Binding Constants of Clinical Drugs and Other Organic Ligands with Human and Mammalian Serum Albumins

A dataset containing the experimental values of the equilibrium binding constants of clinical drugs, and some other organic ligands with human and mammalian (predominantly bovine) serum albumins, is assembled. The affinity of drugs to albumin governs their pharmacokinetic properties, related to permeability through physiological barriers and distribution within the organism. The dataset contains 1755 records gathered from 346 original literature sources describing the albumin affinity of 324 different substances. The data were extracted from both articles and existing protein-binding databases applied strict data selection rules in order to exclude the values influenced by the third-party compounds. The dataset provides the details on the experimental conditions of the measurements, such as temperature; protein and ligand concentrations; buffer pH, composition and concentration; and the method and model used for the binding constant calculations. Analysis of the data reveals discrepancies between the values from different studies, as well as the significant influence of the measurement method. Averaging the values from multiple independent measurements from the dataset may help to determine the reliable values of the binding constants. The dataset can be used as the reference dataset for the development of predictive models to calculate binding constants, and as the choice for the experimental setup in the future albumin-binding studies.

Mapping of the fibrinogen-binding site on the staphylocoagulase C-terminal repeat region

The N-terminus of S. aureus staphylocoagulase (SC) triggers activation of host prothrombin (ProT), and the SCProT* complex cleaves host fibrinogen (Fbg) to form fibrin (Fbn) deposits, a hallmark of SC-positive endocarditis. The C-terminal domain of the prototypical Newman D2 Tager 104 SC contains 1 pseudo-repeat (PR) and 7 repeats (R1R7) that bind Fbg/Fbn Fragment D (Frag D). This work defines affinities and stoichiometries of Frag D binding to single- and multi-repeat C-terminal constructs, using fluorescence equilibrium binding, NMR titration, Ala scanning, and native PAGE. Constructs containing PR and each single repeat bound Frag D with KD ~50 130 nM and a 1:1 stoichiometry, indicating a conserved binding site shared between PR and each repeat. NMR titration of PR-R7 with Frag D revealed that residues 22-49, bridging PR and R7, constituted the minimal peptide (MP) required for binding, corroborated by Ala scanning, and binding of labeled MP to Frag D. MP alignment with the PR-repeat and inter-repeat junctions identified conserved residues critical for binding. Labeled PR-(R1R7) bound Frag D with KD ~7 32 nM and stoichiometry of 1:5; and PR-R1R2R3, PR-R1R6R7, PR-R3R4R7, and PR-R3R6R7 competed with PR-(R1R7) for Frag D binding, with a 1:3 stoichiometry and KD ~7 42 nM. These findings are consistent with binding at the PR-R junctions with modest inter-repeat sequence variability. Circular dichroism of PR-R7 and PR-(R1R7) suggested a largely disordered structure and conformational flexibility, allowing binding of multiple fibrin(ogen) molecules. This property facilitates pathogen localization on host fibrin networks.

Kinetic mechanism of Na+-coupled aspartate transport catalyzed by GltTk

It is well-established that the secondary active transporters GltTk and GltPh catalyze coupled uptake of aspartate and three sodium ions, but insight in the kinetic mechanism of transport is fragmentary. Here, we systematically measured aspartate uptake rates in proteoliposomes containing purified GltTk, and derived the rate equation for a mechanism in which two sodium ions bind before and another after aspartate. Re-analysis of existing data on GltPh using this equation allowed for determination of the turnover number (0.14 s−1), without the need for error-prone protein quantification. To overcome the complication that purified transporters may adopt right-side-out or inside-out membrane orientations upon reconstitution, thereby confounding the kinetic analysis, we employed a rapid method using synthetic nanobodies to inactivate one population. Oppositely oriented GltTk proteins showed the same transport kinetics, consistent with the use of an identical gating element on both sides of the membrane. Our work underlines the value of bona fide transport experiments to reveal mechanistic features of Na+-aspartate symport that cannot be observed in detergent solution. Combined with previous pre-equilibrium binding studies, a full kinetic mechanism of structurally characterized aspartate transporters of the SLC1A family is now emerging.

Efficacy of epetraborole against Mycobacterium abscessus is increased with norvaline

Certain aminoacyl-tRNA synthetases developed a proofreading mechanism to ensure aminoacylation of tRNAs with cognate amino acids. Epetraborole (EPT) was identified as an inhibitor of the leucyl-tRNA synthetase (LeuRS) editing site in Mycobacterium abscessus. EPT displayed enhanced activity against M. abscessus over Mycobacterium tuberculosis. Crystallographic and equilibrium binding data showed that EPT binds LeuRSMabs and LeuRSMtb with similar Kd. Proteomic analysis revealed that when M. abscessus LeuRS mutants were fed the non-proteinogenic amino acid norvaline, leucine residues in proteins were replaced by norvaline, inducing expression of GroEL chaperonins and Clp proteases. In vitro data revealed that supplementation of media with norvaline reduced the emergence of EPT mutants in both M. abscessus and M. tuberculosis. The combination of EPT and norvaline had improved in vivo efficacy compared to EPT in a murine model of M. abscessus infection.

Binding of thermalized and active membrane curvature-inducing proteins

Using analytical and numerical approaches, we find that equilibrium binding of membrane curving proteins on a membrane generates a phase-separated and corrugated phase. Active binding shifts its stability and makes the protein aggregates porous.

PyBindingCurve, simulation and curve fitting to complex binding systems at equilibrium

Understanding multicomponent binding interactions in protein-ligand, protein-protein and competition systems is essential for fundamental biology and drug discovery. Hand deriving equations quickly becomes unfeasible when the number of components is increased, and direct analytical solutions only exist to a certain complexity. To address this problem and allow easy access to simulation, plotting and parameter fitting to complex systems at equilibrium, we present the Python package PyBindingCurve. We apply this software to explore homodimer and heterodimer formation culminating in the discovery that under certain conditions, homodimers are easier to break with an inhibitor than heterodimers and may also be more readily depleted. This is a potentially valuable and overlooked phenomenon of great importance to drug discovery. PyBindingCurve may be expanded to operate on any equilibrium binding system and allows definition of custom systems using a simple syntax. PyBindingCurve is available under the MIT license at: https://github.com/stevenshave/pybindingcurve as Python source code accompanied by examples and as an easily installable package within the Python Package Index.

Targeting the Interaction between the SH3 Domain of Grb2 and Gab2

Gab2 is a scaffolding protein, overexpressed in many types of cancers, that plays a key role in the formation of signaling complexes involved in cellular proliferation, migration, and differentiation. The interaction between Gab2 and the C-terminal SH3 domain of the protein Grb2 is crucial for the activation of the proliferation-signaling pathway Ras/Erk, thus representing a potential pharmacological target. In this study, we identified, by virtual screening, seven potential inhibitor molecules that were experimentally tested through kinetic and equilibrium binding experiments. One compound showed a remarkable effect in lowering the affinity of the C-SH3 domain for Gab2. This inhibitory effect was subsequently validated in cellula by using lung cancer cell lines A549 and H1299. Our results are discussed under the light of previous works on the C-SH3:Gab2 interaction.

Novel fluorescent GPCR biosensor detects retinal equilibrium binding to opsin and active G protein and arrestin signaling conformations

Rhodopsin is a canonical class A photosensitive G protein–coupled receptor (GPCR), yet relatively few pharmaceutical agents targeting this visual receptor have been identified, in part due to the unique characteristics of its light-sensitive, covalently bound retinal ligands. Rhodopsin becomes activated when light isomerizes 11-cis-retinal into an agonist, all-trans-retinal (ATR), which enables the receptor to activate its G protein. We have previously demonstrated that, despite being covalently bound, ATR can display properties of equilibrium binding, yet how this is accomplished is unknown. Here, we describe a new approach for both identifying compounds that can activate and attenuate rhodopsin and testing the hypothesis that opsin binds retinal in equilibrium. Our method uses opsin-based fluorescent sensors, which directly report the formation of active receptor conformations by detecting the binding of G protein or arrestin fragments that have been fused onto the receptor's C terminus. We show that these biosensors can be used to monitor equilibrium binding of the agonist, ATR, as well as the noncovalent binding of β-ionone, an antagonist for G protein activation. Finally, we use these novel biosensors to observe ATR release from an activated, unlabeled receptor and its subsequent transfer to the sensor in real time. Taken together, these data support the retinal equilibrium binding hypothesis. The approach we describe should prove directly translatable to other GPCRs, providing a new tool for ligand discovery and mutant characterization.