DYNAMICS OF DIFFERENT FUNCTIONAL PARTS OF BACTERIORHODOPSIN - H-H-2 LABELING AND NEUTRON-SCATTERING

We show that dynamics of specific amino acids within a protein can be characterized by neutron spectroscopy and hydrogen-deuterium labeling, and we present data on the motions of a selected set of groups within bacteriorhodopsin (BR), the retinal-based proton pump in the purple m embrane of halophilic Archaea, Elastic incoherent neutron scattering e xperiments allow the definition of motions in the nano-to picosecond t ime scale and have revealed a dynamical transition from a harmonic to a softer, anharmonic atomic fluctuation regime in the global behavior of proteins. Biological activity in proteins is correlated with this t ransition, suggesting that flexibility is required for function. Elast ic incoherent neutron scattering is dominated by H atom scattering, an d to study the dynamics of a selected part of BR, fully deuterated pur ple membrane with BR containing H-retinal, H-tryptophan, and H-methion ine was prepared biosynthetically in Halobacterium salinarum. These am ino acids cluster in the functional center of the protein. In contrast to the protein globally, the thermal motions of the labeled atoms wer e found to be shielded from solvent melting effects at 260 K. Above th is temperature, the labeled groups appear as more rigid than the rest of the protein, with a significantly smaller mean square amplitude of motion, These experimental results quantify the dynamical heterogeneit y of BR (which meets the functional requirements of global flexibility ), on the one hand, to allow large conformational changes in the molec ule and of a more rigid region in the protein, on the other, to contro l stereo-specific selection of retinal conformations.