Protein dynamics of a b-sheet protein

Rhodnius prolixus Nitrophorin 4 (abbreviated NP4) is an almost pure b-sheet heme protein. Its dynamics is investigated by X-ray structure determination at eight different temperatures from 122 to 304K and by means of Moessbauer spectroscopy. A comparison of this b-sheet protein with the pure a-helical protein myoglobin (abbreviated Mbmet) is performed. The mean square displacement derived from the Moessbauer spectra increases linearly with temperature below a characteristic temperature T sub(c). It is about 10K larger than that of myoglobin. Above T sub(c) the mean square displacements increase dramatically. The Moessbauer 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 140ns. 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.