Dynamic Behavior of Oligomeric Inorganic Pyrophosphatase (IPPASE) Studied by Quasielastic Neutron Scattering

Protein dynamics or protein motions are believed to ultimately govern the biological function and activities of protein. Quasielastic neutron scattering (QENS) technique has been proven to be an exceptional tool to study dynamics of proteins in the time scale of picosecond (ps) to nanosecond (ns) [1, 2]. In this study, we use QENS to investigate how a large oligomeric protein, Inorganic Pyrophosphatase (IPPase) from Thermococcus thioreducens with quaternary structural complexity, have distinguishable dynamic characteristics compared to those of the small simple monomeric model protein, lysozyme. IPPase derived from thermostable microorganisms is of extreme interest for biophysical studies because of their inherent chemical and thermal stability and high temperature activity. Two QENS instruments, a backscattering spectrometer (BASIS) and a disk chopper spectrometer (DCS) are used in probing the protein dynamics in different time ranges from 1 ps to 1 ns at different temperatures. In addition, the DCS experiment was performed under the pressure of 1000 bar, mimicking the natural living conditions of IPPase. Our results reveal that the dynamics of IPPase is slower than that of lysozyme in the time range of 10 ps to 0.5 ns [1] while it is faster in the time range of 1 ps to 30 ps. These results are consistent between two instruments and such dynamic behaviors in proteins are believed to be contributed by the rotational motion of the side methyl groups [3]. Distinguishable dynamical behavior found between two proteins reveals local flexibility and conformational substates unique to oligomeric structures. Our results greatly help understanding the relation between protein dynamics and their biological functions.[1] X.-Q. Chu, et al, JPCB. 116, 9917 (2012).[2] X.-Q. Chu, et al, Soft Matter 6, 2623 (2010).[3] X.-Q. Chu, et al, JPCL. 4, 936 (2013).