Nanoelectrospray Ionization Mass Spectrometric Study of Mycobacterium tuberculosis CYP121–Ligand Interactions

19F-NMR reveals substrate specificity of CYP121A1 in Mycobacterium tuberculosis

Cytochromes P450 are versatile enzymes that function in endobiotic and xenobiotic metabolism and undergo meaningful structural changes that relate to their function. However, the way in which conformational changes inform the specific recognition of the substrate is often unknown. Here, we demonstrate the utility of fluorine (¹⁹F)-NMR spectroscopy to monitor structural changes in CYP121A1, an essential enzyme from Mycobacterium tuberculosis. CYP121A1 forms functional dimers that catalyze the phenol-coupling reaction of the dipeptide dicyclotyrosine. The thiol-reactive compound 3-bromo-1,1,1-trifluoroacetone was used to label an S171C mutation of the enzyme FG loop, which is located adjacent to the homodimer interface. Substrate titrations and inhibitor-bound ¹⁹F-NMR spectra indicate that ligand binding reduces conformational heterogeneity at the FG loop in both the dimer and in an engineered monomer of CYP121A1. However, only the dimer was found to promote a substrate-bound conformation that was preexisting in the substrate-free spectra, thus confirming a role for the dimer interface in dicyclotyrosine recognition. Moreover, ¹⁹F-NMR spectra in the presence of substrate analogs indicate the hydrogen-bonding feature of the dipeptide aromatic side chain as a dicyclotyrosine specificity criterion. This study demonstrates the utility of ¹⁹F-NMR as applied to a multimeric cytochrome P450, while also revealing mechanistic insights for an essential M. tuberculosis enzyme.

Surface hydrophobics mediate functional dimerization of CYP121A1 of Mycobacterium tuberculosis

Tuberculosis is caused by the pathogenic bacterium Mycobacterium tuberculosis (Mtb) and remains the leading cause of death by infection world-wide. The Mtb genome encodes a disproportionate number of twenty cytochrome P450 enzymes, of which the essential enzyme cytochrome P450 121A1 (CYP121A1) remains a target of drug design efforts. CYP121A1 mediates a phenol coupling reaction of the tyrosine dipeptide cyclo-L-Tyr-L-Tyr (cYY). In this work, a structure and function investigation of dimerization was performed as an overlooked feature of CYP121A1 function. This investigation showed that CYP121A1 dimers form via intermolecular contacts on the distal surface and are mediated by a network of solvent-exposed hydrophobic residues. Disruption of CYP121A1 dimers by site-directed mutagenesis leads to a partial loss of specificity for cYY, resulting in an approximate 75% decrease in catalysis. ¹⁹F labeling and nuclear magnetic resonance of the enzyme FG-loop was also combined with protein docking to develop a working model of a functional CYP121A1 dimer. The results obtained suggest that participation of a homodimer interface in substrate selectivity represents a novel paradigm of substrate binding in CYPs, while also providing important mechanistic insight regarding a relevant drug target in the development of novel anti-tuberculosis agents.

Structure-Activity Relationships of cyclo(l-Tyrosyl-l-tyrosine) Derivatives Binding to Mycobacterium tuberculosis CYP121: Iodinated Analogues Promote Shift to High-Spin Adduct

A series of analogues of cyclo(l-tyrosyl-l-tyrosine), the substrate of the Mycobacterium tuberculosis enzyme CYP121, have been synthesized and analyzed by UV-vis and electron paramagnetic resonance spectroscopy and by X-ray crystallography. The introduction of iodine substituents onto cyclo(l-tyrosyl-l-tyrosine) results in sub-μM binding affinity for the CYP121 enzyme and a complete shift to the high-spin state of the heme Fe^III. The introduction of halogens that are able to interact with heme groups is thus a feasible approach to the development of next-generation, tight binding inhibitors of the CYP121 enzyme, in the search for novel antitubercular compounds.

The computational and experimental studies on a 1, 2, 3-triazole compound and its special binding to three kinds of blood proteins

A newly synthesized compound, ethyl 5-phenyl-2-(p-tolyl)-2H-1, 2, 3-triazole-4-carboxylate (EPPC) may be considered as a drug candidate and was exploited to study the structural and spectral properties by using quantum chemical calculation and multiple spectroscopic techniques. The results on theoretical spectrum of EPPC were consistent with experimental spectrum in great degree. In addition, EPPC has been as a special probe and investigated on the interactions with three kinds of blood proteins including human serum albumin (HSA), human immunoglobulin (HIgG) and bovine hemoglobin (BHb) by using UV-Vis, fluorescence spectroscopy and molecular modeling, respectively. Changes in various fluorescence and UV-Vis spectra were observed upon ligand binding along with a remarkable degree of fluorescence enhancement on complex formation under physiological condition with binding constant about 10⁵ order of magnitudes, which caused the variations of conformation and microenvironment of these proteins in aqueous solution. The obtained results from the thermodynamic parameters calculated according to the van't Hoff equation indicated that the entropy change ΔS° and enthalpy change ΔH° were found to be 0.168 KJ/mol K and 22.154 KJ/mol for EPPC-HSA system, 0.284 KJ/mol K and 54.408 KJ/mol for EPPC-HIgG system, and 0.228 KJ/mol K and 37.548 KJ/mol for EPPC-BHb system, respectively, which demonstrated that the primary binding pattern is determined by hydrophobic interaction. The results of docking and molecular dynamics simulation using three proteins crystal models revealed that EPPC could bind to three proteins well into hydrophobic cavity, which showed good consistence with the spectroscopic measurements.Communicated by Ramaswamy H. Sarma.

Cross-linking of dicyclotyrosine by the cytochrome P450 enzyme CYP121 from Mycobacterium tuberculosis proceeds through a catalytic shunt pathway

CYP121, the cytochrome P450 enzyme in Mycobacterium tuberculosis that catalyzes a single intramolecular C-C cross-linking reaction in the biosynthesis of mycocyclosin, is crucial for the viability of this pathogen. This C-C coupling reaction represents an expansion of the activities carried out by P450 enzymes distinct from oxygen insertion. Although the traditional mechanism for P450 enzymes has been well studied, it is unclear whether CYP121 follows the general P450 mechanism or uses a different catalytic strategy for generating an iron-bound oxidant. To gain mechanistic insight into the CYP121-catalyzed reaction, we tested the peroxide shunt pathway by using rapid kinetic techniques to monitor the enzyme activity with its substrate dicyclotyrosine (cYY) and observed the formation of the cross-linked product mycocyclosin by LC-MS. In stopped-flow experiments, we observed that cYY binding to CYP121 proceeds in a two-step process, and EPR spectroscopy indicates that the binding induces active site reorganization and uniformity. Using rapid freeze-quenching EPR, we observed the formation of a high-spin intermediate upon the addition of peracetic acid to the enzyme-substrate complex. This intermediate exhibits a high-spin (S = 5/2) signal with g values of 2.00, 5.77, and 6.87. Likewise, iodosylbenzene could also produce mycocyclosin, implicating compound I as the initial oxidizing species. Moreover, we also demonstrated that CYP121 performs a standard peroxidase type of reaction by observing substrate-based radicals. On the basis of these results, we propose plausible free radical-based mechanisms for the C-C bond coupling reaction.

Sizing Up Protein-Ligand Complexes: The Rise of Structural Mass Spectrometry Approaches in the Pharmaceutical Sciences

Capturing the dynamic interplay between proteins and their myriad interaction partners is critically important for advancing our understanding of almost every biochemical process and human disease. The importance of this general area has spawned many measurement methods capable of assaying such protein complexes, and the mass spectrometry-based structural biology methods described in this review form an important part of that analytical arsenal. Here, we survey the basic principles of such measurements, cover recent applications of the technology that have focused on protein-small-molecule complexes, and discuss the bright future awaiting this group of technologies.

Structural Characterization and Ligand/Inhibitor Identification Provide Functional Insights into the Mycobacterium tuberculosis Cytochrome P450 CYP126A1

The Mycobacterium tuberculosis H37Rv genome encodes 20 cytochromes P450, including P450s crucial to infection and bacterial viability. Many M. tuberculosis P450s remain uncharacterized, suggesting that their further analysis may provide new insights into M. tuberculosis metabolic processes and new targets for drug discovery. CYP126A1 is representative of a P450 family widely distributed in mycobacteria and other bacteria. Here we explore the biochemical and structural properties of CYP126A1, including its interactions with new chemical ligands. A survey of azole antifungal drugs showed that CYP126A1 is inhibited strongly by azoles containing an imidazole ring but not by those tested containing a triazole ring. To further explore the molecular preferences of CYP126A1 and search for probes of enzyme function, we conducted a high throughput screen. Compounds containing three or more ring structures dominated the screening hits, including nitroaromatic compounds that induce substrate-like shifts in the heme spectrum of CYP126A1. Spectroelectrochemical measurements revealed a 155-mV increase in heme iron potential when bound to one of the newly identified nitroaromatic drugs. CYP126A1 dimers were observed in crystal structures of ligand-free CYP126A1 and for CYP126A1 bound to compounds discovered in the screen. However, ketoconazole binds in an orientation that disrupts the BC-loop regions at the P450 dimer interface and results in a CYP126A1 monomeric crystal form. Structural data also reveal that nitroaromatic ligands "moonlight" as substrates by displacing the CYP126A1 distal water but inhibit enzyme activity. The relatively polar active site of CYP126A1 distinguishes it from its most closely related sterol-binding P450s in M. tuberculosis, suggesting that further investigations will reveal its diverse substrate selectivity.

Fragment-Based Approaches to the Development of Mycobacterium tuberculosis CYP121 Inhibitors

The essential enzyme CYP121 is a target for drug development against antibiotic resistant strains of Mycobacterium tuberculosis. A triazol-1-yl phenol fragment 1 was identified to bind to CYP121 using a cascade of biophysical assays. Synthetic merging and optimization of 1 produced a 100-fold improvement in binding affinity, yielding lead compound 2 (KD = 15 μM). Deconstruction of 2 into its component retrofragments allowed the group efficiency of structural motifs to be assessed, the identification of more LE scaffolds for optimization and highlighted binding affinity hotspots. Structure-guided addition of a metal-binding pharmacophore onto LE retrofragment scaffolds produced low nanomolar (KD = 15 nM) CYP121 ligands. Elaboration of these compounds to target binding hotspots in the distal active site afforded compounds with excellent selectivity against human drug-metabolizing P450s. Analysis of the factors governing ligand potency and selectivity using X-ray crystallography, UV-vis spectroscopy, and native mass spectrometry provides insight for subsequent drug development.

A ligand-observed mass spectrometry approach integrated into the fragment based lead discovery pipeline

In fragment-based lead discovery (FBLD), a cascade combining multiple orthogonal technologies is required for reliable detection and characterization of fragment binding to the target. Given the limitations of the mainstream screening techniques, we presented a ligand-observed mass spectrometry approach to expand the toolkits and increase the flexibility of building a FBLD pipeline especially for tough targets. In this study, this approach was integrated into a FBLD program targeting the HCV RNA polymerase NS5B. Our ligand-observed mass spectrometry analysis resulted in the discovery of 10 hits from a 384-member fragment library through two independent screens of complex cocktails and a follow-up validation assay. Moreover, this MS-based approach enabled quantitative measurement of weak binding affinities of fragments which was in general consistent with SPR analysis. Five out of the ten hits were then successfully translated to X-ray structures of fragment-bound complexes to lay a foundation for structure-based inhibitor design. With distinctive strengths in terms of high capacity and speed, minimal method development, easy sample preparation, low material consumption and quantitative capability, this MS-based assay is anticipated to be a valuable addition to the repertoire of current fragment screening techniques.

Identification of inhibitors of the antibiotic-resistance target New Delhi metallo-β-lactamase 1 by both nanoelectrospray ionization mass spectrometry and ultrafiltration liquid chromatography/mass spectrometry approaches

Mass spectrometry-based platforms have gained increasing success in discovery of ligands bound to therapeutic targets as drug candidates. We established both a nanoelectrospray ionization mass spectrometry (nanoESI-MS) assay and an ultrafiltration liquid chromatography/mass spectrometry (LC/MS) assay to identify new ligands for New Delhi metallo-β-lactamase 1 (NDM-1), responsible for worldwide antibiotic resistance. To alleviate nonspecific binding of hydrophobic compounds and eliminate false positives typically encountered in the indirect LC/MS-based assay, we introduced a blocking protein in the control, which remarkably enhances the selectivity and accuracy of the indirect approach. Side-by-side comparison of the two MS-based approaches for the first time further reveals unique advantages of the indirect approach, including better reproducibility and tolerance of interference. Moreover, the success of fishing out a potent ligand from a mixture of small-molecule fragments demonstrates great potential of the indirect LC/MS-based approach for constructing a robust screening platform against combinatorial libraries or natural product extracts. More importantly, by combining the results of MS-based analyses, enzymatic activity assay, competition experiments, and structural simulation, we discovered a new compound as a promising drug candidate targeting NDM-1.