Conformational changes are essential for the activity
of many proteins. If, or how fast, internal fluctuations
are related to slow conformational changes that mediate
protein function is not understood. In this study, we measure
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 α-helices are enhanced when the enzyme
transports substrate. This correlates with previously published
kinetic data from transport measurements showing that millisecond
conformational transitions of the substrate-loaded carrier
are faster than those in the absence of substrate. These
findings corroborate the hypothesis of the hierarchical
model of protein dynamics that predicts that slow conformational
transitions are based on fast, thermally activated internal
motions.