Enhanced internal dynamics of a membrane transport protein during substrate translocation

Conformational changes are essential for the activity of many proteins. If, or how fast, internal fluctuations are related to slow conformational chan ges that mediate protein function is not understood. In this study, we meas ure internal fluctuations of the transport protein lactose permease in the presence and absence of substrate by tryptophan fluorescence spectroscopy. We demonstrate that nanosecond fluctuations of alpha -helices are enhanced when the enzyme transports substrate. This correlates with previously publi shed kinetic data from transport measurements showing that millisecond conf ormational transitions of the substrate-loaded carrier are faster than thos e in the absence of substrate. These findings corroborate the hypothesis of the hierarchical model of protein dynamics that predicts that slow conform ational transitions are based on fast, thermally activated internal motions .