Protein dynamics of a beta-sheet protein

Rhodnius prolixus Nitrophorin 4 (abbreviated NP4) is an almost pure beta-sheet heme protein. Its dynamics is investigated by X-ray structure determination at eight different temperatures from 122 to 304 K and by means of Mössbauer spectroscopy. A comparison of this beta-sheet protein with the pure alpha-helical protein myoglobin (abbreviated Mbmet) is performed. The mean square displacement derived from the Mössbauer spectra increases linearly with temperature below a characteristic temperature T(c). It is about 10 K larger than that of myoglobin. Above T(c) the mean square displacements increase dramatically. The Mössbauer spectra are analyzed by a two state model. The increased mean square displacements are caused by very slow motions occurring on a time scale faster than 140 ns. With respect to these motions NP4 shows the same protein specific modes as Mbmet. There is, however, a difference in the fast vibration regime. The B values found in the X-ray structures vary linearly over the entire temperature range. The mean square displacements in NP4 increase with slopes which are 60% larger than those observed for Mbmet. This indicates that nitrophorin has a larger structural distribution which makes it more flexible than myoglobin.