene) to prepare six new complexes (Tb1-Tb6) with bathochromic shifts that extended into the visible region. Determination of their quantum yields for singlet oxygen (1O2) production at 350 and 420 nm showed significant enhancements from the parent molecule in all cases. Cell viability studies on cervical cancer cells (HeLa) and noncancerous MRC-5 cells showed no measurable cytotoxicity for all complexes prior to light irradiation. However, after irradiation at 420 nm (20 min, 9.27 J cm-2), Tb3-Tb6 were phototoxic to HeLa cells with IC50 values between 14.3-32.3 µM. Cell morphological studies and fluorescence microscopy with live/dead cell stains confirmed these findings. In addition, these complexes were highly stable in human blood plasma, with no significant degradation observed after 96 h at 37 °C. This excellent phototoxicity profile and high stability in blood plasma, coupled with the moderately lipophilic nature of the complexes, favorably indicate the potential of DO3A-Tb as a heavy atom-bearing moiety for modification of potential photosensitizers into ideal phototherapeutic drug candidates with longer excitation wavelengths for in vivo application.

Real-time examination of aminoglycoside activity towards bacterial mimetic membranes using Quartz Crystal Microbalance with Dissipation monitoring (QCM-D).

The rapid increase in multi-drug resistant bacteria has resulted in previously discontinued treatments being revisited. Aminoglycosides are effective "old" antibacterial agents that fall within this category. Despite extensive usage and understanding of their intracellular targets, there is limited mechanistic knowledge regarding how aminoglycosides penetrate bacterial membranes. Thus, the activity of two well-known aminoglycosides, kanamycin A and neomycin B, towards a bacterial mimetic membrane (DMPC:DMPG (4:1)) was examined using a Quartz Crystal Microbalance with Dissipation monitoring (QCM-D). The macroscopic effect of increasing the aminoglycoside concentration showed that kanamycin A exerts a threshold response, switching from binding to the membrane to disruption of the surface. Neomycin B, however, disrupted the membrane at all concentrations examined. At concentrations above the threshold value observed for kanamycin A, both aminoglycosides revealed similar mechanistic details. That is, they both inserted into the bacterial mimetic lipid bilayer, prior to disruption via loss of materials, presumably aminoglycoside-membrane composites. Depth profile analysis of this membrane interaction was achieved using the overtones of the quartz crystal sensor. The measured data is consistent with a two-stage process in which insertion of the aminoglycoside precedes the 'detergent-like' removal of membranes from the sensor. The results of this study contribute to the insight required for aminoglycosides to be reconsidered as active antimicrobial agents/co-agents by providing details of activity at the bacterial membrane. Kanamycin and neomycin still offer potential as antimicrobial therapeutics for the future and the QCM-D method illustrates great promise for screening new antibacterial or antiviral drug candidates.

Sensitive NMR Approach for Determining the Binding Mode of Tightly Binding Ligand Molecules to Protein Targets.

Structure-guided drug design relies on detailed structural knowledge of protein-ligand complexes, but crystallization of cocomplexes is not always possible. Here we present a sensitive nuclear magnetic resonance (NMR) approach to determine the binding mode of tightly binding lead compounds in complex with difficult target proteins. In contrast to established NMR methods, it does not depend on rapid exchange between bound and free ligand or on stable isotope labeling, relying instead on a tert-butyl group as a chemical label. tert-Butyl groups are found in numerous protein ligands and deliver an exceptionally narrow and tall (1)H NMR signal. We show that a tert-butyl group also produces outstandingly intense intra- and intermolecular NOESY cross-peaks. These enable measurements of pseudocontact shifts generated by lanthanide tags attached to the protein, which in turn allows positioning of the ligand on the protein. Once the ligand has been located, assignments of intermolecular NOEs become possible even without prior resonance assignments of protein side chains. The approach is demonstrated with the dengue virus NS2B-NS3 protease in complex with a high-affinity ligand containing a tert-butyl group.