Main Protease Research Articles

Prediction of Binding Pose and Affinity of Nelfinavir, a SARS-CoV-2 Main Protease Repositioned Drug, by Combining Docking, Molecular Dynamics, and Fragment Molecular Orbital Calculations.

A novel in silico drug design procedure is described targeting the Main protease (Mpro) of the SARS-CoV-2 virus. The procedure combines molecular docking, molecular dynamics (MD), and fragment molecular orbital (FMO) calculations. The binding structure and properties of Mpro were predicted for Nelfinavir (NFV), which had been identified as a candidate compound through drug repositioning, targeting Mpro. Several poses of the Mpro and NFV complexes were generated by docking, from which four docking poses were selected by scoring with FMO energy. Then, each pose was subjected to MD simulation, 100 snapshot structures were sampled from each of the generated MD trajectories, and the structures were evaluated by FMO calculations to rank the pose based on binding energy. Several residues were found to be important in ligand recognition, including Glu47, Asp48, Glu166, Asp187, and Gln189, all of which interacted strongly with NFV. Asn142 is presumably regarded to form hydrogen bonds or CH/π interaction with NFV; however, in the present calculation, their interactions were transient. Moreover, the tert-butyl group of NFV had no interaction with Mpro. Identifying such strong and weak interactions provides candidates for maintaining and substituting ligand functional groups and important suggestions for drug discovery using drug repositioning. Besides the interaction between NFV and the amino acid residues of Mpro, the desolvation effect of the binding pocket also affected the ranking order. A similar procedure of drug design was applied to Lopinavir, and the calculated interaction energy and experimental inhibitory activity value trends were consistent. Our approach provides a new guideline for structure-based drug design starting from a candidate compound whose complex crystal structure has not been obtained.

Design, synthesis and computational study of benzimidazole derivatives as potential anti-SARS-CoV-2 agents

As the human world has constantly been plagued by the COVID-19 pandemic since its emergence, researchers have been putting mountain-high efforts into designing new weapons to combat the virus. To join this battle against coronavirus, the present communication explores the synthesis of four benzimidazoles (1–4) from aromatic diamines utilizing two separate synthetic strategies. Afterwards, the synthesized four benzimidazole analogues along with sixteen previously synthesized benzimidazole analogues from our laboratory were subjected to in silico prediction of drug-likeness properties and ADMET (absorption, distribution, metabolism, excretion and toxicity) parameters. Additionally, all the compounds were computationally evaluated through molecular docking analysis against four vital target proteins of SARS-CoV-2 namely the main protease, the papain-like protease, the RNA-dependent RNA polymerase and the spike receptor binding domain to gain insight into the interaction pattern of the ligands with the binding site residues of the protein targets and thus to analyze the SARS-CoV-2 inhibitory proficiency of the synthesized derivatives. Among the studied ligands, compounds 1, 2, 13, 14, 15, 16 and 18 displayed excellent docking scores against two or more protein targets as compared to the established antiviral drugs Nirmatrelvir and Remdesivir. Based on the docking scores, the best four complexes (compound 18-Mpro, compound 2-PLpro, compound 1-RdRp and compound 2-spike-RBD) were subjected to molecular dynamic (MD) simulation to examine the stability of the complexes during the simulation period. This integrated molecular docking and MD simulation study suggested two compounds (1 and 18) that could be evolved as promising candidates for the design and discovery of new anti-SARS-CoV-2 agents.

Computational Repurposing of Potential Dimerization Inhibitors against SARS-CoV-2 Main Protease

Background: The screening, design, and synthesis of various dimerization inhibitors have been an active area of interest for structure-based drug design efforts. Functionally important dimers, such as human immunodeficiency virus (HIV) protease and surviving, are being targeted for such studies over time. Computational repurposing of potential drug candidates provides a cost and time-efficient way in the drug discovery life cycle. Objective: Concerning the current coronavirus disease (COVID-19) scenario, the functionally active dimer of SARS-CoV-2 (severe acute respiratory syndrome) main protease (Mpro) is used as a target to screen possible dimerization inhibitors. Methods: A database of small molecule protein-protein interaction inhibitors was screened for the study. This study used molecular docking, followed by molecular dynamics (MD) simulation and postsimulation binding energy predictions. Results: From the selected 183 compounds, a diazene-based compound and a salicylic-type compound were identified as possible dimerization inhibitors in this study. These two compounds formed stable complexes with the Mpro during the MD simulations. The complexes formed by these two compounds were also unable to form important salt bridge interactions required for the dimerization of the protomers. Conclusion: Experimental studies on both compounds were previously conducted as dimerization inhibitors in HIV. The data led to the possibility of exploring the identified compounds as dimerization inhibitors, which could be important for SARS-CoV-2 therapeutics.

Improved fluorescence-based assay for rapid screening and evaluation of SARS-CoV-2 main protease inhibitors.

The outbreak of coronavirus disease 2019 (COVID-19) pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a global threat to human health. In parallel with vaccines, efficacious antivirals are urgently needed. SARS-CoV-2 main protease (Mpro) is an attractive drug target for antiviral development owing to its key roles in virus replication and host immune evasion. Due to the limitations of currently available methods, the development of novel high-throughput screeningassays is of the highest importance for the discovery of Mpro inhibitors. In this study, we first developed an improved fluorescence-based assay for rapid screening of Mpro inhibitors from an anti-infection compound library using a versatile dimerization-dependent red fluorescent protein (ddRFP) biosensor. Utilizing this assay, we identified MG-101 as a competitive Mpro inhibitor in vitro. Moreover, our results revealed that ensitrelvir is a potent Mpro inhibitor, but baicalein, chloroquine, ebselen, echinatin, and silibinin are not. Therefore, this robust ddRFP assay provides a faithful avenue for rapid screening and evaluation of Mpro inhibitors to fight against COVID-19.

Identification of Dual Inhibitors Targeting Main Protease (Mpro) and Cathepsin L as Potential Anti-SARS-CoV-2 Agents.

In this structure-activity relationship (SAR) study, we report the development of dual inhibitors with antiviral properties targeting the SARS-CoV-2 main protease (Mpro) and human cathepsin L (hCatL). The novel molecules differ in the aliphatic amino acids at the P2 site and the fluorine position on the phenyl ring at the P3 site. The identified dual inhibitors showed Ki values within 1.61 and 10.72 μM against SARS-CoV-2 Mpro; meanwhile, Ki values ranging from 0.004 to 0.701 μM toward hCatL were observed. A great interdependency between the nature of the side chain at the P2 site and the position of the fluorine atom was found. Three dual-targeting inhibitors exhibited antiviral activity in the low micromolar range with CC50 values >100 μM. Docking simulations were executed to gain a deeper understanding of the SAR profile. The findings herein collected should be taken into consideration for the future development of dual SARS-CoV-2 Mpro/hCatL inhibitors.

Development of Fluorescence-Based Assays for Key Viral Proteins in the SARS-CoV-2 Infection Process and Lifecycle.

Since the appearance of SARS-CoV-2 in 2019, the ensuing COVID-19 (Corona Virus Disease 2019) pandemic has posed a significant threat to the global public health system, human health, life, and economic well-being. Researchers worldwide have devoted considerable efforts to curb its spread and development. The latest studies have identified five viral proteins, spike protein (Spike), viral main protease (3CLpro), papain-like protease (PLpro), RNA-dependent RNA polymerase (RdRp), and viral helicase (Helicase), which play crucial roles in the invasion of SARS-CoV-2 into the human body and its lifecycle. The development of novel anti-SARS-CoV-2 drugs targeting these five viral proteins holds immense promise. Therefore, the development of efficient, high-throughput screening methodologies specifically designed for these viral proteins is of utmost importance. Currently, a plethora of screening techniques exists, with fluorescence-based assays emerging as predominant contenders. In this review, we elucidate the foundational principles and methodologies underpinning fluorescence-based screening approaches directed at these pivotal viral targets, hoping to guide researchers in the judicious selection and refinement of screening strategies, thereby facilitating the discovery and development of lead compounds for anti-SARS-CoV-2 pharmaceuticals.

On Inactivation of the Coronavirus Main Protease.

A deeper understanding of the inactive conformations of the coronavirus main protease (MPro) could inform the design of allosteric drugs. Based on extensive molecular dynamics simulations, we built a Markov State Model to investigate structural changes that can inactivate the SARS-CoV-2 MPro. In a subset of structures, one subunit of the homodimer assumes an inactive conformation that resembles an inactive crystal structure. However, contradicting the widely held half-of-sites activity hypothesis, the most populated enzyme structures have two active subunits. We then used transition path theory (TPT) and the Jensen-Shannon Divergence (JSD) to pinpoint residues involved in the inactivation process. A π stack between Phe140 and His163 is a key feature that can distinguish active and inactive conformations of MPro. Each subunit has unique inactive conformations stabilized by π stacking interactions involving residues Phe140, Tyr118, His163, and His172, a hydrogen bonding network centered around His163 and His172, and a modified network of interactions in the dimer interface. The importance of these residues in maintaining an active structure explains the sensitivity of enzymatic activity to site-directed mutagenesis.

Investigation of Active Compounds in Propolis Structure Against Sars Cov-2 Main Protease by Molecular Docking Method: In Silico Study

It was aimed to investigate the active ingredients limonin, quercetin and kaempferol in propolis against SARS-CoV-2 main protease(MPro) using in silico methods. Absorption, distribution, metabolism, excretion, and toxicity (ADMET) screening of ligands assists US to state their absorption properties, toxicity, and drug-likeness. Ligand molecules obtained from PubChem in smiles format were loaded on SWISSADME and PROTOX-II webservers for ADMET screening. The three compounds in propolis were obtained from the PubChem database. Compounds were located at the active site of the SARS-CoV-2 MPro receptor with PDB ID:6LU7. Molecular docking work was done with Autodock program. Molecular docking results were found as -8.7 kcal/mol in limonin, -7.5 kcal/mol in quercetin and -7.7 kcal/mol in kaempferol. In silico ADMET estimation showed they have a potential for antiviral therapy. In conclusion, we thought that propolis active components limonin, quercetin and kaempferol have the potential to be a SARS CoV-2 MPro inhibitor.

COMPUTATIONAL IDENTIFICATION OF SELECTED BIOACTIVE COMPOUNDS FROM CEDRUS DEODARA AS INHIBITORS AGAINST SARS-COV-2 MAIN PROTEASE: A PHARMACOINFORMATICS STUDY

Amid the ongoing Covid-19 pandemic, the quest for potent antiviral treatments intensifies. This study focuses on the potential of bioactive compounds from the Himalayan cedar Cedrus deodara against the SARS-CoV-2 virus. Specifically targeting the main protease (MPro) and spike protein, the study employs docking trials and molecular dynamics simulations. Compounds such as quercetin, dihydrodehydrodiconiferyl alcohol, and cedeodarin exhibit notable binding affinity, surpassing the reference drug favipiravir. Molecular dynamics simulations affirm the stability of these complexes throughout the simulation period. While these findings underscore promising interactions, it is crucial to emphasize the need for further research and experimental validation to fully explore the therapeutic capabilities of C. deodara in combatting Covid-19.

Synthesis, SARS-CoV-2 main protease inhibition, molecular docking and in silico ADME studies of furanochromene-quinoline hydrazone derivatives

Seven furanochromene-quinoline derivatives containing a hydrazone linker were synthesized by condensing a furanochromene hydrazide with quinoline 2-, 3-, 4-, 5-, 6-, and 8-carbaldehydes, including 8-hydroxyquinoline-2-carbaldehye. Structure-activity correlations were investigated to determine the influence of the location of the hydrazone linker on the quinoline unit on SARS-CoV-2 Mpro enzyme inhibition. The 3-, 5-, 6- and 8-substituted derivatives showed moderate inhibition of SARS-CoV-2 Mpro with IC50 values ranging from 16 to 44 μM. Additionally, all of the derivatives showed strong interaction with the SARS-CoV-2 Mpro substrate binding pocket, with docking energy scores ranging from -8.0 to -8.5 kcal/mol. These values are comparable to that of N3 peptide (-8.1 kcal/mol) and more favorable than GC-373 (-7.6 kcal/mol) and ML-188 (-7.5 kcal/mol), all of which are known SARS-CoV-2 Mpro inhibitors. Furthermore, in silico absorption, distribution, metabolism, and excretion (ADME) profiles indicate that the derivatives have good drug-likeness properties. Overall, this study highlights the potential of the furanochromene-quinoline hydrazone scaffold as a SARS-CoV-2 Mpro inhibitor.

Compromise in Docking Power of Liganded Crystal Structures of Mpro SARS-CoV-2 Surpasses 90% Success Rate.

Herein, we present the capacity of three different molecular docking programs (AutoDock, AutoDock Vina, and PLANTS) to identify and reproduce the binding modes of ligands present in 247 covalent and 169 noncovalent complex crystal structures of the severe acute respiratory syndrome coronavirus 2 main protease (Mpro). The compromise in docking power is evaluated with respect to their ability to generate poses similar to the crystal structure binding mode (heavy atoms' root-mean-square deviation < 2 Å) and their ability to recognize the native binding mode with an included compensation for the scoring function error. Noncovalently bound inhibitors are best modeled by AutoDock Vina (90.6% success rate in the active site), while the most relevant results for covalently bound inhibitors are produced by PLANTS (93.0%). AutoDock shows acceptable performance for both types of ligands, 81.1 and 76.4% for noncovalent and covalent complexes, respectively. All three programs manifest worse performance when reproducing surface-bound ligands. Comparison with other works illustrates the importance of crystal structure processing (12% of noncovalent and 26% of covalent ligands had to be manually corrected), proper sampling protocol settings, and inclusion of root-mean-square deviation (RMSD)/scoring function error compensations in crystal structure pose identification. Results are analyzed with respect to a clustering scheme of the noncovalently bound ligands and the chemical reaction type of the covalent ligand bound to the Cys145 residue. A comparison of screening power based on the docking scores of noncovalent ligands from the crystal structures with a "Directory of Useful Decoys, Enhanced" set of known decoys (6562 compounds) and ZINC15 in vivo subset (60,394 compounds) is provided. Ligand and protein input files are provided for future benchmarking purposes.

Preparation and characterization of a fluorogenic ddRFP-M biosensor as a specific SARS-CoV-2 main protease substrate

The conventional peptide substrates of SARS-CoV-2 main protease (Mpro) are frequently associated with high cost, unstable kinetics, and multistep synthesis. Hence, there is an urgent need to design affordable and stable Mpro substrates for pharmacological research. Herein, we designed a functional Mpro substrate based on a dimerization-dependent red fluorescent protein (ddRFP) for the evaluation of Mpro inhibitors in vitro. The codon-optimized DNA fragment encoding RFP-A1 domain, a polypeptide linker containing Mpro cleavage sequence (AVLQS), and the RFP-B1 domain was subcloned into the pET-28a vector. After transformation into Escherichia coli Rosetta(DE3) cells, the kanamycin resistant transformants were selected. Using a low temperature induction strategy, most of the target proteins (ddRFP-M) presented in the supernatant fractions were collected and purified by a HisTrapTM chelating column. Subsequently, the inhibition of Mpro by ensitrelvir and baicalein was assessed using ddRFP-M assay, and the biochemical properties of ddRFP-M substrate were analyzed. Our results showed that the fluorogenic substrate ddRFP-M was successfully prepared from E. coli cells, and this biosensor exhibited the expected specificity, sensitivity, and reliability. In conclusion, the production of the fluorogenic substrate ddRFP-M provides an expedient avenue for the assessment of Mpro inhibitors in vitro.

Synthesis and biological evaluation of novel peptidomimetic inhibitors of the coronavirus 3C-like protease

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and related variants, are responsible for the devastating coronavirus disease 2019 (COVID-19) pandemic. The SARS-CoV-2 main protease (Mpro) plays a central role in the replication of the virus and represents an attractive drug target. Herein, we report the discovery of novel SARS-CoV-2 Mpro covalent inhibitors, including highly effective compound NIP-22c which displays high potency against several key variants and clinically relevant nirmatrelvir Mpro E166V mutants.

Harnessing Brazilian biodiversity database: identification of flavonoids as potential inhibitors of SARS-CoV-2 main protease using computational approaches and all-atom molecular dynamics simulation.

SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus 2) is the etiological agent responsible for the global outbreak of COVID-19 (Coronavirus Disease 2019). The main protease of SARS-CoV-2, Mpro, is a key enzyme that plays a vital role in mediating viral replication and transcription. In this study, a comprehensive computational approach was employed to investigate the binding affinity, selectivity, and stability of natural product candidates as potential new antivirals acting on the viral polyprotein processing mediated by SARS-CoV-2 Mpro. A library of 288 flavonoids extracted from Brazilian biodiversity was screened to select potential Mpro inhibitors. An initial filter based on Lipinski's rule of five was applied, and 204 compounds that did not violate any of the Lipinski rules were selected. The compounds were then docked into the active site of Mpro using the GOLD program, and the poses were subsequently re-scored using MM-GBSA (Molecular Mechanics Generalized Born Surface Area) binding free energy calculations performed by AmberTools23. The top five flavonoids with the best MM-GBSA binding free energy values were selected for analysis of their interactions with the active site residues of the protein. Next, we conducted a toxicity and drug-likeness analysis, and non-toxic compounds were subjected to molecular dynamics simulation and free energy calculation using the MM-PBSA (Molecular Mechanics Poisson-Boltzmann Surface Area) method. It was observed that the five selected flavonoids had lower MM-GBSA binding free energy with Mpro than the co-crystal ligand. Furthermore, these compounds also formed hydrogen bonds with two important residues, Cys145 and Glu166, in the active site of Mpro. Two compounds that passed the drug-likeness filter showed stable conformations during the molecular dynamics simulations. Among these, NuBBE_867 exhibited the best MM-PBSA binding free energy value compared to the crystallographic inhibitor. Therefore, this study suggests that NuBBE_867 could be a potential inhibitor against the main protease of SARS-CoV-2 and may be further examined to confirm our results.

Ubiquitin Ligase Parkin Regulates the Stability of SARS-CoV-2 Main Protease and Suppresses Viral Replication.

The highly infectious coronavirus SARS-CoV-2 relies on the viral main protease (Mpro, also known as 3CLpro or Nsp5) to proteolytically process the polyproteins encoded by the viral genome for the release of functional units in the host cells to initiate viral replication. Mpro also interacts with host proteins of the innate immune pathways, such as IRF3 and STAT1, to suppress their activities and facilitate virus survival and proliferation. To identify the host mechanism for regulating Mpro, we screened various classes of E3 ubiquitin ligases and found that Parkin of the RING-between-RING family can induce the ubiquitination and degradation of Mpro in the cell. Furthermore, when the cells undergo mitophagy, the PINK1 kinase activates Parkin and enhances the ubiquitination of Mpro. We also found that elevated expression of Parkin in the cells significantly decreased the replication of SARS-CoV-2 virus. Interestingly, SARS-CoV-2 infection downregulates Parkin expression in the mouse lung tissues compared to healthy controls. These results suggest an antiviral role of Parkin as a ubiquitin ligase targeting Mpro and the potential for exploiting the virus-host interaction mediated by Parkin to treat SARS-CoV-2 infection.

In silico elucidation for the identification of potential phytochemical against ACE-II inhibitors.

The present study aims to investigate the therapeutic potential of phytocompounds derived from Annona reticulata leaves for the treatment of hypertension, utilizing computational methodologies. Gaining a comprehensive understanding of the molecular interactions between neophytadiene and γ-sitosterol holds significant importance in the advancement of innovative therapeutic approaches. This study aims to examine the inhibitory effects of neophytadiene and γ-sitosterol using molecular docking and dynamics simulations. Additionally, we will evaluate their stability and predict their drug-like properties as well as their ADME/toxicity profiles. Neophytadiene and γ-sitosterol have a substantial binding affinity with 1O8A, as shown by the docking study. The stability of the complexes was confirmed through molecular dynamics simulations, while distinct clusters were identified using PCA. These findings suggest the presence of potential stabilizers. The drug-likeness and ADME/toxicity predictions revealed positive characteristics, such as efficient absorption rates, limited distribution volume and non-hazardous profiles. The neophytadiene and γ-sitosterol exhibit potential as hypertension medication options. Computational investigations reveal that these compounds exhibit high affinity for binding, stability and favourable pharmacokinetic properties. The results of this study lay the groundwork for additional experimental verification and highlight the promising prospects of utilizing natural compounds in the field of pharmaceutical research. Target proteins (1O8A) were used to perform molecular docking with representative molecules. Stability, conformational changes and binding energies were assessed through molecular dynamics simulations lasting 100ns. Principal component analysis (PCA) was utilized to analyze molecular dynamics (MD) simulation data, to identify potential compounds that could stabilize the main protease. The safety and pharmacokinetic profiles of the compounds were evaluated through drug-likeness and ADME/toxicity predictions.

A Theoretical Study To Understand the Impact of Mpro of nCoV on the Hormones

AbstractAmidst the unprecedented 2019 coronavirus outbreak and the substantial loss of lives due to the scarcity of information about the virus and lack of viable drugs, understanding the impact of the nCoV Main Protease (Mpro) on the endocrine system has emerged as a critical topic to pursue. This study explores the potential interactions between four key hormones (cortisol, cortisone, estradiol, and estrone) and the Mpro of nCoV using various in‐silico methodologies. Density Functional Theory (DFT) calculations scrutinized the electron density delocalization of these hormones. Through molecular docking, the molecular‐level binding of Mpro of nCoV with these hormones was studied, revealing cortisol as the hormone exhibiting the most promising binding affinity (−100.05 kcal/mol). Furthermore, molecular dynamics (MD) simulations, emulating human body conditions at distinct temperatures, were employed to study the implications of these interactions. The study analysed the various trajectories such as root mean square deviation (RMSD), root mean square fluctuations (RMSF), radius of gyration (Rg), and H‐bond trajectories to understand the potential impact on biological systems. The findings demonstrate the binding potential of the Mpro of nCoV with these hormones, raising plausible concerns regarding consequential physiological changes within the human body.

Identification of Antibiotic Drugs against SARS-CoV2 Mpro: A Computational Approach for Drug Repurposing

The coronavirus pandemic has posed a significant challenge for researchers seeking to develop new compounds and repurpose existing drugs to manage this disease. It has been found that the Main protease enzyme (Mpro) is critical to the replication of the virus, making it an attractive target for drug development. Different antibiotics have been proven effective against different viruses, leading to their recommendation for COVID-19. In this study, virtual screening, pharmacokinetics, QSAR, and molecular docking techniques were used to investigate the best antibiotic drugs for COVID-19 by targeting the active and inactive conformations of the Mpro enzyme. The results of the study demonstrate that Praziquantel is a promising candidate for COVID-19 treatment. This is due to several reasons: First, Praziquantel exhibits better binding energy in both the conformations of Mpro. Second, it binds in S-3A site in native conformation and S-1B in active state. Third, Praziquantel has excellent absorption properties, strong blood-brain barrier penetration power, and reasonably good solubility. Therefore, the study nominates Praziquantel as the best option for future experimental and pre-clinical investigations for COVID-19.

Invention of MK-7845, a SARS-CoV-2 3CL Protease Inhibitor Employing a Novel Difluorinated Glutamine Mimic.

As SARS-CoV-2 continues to circulate, antiviral treatments are needed to complement vaccines. The virus's main protease, 3CLPro, is an attractive drug target in part because it recognizes a unique cleavage site, which features a glutamine residue at the P1 position and is not utilized by human proteases. Herein, we report the invention of MK-7845, a novel reversible covalent 3CLPro inhibitor. While most covalent inhibitors of SARS-CoV-2 3CLPro reported to date contain an amide as a Gln mimic at P1, MK-7845 bears a difluorobutyl substituent at this position. SAR analysis and X-ray crystallographic studies indicate that this group interacts with His163, the same residue that forms a hydrogen bond with the amide substituents typically found at P1. In addition to promising in vivo efficacy and an acceptable projected human dose with unboosted pharmacokinetics, MK-7845 exhibits favorable properties for both solubility and absorption that may be attributable to the unusual difluorobutyl substituent.

Correlating structure and activity of pepsin enzyme in H2O and D2O for the study of gastric digestion

D2O, an isotope of H2O, is commonly used as a solvent in neutron scattering; the large difference in scattering length density between H and D can provide better contrast between the sample and the solvent. However, this is of concern for studies using enzymes as the use of D2O can influence protein interactions (due to differences in hydrogen bonding) and is therefore expected to affect the function, activity and solubility of enzymes. Neutron-based in vitro digestion assays on proteins, including those found in food or as oral protein and peptide drugs, often involve different solvents or pH conditions where the activity of the digestive enzyme may not be optimal. Herein, we investigate the structure and activity of the main gastric protease, porcine pepsin, in both H2O and D2O at pH values in the range 1 – 8. We showed that the activity of pepsin was lower in D2O, although the relative change in activity with pH was similar for both solvents. We demonstrated using a combination of SAXS and CD that this relative change in activity was not related to any structural change within the protein but was, rather, linked to relative changes in solubility of the protein.