Dynamics and Binding Affinity of Spin-Labeled Stearic Acids in beta-Lactoglobulin: Evidences from EPR Spectroscopy and Molecular Dynamics Simulation

β-Lactoglobulin (β-LG) is a member of the lipocalinprotein family involved in the transport of fatty acids and other smallhydrophobic molecules. The main binding site is at a central cavity,referred to as “calyx”, formed by the protein β-barrel sandwich.Continuous-wave and pulsed Fourier transform electron spinresonance (cw- and FT-EPR) spectroscopy and molecular dynamics(MD) simulation were combined to investigate the interaction offatty acids with bovine β-LG. Stearic acid bearing the nitroxide labelat different positions, n, along the acyl chain (n-SASL, n = 5, 7, 10,12, 16) were used. The EPR data show that the protein affinity for SASL decreases on going from n = 5 to 16. This behavior isdue to the accommodation of the SASL in the protein calyx, which is hampered by steric hindrance of the doxyl ring for n ≥ 10,as evidenced by MD data. Conformation and dynamics of 5-SASL are similar to those of the unlabeled stearate molecule. 5-SASLin the protein binding site undergoes librational motion of small amplitude on the nanosecond time scale at cryogenictemperature and rotational dynamics with correlation time of 4.2 ns at physiological temperature. The results highlight thedynamical features of fatty acids/β-LG interaction.