Computational Modeling of the SARS-CoV-2 Main Protease Inhibition by the Covalent Binding of Prospective Drug Molecules

We illustrate modern modeling tools applied in the computational design of drugs acting as covalent inhibitors of enzymes. We take the Main protease (M pro ) from the SARS-CoV-2 virus as an important present-day representative. In this work, we construct a compound capable to block M pro , which is composed of fragments of antimalarial drugs and covalent inhibitors of cysteine proteases. To characterize the mechanism of its interaction with the enzyme, the algorithms based on force fields, including molecular mechanics (MM), molecular dynamics (MD) and molecular docking, as well as quantum-based approaches, including quantum chemistry and quantum mechanics/molecular mechanics (QM/MM) methods, should be applied. The use of supercomputers is indispensably important at least in the latter approach. Its application to enzymes assumes that energies and forces in the active sites are computed using methods of quantum chemistry, whereas the rest of protein matrix is described using conventional force fields. For the proposed compound, containing the benzoisothiazolone fragment and the substitute at the uracil ring, we show that it can form a stable covalently bound adduct with the target enzyme, and thus can be recommended for experimental trials

Modeling Light-Induced Chromophore Hydration in the Reversibly Photoswitchable Fluorescent Protein Dreiklang

We report the results of a computational study of the mechanism of the light-induced chemical reaction of chromophore hydration in the fluorescent protein Dreiklang, responsible for its switching from the fluorescent ON-state to the dark OFF-state. We explore the relief of the charge-transfer excited-state potential energy surface in the ON-state to locate minimum energy conical intersection points with the ground-state energy surface. Simulations of the further evolution of model systems allow us to characterize the ground-state reaction intermediate tentatively suggested in the femtosecond studies of the light-induced dynamics in Dreiklang and finally to arrive at the reaction product. The obtained results clarify the details of the photoswitching mechanism in Dreiklang, which is governed by the chemical modification of its chromophore

Two Sides of Quantum-Based Modeling of Enzyme-Catalyzed Reactions: Mechanistic and Electronic Structure Aspects of the Hydrolysis by Glutamate Carboxypeptidase

We report the results of a computational study of the hydrolysis reaction mechanism of N-acetyl-l-aspartyl-l-glutamate (NAAG) catalyzed by glutamate carboxypeptidase II. Analysis of both mechanistic and electronic structure aspects of this multistep reaction is in the focus of this work. In these simulations, model systems are constructed using the relevant crystal structure of the mutated inactive enzyme. After selection of reaction coordinates, the Gibbs energy profiles of elementary steps of the reaction are computed using molecular dynamics simulations with ab initio type QM/MM potentials (QM/MM MD). Energies and forces in the large QM subsystem are estimated in the DFT(PBE0-D3/6-31G**) approximation. The established mechanism includes four elementary steps with the activation energy barriers not exceeding 7 kcal/mol. The models explain the role of point mutations in the enzyme observed in the experimental kinetic studies; namely, the Tyr552Ile substitution disturbs the “oxyanion hole”, and the Glu424Gln replacement increases the distance of the nucleophilic attack. Both issues diminish the substrate activation in the enzyme active site. To quantify the substrate activation, we apply the QTAIM-based approaches and the NBO analysis of dynamic features of the corresponding enzyme-substrate complexes. Analysis of the 2D Laplacian of electron density maps allows one to define structures with the electron density deconcentration on the substrate carbon atom, i.e., at the electrophilic site of reactants. The similar electronic structure element in the NBO approach is a lone vacancy on the carbonyl carbon atom in the reactive species. The electronic structure patterns revealed in the NBO and QTAIM-based analyses consistently clarify the reactivity issues in this system

Modeling the Transient Kinetics of the L1 Metallo-β-Lactamase

The transient absorption spectroscopy of hydrolysis of the chromogenic substrate nitrocefin by the L1 metallo-β-lactamase (MβL), a bacterial enzyme responsible for destruction of β-lactam antibiotics molecules, showed formation and decay of a plausible red-shifted reaction intermediate. We propose a mechanism of this important reaction consistent with the transient kinetic data. Quantum mechanics/molecular mechanics (QM/MM) simulations of the reaction pathway revealed occurrence of two reaction intermediates (I1, I2) between the enzyme–substrate (ES) and enzyme–product (EP) complexes. The vertical S₀–S₁ transition energies calculated at the minimum energy structures (ES, I1, I2, EP) using the time dependent DFT (TD-DFT) method allowed us to assign the experimental absorption bands to all reacting species. We numerically solved the equations of chemical kinetics with the rate constants of all elementary steps evaluated with the transition state theory and simulated the kinetic curves as well as the evolution of the absorption bands of ES, I2, and EP. Direct comparison to the experimental data allowed us to identify the I2 intermediate as the transient red-shifted species detected experimentally. In agreement with the experimental observations, the recomputed energy profiles for the D120N and D120C mutants of L1 reacting with nitrocefin showed absence of a stable intermediate I2. According to the consistent experimental and theoretical results, the breakdown of the intermediate I2 corresponds to the rate-limiting stage of the chemical transformations in the active site of the L1 metallo-β-lactamase. On this basis, we established a QSAR-type correlation between the observed reaction rates (<i>k</i>_cat) of three cephalosporin antibiotics (cefotixin, nitrocefin, cefepime) showing different hydrolysis rates by the L1 metallo-β-lactamase and different structures of the corresponding intermediates of the I2 type. This correlation can be employed for a rational design of novel antibiotics, which are not decomposed by metallo-β-lactamases

Theoretical characterization of the photochemical reaction CO2+O(3P)→CO+O2 related to experiments in solid krypton

Formation and decomposition of the complex of carbon dioxide and atomic oxygen are characterized by quantum chemistry methods aiming to rationalize experimental studies in solid krypton. The observed FTIR spectra reflected the temporal evolution of the system after irradiation showing the bands of reactants, intermediates and products. Advanced quantum chemistry calculations show that the T-shape complex CO2…O(3P) can be formed in the matrix. Its excitation by the 193 nm light results in the charge-transfer state CO2+…O-, which evolves to the reaction intermediate CO3. The latter species decomposes to CO + O2 following pathways on the excited state energy surfaces.peerReviewe

Computational Characterization of Ketone–Ketal Transformations at the Active Site of Matrix Metalloproteinases

We modeled the first steps of hydrolysis reactions of a natural oligopeptide substrate of matrix metalloproteinase MMP-2 as well as of a substrate analogue. In the latter, the scissile amide group is substituted by a ketomethylene group which can be transformed to the ketal group upon binding of this compound to the enzyme active site. According to our quantum mechanical–molecular mechanical (QM/MM) calculations, the reaction of the ketone–ketal transformation proceeds with a low energy barrier (3.4 kcal/mol) and a high equilibrium constant (10⁴). The reaction product with the ketal group formed directly at the active site of the enzyme works as an inhibitor that chelates the zinc ion. On the other hand, the oligopeptide mimetic retains molecular groups responsible for binding of this compound to the enzyme active site. This example illustrates a strategy to design MMP inhibitors <i>in situ</i> by using data on binding specificity of substrates to a particular type of MMP and details of the reaction mechanism

Optimization of Cholinesterase-Based Catalytic Bioscavengers Against Organophosphorus Agents

Organophosphorus agents (OPs) are irreversible inhibitors of acetylcholinesterase (AChE). OP poisoning causes major cholinergic syndrome. Current medical counter-measures mitigate the acute effects but have limited action against OP-induced brain damage. Bioscavengers are appealing alternative therapeutic approach because they neutralize OPs in bloodstream before they reach physiological targets. First generation bioscavengers are stoichiometric bioscavengers. However, stoichiometric neutralization requires administration of huge doses of enzyme. Second generation bioscavengers are catalytic bioscavengers capable of detoxifying OPs with a turnover. High bimolecular rate constants (kcat/Km &gt; 106 M−1min−1) are required, so that low enzyme doses can be administered. Cholinesterases (ChE) are attractive candidates because OPs are hemi-substrates. Moderate OP hydrolase (OPase) activity has been observed for certain natural ChEs and for G117H-based human BChE mutants made by site-directed mutagenesis. However, before mutated ChEs can become operational catalytic bioscavengers their dephosphylation rate constant must be increased by several orders of magnitude. New strategies for converting ChEs into fast OPase are based either on combinational approaches or on computer redesign of enzyme. The keystone for rational conversion of ChEs into OPases is to understand the reaction mechanisms with OPs. In the present work we propose that efficient OP hydrolysis can be achieved by re-designing the configuration of enzyme active center residues and by creating specific routes for attack of water molecules and proton transfer. Four directions for nucleophilic attack of water on phosphorus atom were defined. Changes must lead to a novel enzyme, wherein OP hydrolysis wins over competing aging reactions. Kinetic, crystallographic, and computational data have been accumulated that describe mechanisms of reactions involving ChEs. From these studies, it appears that introducing new groups that create a stable H-bonded network susceptible to activate and orient water molecule, stabilize transition states (TS), and intermediates may determine whether dephosphylation is favored over aging. Mutations on key residues (L286, F329, F398) were considered. QM/MM calculations suggest that mutation L286H combined to other mutations favors water attack from apical position. However, the aging reaction is competing. Axial direction of water attack is not favorable to aging. QM/MM calculation shows that F329H+F398H-based multiple mutants display favorable energy barrier for fast reactivation without aging

Unusual Emitting States of the Kindling Fluorescent Protein: Appearance of the Cationic Chromophore in the GFP Family

The kindling fluorescent protein (KFP), the Ala143Gly variant of the natural chromoprotein asFP595, is a prospective biomarker in live cells. Following the results of QM/MM calculations, we predict that excitation of the protein under certain conditions, favoring formation of KFP fractions with the neutral chromophore, should result in fluorescence from the cationic form of the chromophore which is unusual for the members of the green fluorescent protein family. Occurrence of the neutral form is due to a water wire connecting the chromophore with the exterior of the protein. Occurrence of the cationic form is due to the excited-state proton transfer from the conserved Glu215 to the imidazolinone ring nitrogen of the chromophore. The emission band from conformations with the trans cationic chromophore should be noticeably shifted to the blue side around 520 nm compared to the well-known red fluorescence around 600 nm arising from the cis anionic species

Molecular Modeling Clarifies the Mechanism of Chromophore Maturation in the Green Fluorescent Protein

We report the first complete theoretical description of the chain of elementary reactions resulting in chromophore maturation in the green fluorescent protein (GFP). All reaction steps including cyclization, dehydration, and oxidation are characterized at the uniform quantum mechanics/molecular mechanics (QM/MM) computational level using density functional theory in quantum subsystems. Starting from a structure of the wild-type protein with the noncyclized Ser65-Tyr66-Gly67 tripeptide, we modeled cyclization and dehydration reactions. We then added molecular oxygen to the system and modeled the oxidation reaction resulting in the mature protein-bound chromophore. Computationally derived structures of the reaction product and several reaction intermediates agree well with the relevant crystal structures, validating the computational protocol. The highest computed energy barriers at the cyclization–dehydration (17 kcal/mol) and oxidation (21 kcal/mol) steps agree well with the values derived from the kinetics measurements (20.7 and 22.7 kcal/mol, respectively). The simulations provide strong support to the mechanism involving the cyclization–dehydration–oxidation sequence of the chromophore’s maturation reactions. The results also establish a solid basis for predictions of maturation mechanisms in other fluorescent proteins