Long-Range Correlated Motion Changes with Protein-Ligand Binding

Molecular dynamics calculations have long predicted large scale protein structural vibrations lie in the terahertz (THz) frequency range (5-100 cm−1), and these vibrations are related to protein function. Measuring these vibrational modes has been challenged by the large glass-like contribution from solvent and side chain librational motions. We remove this isotropic background, using anisotropic THz near field microscopy measurements of protein crystals. The technique reveals for the first time narrow band protein excitations in this frequency range. To determine if these features arise from the internal molecular motions, we measured ligand binding dependence using a faster data acquisition technique. The measurements performed on tetragonal chicken-egg white lysozyme (CEWL) single crystals and tetragonal CEWL tri-N-acetylglucosamine inhibitor bound crystals (CEWL+3NAG) show reproducible spectra that change dramatically with inhibitor binding. The large shifts observed indicate the features arise from the protein intramolecular motions and not from crystal phonons, which would have frequency shifts of only ≤ 2% with binding. The results validate that the technique can be used to determine ligand binding for inhibitor screening and to understand the role of intramolecular motions in protein function. This work supported by NSF MRI∧2 grant DBI295998.View Large Image | View Hi-Res Image | Download PowerPoint Slide