Binding of Hydroxyquinoline Probes to Human Serum Albumin: Combining Molecular Modeling and Forster's Resonance Energy Transfer Spectroscopy to Understand Flexible Ligand Binding

Human serum albumin (HSA) is the most abundant protein in blood plasma. It has high relevance for the lipid metabolism, and its ability to bind a large variety of natural and pharmaceutical compounds makes it a crucial determinant of drug pharmaco-kinetics and -dynamics. The drug binding properties of HSA can be characterized by spectroscopic analysis of bound probe molecules. We have recently characterized the subdomain IIA binding site of HSA using three hydroxyquinoline derivatives. In this work, we extend our study by combining data from energy transfer experiments, ligand docking, and long molecular dynamics (MD) simulations. Multiple possible binding locations are found within the subdomain IIA site, and their solvent accessibility and interactions with ligands are analyzed in detail. Binding pockets appear well hydrated during simulations, with ligands in direct contact to water molecules at all times. Binding free energies in good agreement to experiment are calculated. The HSA apoprotein is found to exhibit significant conformational flexibility over 250 ns of simulation time, but individual domains remain structurally stable. Two rotamers of Trp214 were observed on a time scale longer than 50 ns in the MD simulations, supporting the experimental observation of two fluorescence lifetime components. The flexible protein structure and heterogeneous nature of its binding sites explain the ability of HSA to act as a versatile molecular transporter. The combination of experimental and computational molecular distance information allows the conclusion that hydroxyquinoline probes bind in a binding mode similar to the anticoagulant drug warfarin.