@article {2018|2059,
	title = {A Streamlined, General Approach for Computing Ligand Binding Free Energies and Its Application to GPCR-Bound Cholesterol.},
	journal = {Journal of Chemical Theory and Computation},
	volume = {14},
	year = {2018},
	pages = {6560{\textendash}6573},
	abstract = {<p>The theory of receptor-ligand binding equilibria has long been well-established in biochemistry, and was primarily constructed to describe dilute aqueous solutions. Accordingly, few computational approaches have been developed for making quantitative predictions of binding probabilities in environments other than dilute isotropic solution. Existing techniques, ranging from simple automated docking procedures to sophisticated thermodynamics-based methods, have been developed with soluble proteins in mind. Biologically and pharmacologically relevant protein-ligand interactions often occur in complex environments, including lamellar phases like membranes and crowded, nondilute solutions. Here, we revisit the theoretical bases of ligand binding equilibria, avoiding overly specific assumptions that are nearly always made when describing receptor-ligand binding. Building on this formalism, we extend the asymptotically exact Alchemical Free Energy Perturbation technique to quantifying occupancies of sites on proteins in a complex bulk, including phase-separated, anisotropic, or nondilute solutions, using a thermodynamically consistent and easily generalized approach that resolves several ambiguities of current frameworks. To incorporate the complex bulk without overcomplicating the overall thermodynamic cycle, we simplify the common approach for ligand restraints by using a single distance-from-bound-configuration (DBC) ligand restraint during AFEP decoupling from protein. DBC restraints should be generalizable to binding modes of most small molecules, even those with strong orientational dependence. We apply this approach to compute the likelihood that membrane cholesterol binds to known crystallographic sites on three GPCRs (β -adrenergic, 5HT-2B, and μ-opioid) at a range of concentrations. Nonideality of cholesterol in a binary cholesterol:phosphatidylcholine (POPC) bilayer is characterized and consistently incorporated into the interpretation. We find that the three sites exhibit very different affinities for cholesterol: The site on the adrenergic receptor is predicted to be high affinity, with 50\% occupancy for 1:10 CHOL:POPC mixtures. The sites on the 5HT-2B and μ-opioid receptor are predicted to be lower affinity, with 50\% occupancy for 1:10 CHOL:POPC and 1:10 CHOL:POPC, respectively. These results could not have been predicted from the crystal structures alone.</p>
},
	issn = {1549-9626},
	doi = {10.1021/acs.jctc.8b00447},
	author = {Salari, Reza and Joseph, Thomas and Lohia, Ruchi and J{\'e}r{\^o}me H{\'e}nin and Brannigan, Grace}
}
@article {2016|1672,
	title = {A Novel Bifunctional Alkylphenol Anesthetic Allows Characterization of gamma-Aminobutyric Acid, Type A (GABAA), Receptor Subunit Binding Selectivity in Synaptosomes.},
	journal = {J. Biol. Chem},
	volume = {291},
	year = {2016},
	month = {sep},
	pages = {20473{\textendash}86},
	abstract = {<p>Propofol, an intravenous anesthetic, is a positive modulator of the GABAA receptor, but the mechanistic details, including the relevant binding sites and alternative targets, remain disputed. Here we undertook an in-depth study of alkylphenol-based anesthetic binding to synaptic membranes. We designed, synthesized, and characterized a chemically active alkylphenol anesthetic (2-((prop-2-yn-1-yloxy)methyl)-5-(3-(trifluoromethyl)-3H-diazirin-3-yl)phenol, AziPm-click (1)), for affinity-based protein profiling (ABPP) of propofol-binding proteins in their native state within mouse synaptosomes. The ABPP strategy captured approximately 4\% of the synaptosomal proteome, including the unbiased capture of five alpha or beta GABAA receptor subunits. Lack of gamma2 subunit capture was not due to low abundance. Consistent with this, independent molecular dynamics simulations with alchemical free energy perturbation calculations predicted selective propofol binding to interfacial sites, with higher affinities for alpha/beta than gamma-containing interfaces. The simulations indicated hydrogen bonding is a key component leading to propofol-selective binding within GABAA receptor subunit interfaces, with stable hydrogen bonds observed between propofol and alpha/beta cavity residues but not gamma cavity residues. We confirmed this by introducing a hydrogen bond-null propofol analogue as a protecting ligand for targeted-ABPP and observed a lack of GABAA receptor subunit protection. This investigation demonstrates striking interfacial GABAA receptor subunit selectivity in the native milieu, suggesting that asymmetric occupancy of heteropentameric ion channels by alkylphenol-based anesthetics is sufficient to induce modulation of activity.</p>},
	keywords = {anesthesia, anesthetic, click chemistry, GABA receptor, photoaffinity labeling},
	doi = {10.1074/jbc.M116.736975},
	author = {Woll, Kellie A. and Murlidaran, Sruthi and Pinch, Benika J. and J{\'e}r{\^o}me H{\'e}nin and Wang, Xiaoshi and Salari, Reza and Covarrubias, Manuel and Dailey, William P. and Grace Brannigan and Garcia, Benjamin A. and Roderic G Eckenhoff}
}
@article {2014|1598,
	title = {A predicted binding site for cholesterol on the GABAA receptor.},
	journal = {Biophys. J.},
	volume = {106},
	number = {9},
	year = {2014},
	month = {may},
	pages = {1938{\textendash}1949},
	publisher = {Department of Physics, Rutgers University-Camden, Camden, New Jersey; Center for Computational and Integrative Biology, Rutgers University-Camden, Camden, New Jersey. Electronic address: Grace.Brannigan@rutgers.edu.},
	abstract = {Modulation of the GABA type A receptor (GABAAR) function by cholesterol and other steroids is documented at the functional level, yet its structural basis is largely unknown. Current data on structurally related modulators suggest that cholesterol binds to subunit interfaces between transmembrane domains of the GABAAR. We construct homology models of a human GABAAR based on the structure of the glutamate-gated chloride channel GluCl of Caenorhabditis elegans. The models show the possibility of previously unreported disulfide bridges linking the M1 and M3 transmembrane helices in the α and γ subunits. We discuss the biological relevance of such disulfide bridges. Using our models, we investigate cholesterol binding to intersubunit cavities of the GABAAR transmembrane domain. We find that very similar binding modes are predicted independently by three approaches: analogy with ivermectin in the GluCl crystal structure, automated docking by AutoDock, and spontaneous rebinding events in unbiased molecular dynamics simulations. Taken together, the models and atomistic simulations suggest a somewhat flexible binding mode, with several possible orientations. Finally, we explore the possibility that cholesterol promotes pore opening through a wedge mechanism.},
	keywords = {Amino Acid, Binding Sites, Caenorhabditis elegans Proteins, chemistry, chemistry/metabolism, Chloride Channels, Cholesterol, GABA-A, Humans, Hydrogen Bonding, Ivermectin, metabolism, Molecular Docking Simulation, Molecular Dynamics Simulation, Porosity, Protein Binding, Protein Conformation, Receptors, Sequence Homology, Substrate Specificity},
	doi = {10.1016/j.bpj.2014.03.024},
	author = {J{\'e}r{\^o}me H{\'e}nin and Salari, Reza and Murlidaran, Sruthi and Grace Brannigan}
}
