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Computational Research in Molecular Chemistry
Gaussian Accelerated Molecular Dynamics in NAMDPang, Y.T., Y. Miao, Y Wang, J.A. McCammonJ. Chem. Theory Comput., 13 (1), pp9-19 (2017)    
Gaussian accelerated molecular dynamics (GaMD) is a recently developed enhanced sampling technique that provides efficient free energy calculations of biomolecules. Like the previous accelerated molecular dynamics (aMD), GaMD allows for "unconstrained" enhanced sampling without the need to set predefined collective variables and so is useful for studying complex biomolecular conformational changes such as protein folding and ligand binding. Furthermore, because the boost potential is constructed using a harmonic function that follows Gaussian distribution in GaMD, cumulant expansion to the second order can be applied to recover the original free energy profiles of proteins and other large biomolecules, which solves a long-standing energetic reweighting problem of the previous aMD method. Taken together, GaMD offers major advantages for both unconstrained enhanced sampling and free energy calculations of large biomolecules. Here, we have implemented GaMD in the NAMD package on top of the existing aMD feature and validated it on three model systems: alanine dipeptide, the chignolin fast-folding protein, and the M muscarinic G protein-coupled receptor (GPCR). For alanine dipeptide, while conventional molecular dynamics (cMD) simulations performed for 30 ns are poorly converged, GaMD simulations of the same length yield free energy profiles that agree quantitatively with those of 1000 ns cMD simulation. Further GaMD simulations have captured folding of the chignolin and binding of the acetylcholine (ACh) endogenous agonist to the M muscarinic receptor. The reweighted free energy profiles are used to characterize the protein folding and ligand binding pathways quantitatively. GaMD implemented in the scalable NAMD is widely applicable to enhanced sampling and free energy calculations of large biomolecules.
NMR Structure-Based Optimization of Staphylococcus aureus Sortase A Pyridazinone InhibitorsChan, A.H., S.W. Yi, E.M. Weiner, B.R. Amer, C.K. Sue, J. Wereszczynski, C.A. Dillen, S. Senese, J.Z. Torres, J.A.McCammon, L.S. Miller, M.E. Jung, R.T. ClubbChem. Biol. Drug Design in press (2017)    
Staphylococcus aureus is a leading cause of hospital-acquired infections in the USA and is a major health concern as methicillin-resistant S. aureus and other antibiotic-resistant strains are common. Compounds that inhibit the S. aureus sortase (SrtA) cysteine transpeptidase may function as potent anti-infective agents as this enzyme attaches virulence factors to the bacterial cell wall. While a variety of SrtA inhibitors have been discovered, the vast majority of these small molecules have not been optimized using structure-based approaches. Here we have used NMR spectroscopy to determine the molecular basis through which pyridazinone-based small molecules inhibit SrtA. These inhibitors covalently modify the active cysteine thiol and partially mimic the natural substrate of SrtA by inducing the closure of an active site loop. Computational and synthetic chemistry methods led to second-generation analogues that are ~70-fold more potent than the lead molecule. These optimized molecules exhibit broad-spectrum activity against other types of class A sortases, have reduced cytotoxicity, and impair SrtA-mediated protein display on S. aureus cell surface. Our work shows that pyridazinone analogues are attractive candidates for further development into anti-infective agents, and highlights the utility of employing NMR spectroscopy and solubility-optimized small molecules in structure-based drug discovery.
Spectroscopic and Computational Investigations of Ligand Binding to IspH: Discovery of Non-Diphosphate Inhibitors.O'Dowd, B., S. Williams, H. Wang, J. No, G. Rao, W. Wang, J.A. McCammon, S.P. Cramer, E. OldfieldChemBioChem. In press (2017)    
Isoprenoid biosynthesis is an important area for anti-infective drug development and one target is IspH, (E)-1-hydroxy-2-methyl-but-2-enyl 4-diphosphate (HMBPP) reductase, which forms isopentenyl diphosphate and dimethylallyl diphosphate from HMBPP in a 2H+/2e- reduction. IspH contains a 4Fe-4S cluster and here, we first investigated how small molecules can bind to the cluster using HYSCORE and NRVS spectroscopies. The results of these as well as other structural and spectroscopic investigations led to the conclusion that in most cases, ligands bind to IspH 4Fe-4S clusters via h1 coordination, forming tetrahedral geometries at the unique 4th Fe, ligand side-chains preventing further ligand (e.g. H2O, O2) binding. Based on these ideas, we sought using in silico methods to find drug-like inhibitors that might occupy the HMBPP substrate binding pocket and bind to Fe, leading to the discovery of a barbituric acid analog having a Ki ~ 500 nM against Pseudomonas aeruginosa IspH.
Activation Mechanisms of the Sphingosine-1-Phosphate ReceptorCaliman, A.D., Y. Miao, J.A. McCammonProtein Sci. In press (2017)    
Activation of the first sphingosine-1-phosphate receptor (S1PR1) promotes permeability of the blood brain barrier, astrocyte and neuronal protection, and lymphocyte egress from secondary lymphoid tissues. Although an agonist often activates the S1PR1, the receptor exhibits high levels of basal activity. In this study, we performed long-timescale molecular dynamics and accelerated molecular dynamics (aMD) simulations to investigate activation mechanisms of the ligand-free (apo) S1PR1. In the aMD enhanced sampling simulations, we observed four independent events of activation, which is characterized by close interaction between Y3117.53 and Y2215.58 and increased distance between the intracellular ends of transmembrane (TM) helices 3 and 6. Although transmembrane helices TM3, TM6, TM5 and, TM7 are associated with GPCR activation, we discovered that their movements are not necessarily correlated during activation. Instead, TM5 showed a decreased correlation with each of these regions during activation. During activation of the apo receptor, Y2215.58 and Y3117.53 became more solvated, because a water channel formed in the intracellular pocket. Additionally, a lipid molecule repeatedly entered the receptor between the extracellular ends of TM1 and TM7, providing important insights into the pathway of ligand entry into the S1PR1. This article is protected by copyright. All rights reserved.
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