A comparison of the binding sites of matrix metalloproteinases and tumor necrosis factor-alpha converting enzyme: implications for selectivity

Biomolecular basis of matrix metallo proteinase-9 activity

Matrix metalloproteinases (MMPs) are structurally related endopeptidases. They are also known as metzincins due to their interaction with zinc ion of the conserved methionine (Met) at the active site. MMPs play an important role in physiological and signaling processes of wound healing, bone resorption and angiogenesis. The structure of MMPs consists of signal peptide, propeptide, catalytic domain, hinge region and hemopexin-like domain. MMP-9 shares high structural and functional similarities with MMP-2, therefore designing selective MMP-9 inhibitors (MMPIs) is challenging. The selectivity can be achieved by targeting S2 subsite of MMP-9 that is having difference with MMP-2. Further, targeting its exosite and protein disulfide isomerase may also provide selective MMPIs. The review highlights the molecular features and basis of MMP-9 enzyme action. The MMPIs reported in the recent years have also been included.

Improved QM/MM Linear-Interaction Energy Model for Substrate Recognition in Zinc-Containing Metalloenzymes

One of the essential challenges in the description of receptor-drug interactions in the presence of various polyvalent cations (such as zinc, magnesium, or iron) is the accurate assessment of the electronic effects due to cofactor binding. The effects can range from partial electronic polarization of the proximal atoms in a receptor and bound substrate to long-range effects related to partial charge transfer and electronic delocalization effects between the cofactor and the drug. Here, we examine the role of the explicit account for electronic effects for a panel of small-molecule inhibitors binding to the zinc-aminopeptidase PfA-M1, an essential target for antimalarial drug development. Our study on PfA-M1:inhibitor interactions at the QM level reveals that the partial charge and proton transfer due to bound zinc ion are important mechanisms in the inhibitors' recognition and catalysis. The combination of classical MD simulations with a posteriori QM/MM corrections with novel DFTB parameters for the zinc cation and the linear-interaction energy (LIE) approach offers by far the most accurate estimates for the PfA-M1:inhibitor binding affinities, opening the door for future inhibitor design.

Synthesis, Structure, and SAR of Tetrahydropyran-Based LpxC Inhibitors

In the search for novel Gram-negative agents, we performed a comprehensive search of the AstraZeneca collection and identified a tetrahydropyran-based matrix metalloprotease (MMP) inhibitor that demonstrated nanomolar inhibition of UDP-3-O-(acyl)-N-acetylglucosamine deacetylase (LpxC). Crystallographic studies in Aquifex aeolicus LpxC indicated the tetrahydropyran engaged in the same hydrogen bonds and van der Waals interactions as other known inhibitors. Systematic optimization of three locales on the scaffold provided compounds with improved Gram-negative activity. However, the optimization of LpxC activity was not accompanied by reduced inhibition of MMPs. Comparison of the crystal structure of the native product, UDP-3-O-(acyl)-glucosamine, in Aquifex aeolicus to the structure of a tetrahydropyran-based inhibitor indicates pathways for future optimization.

Novel methods and strategies in the discovery of TACE inhibitors

INTRODUCTION

Tumor necrosis factor-α (TNF-α) is a key player in inflammation and joint damage in rheumatoid arthritis (RA). One treatment approach to exclude TNF-α from the biological system is by inhibiting tumor necrosis factor-alpha converting enzyme (TACE), the enzyme responsible for the production of its active form. To date, a number of TACE inhibitors have been reported in the literature from various strategies and methods.

AREAS COVERED

The following article presents the design and development strategies for the discovery of novel TACE inhibitors which could be of therapeutic utility for the alleviation of inflammatory conditions. The review is based on literature of the subject from 2005 onward.

EXPERT OPINION

Discovery of a selective TACE inhibitor has remained a major goal for many academic and pharmaceutical industrial research laboratories for quite some time. Identification of selective TACE inhibitors has proved elusive until recently due to structural similarities between TACE and MMPs. The differences in the shape and size of the S1' pocket of TACE and MMPs could be exploited to design selective TACE inhibitors devoid of any MMP inhibitory activity in the near future. It would be a Herculean task to develop a specific TACE inhibitor for clinical treatment of RA because binding subsites of TACE and MMPs are quite similar. However, developments taking place currently in the field as well as in the application of molecular modeling techniques at a wider scale could yet provide clinically useful selective TACE inhibitors in the not too distant future.

Molecular modeling and biological effects of peptidomimetic inhibitors of TACE activity

We investigated the molecular basis of specificity for the interaction between tumor necrosis factor-alpha converting enzyme (TACE) and peptidomimetic inhibitors. Four novel peptidomimetic TACE inhibitors (8a-d) were designed and synthesized by introducing a substituted sulfur group and a hydrophobic group to a novel matrix metalloprotease (MMP) inhibitor. Inhibition was determined by in vitro lipopolysaccharide (LPS) cytotoxicity tests in HL-60 cell lines and by measuring the expression of mTNF-alpha using FCM techniques and immunohistochemistry in vivo. We simulated the interaction of the inhibitors with the 3D structure of the TACE active site in the Brookhaven Protein Database (PDB). The four inhibitors (8a-d) inhibited activity by 9.1%, 54.5%, 27.3%, and 54.5%, respectively. 8b and 8d showed significant in vitro inhibition in cytotoxicity tests, which corresponded to the molecular docking results. 8d also showed inhibitory activity in vivo. We explored the interface between enzyme and substrate by combining bioinformatics with experimental observations to further the development of specific TACE inhibitors to reduce inflammatory responses.

Matrix metalloproteinase 2 inhibition: combined quantum mechanics and molecular mechanics studies of the inhibition mechanism of (4-phenoxyphenylsulfonyl)methylthiirane and its oxirane analogue

The inhibition mechanism of matrix metalloproteinase 2 (MMP2) by the selective inhibitor (4-phenoxyphenylsulfonyl)methylthiirane (SB-3CT) and its oxirane analogue is investigated computationally. The inhibition mechanism involves C-H deprotonation with concomitant opening of the three-membered heterocycle. SB-3CT was docked into the active site of MMP2, followed by molecular dynamics simulation to prepare the complex for combined quantum mechanics and molecular mechanics (QM/MM) calculations. QM/MM calculations with B3LYP/6-311+G(d,p) for the QM part and the AMBER force field for the MM part were used to examine the reaction of these two inhibitors in the active site of MMP2. The calculations show that the reaction barrier for transformation of SB-3CT is 1.6 kcal/mol lower than its oxirane analogue, and the ring-opening reaction energy of SB-3CT is 8.0 kcal/mol more exothermic than that of its oxirane analogue. Calculations also show that protonation of the ring-opened product by water is thermodynamically much more favorable for the alkoxide obtained from the oxirane than for the thiolate obtained from the thiirane. A six-step partial charge fitting procedure is introduced for the QM/MM calculations to update atomic partial charges of the quantum mechanics region and to ensure consistent electrostatic energies for reactants, transition states, and products.

QM/MM linear response method distinguishes ligand affinities for closely related metalloproteins

Design of selective ligands for closely related targets is becoming one of the most important tasks in the drug development. New tools, more precise than fast scoring functions and less demanding than sophisticated Free Energy Perturbation methods, are necessary to help accomplish this goal. The methods of intermediate complexity, characterizing individual contributions to the binding energy, have been an area of intense research in the past few years. Our recently developed quantum mechanical/molecular mechanical (QM/MM) modification of the Linear Response (LR) method describes the binding free energies as the sum of empirically weighted contributions of the QM/MM interaction energies and solvent-accessible surface areas for the time-averaged structures of hydrated complexes, obtained by molecular dynamics (MD) simulations. The method was applied to published data on 27 inhibitors of matrix metalloproteinase-3 (MMP-3). The two descriptors explained 90% of variance in the inhibition constants with RMSE of 0.245 log units. The QM/MM treatment is indispensable for characterization of the systems lacking suitable force-field expressions. In this case, it provided characteristics of H-bonds of the inhibitors to Glu202, charges of binding site atoms, and accurate coordination geometries of the ligands to catalytic zinc. The geometries were constrained during the MD simulations, which characterized conformational flexibility of the complexes and helped in the elucidation of the binding differences for related compounds. A comparison of the presented QM/MM LR results with those previously published for inhibition of MMP-9 by the same set of ligands showed that the QM/MM LR approach was able to distinguish subtle differences in binding affinities for MMP-3 and MMP-9, which did not exceed one order of magnitude. This precision level makes the approach a useful tool for design of selective ligands to similar targets, because the results can be safely extrapolated to maximize selectivity.

Discovery of novel hydroxamates as highly potent tumor necrosis factor-alpha converting enzyme inhibitors. Part II: optimization of the S3' pocket

A series of cyclopropyl hydroxamic acids were prepared. Many of the compounds displayed picomolar affinity for the TACE enzyme while maintaining good to excellent selectivity profiles versus MMP-1, -2, -3, -7, -14, and ADAM-10. X-ray analysis of an inhibitor in the TACE active site indicated that the molecules bound to the enzyme in the S1'-S3' pocket.

Identification of potent and selective TACE inhibitors via the S1 pocket

By focusing on the P1 portion of the piperidine beta-sulfone ligands we identified a motif that induces selectivity and resulted in a series of TACE inhibitors that demonstrated excellent in vitro potency against isolated TACE enzyme and excellent selectivity over MMPs 1, 2, 9, 13, and 14.

Recent advances in MMP inhibitor design

The search for an MMP inhibitor with anticancer efficacy is a nearly three-decade endeavor. This inhibitor is yet to be found. The reasons for this failure include shortcomings in the chemistry of these compounds (including broad MMP sub-type selectivity, metabolic lability, and toxicity) as well as the emerging, and arguably extraordinary, complexity of MMP cell (and cancer) biology. Together these suggest that the successful anticancer inhibitor must possess MMP selectivity against the MMP subtype whose involvement is critical, yet highly temporally (with respect to metastatic progression) and mechanistically (with respect to matrix degradation) regulated. This review summarizes the progression of chemical structure and mechanistic thinking toward these objectives, with emphasis on the disappointment, the perseverance, and the resilient optimism that such an inhibitor is there to be discovered.