Effects of ligand binding on the internal dynamics of maltose-binding protein

Ligand binding to proteins often causes large conformational changes. A typical example is maltose-binding protein (MBP), a member of the family of periplasmic binding proteins of Gram-negative bacteria. Upon binding of maltose, MBP undergoes a large structural change that closes the binding cleft, i.e. the distance between its two domains decreases. In contrast, binding of the larger, nonphysiological ligand beta-cyclodextrin does not result in closure of the binding cleft. We have investigated the dynamic properties of MBP in its different states using time-resolved tryptophan fluorescence anisotropy. We found that the 'empty' protein exhibits strong internal fluctuations that almost vanish upon ligand binding. The measured relaxation times corresponding to internal fluctuations can be interpreted as originating from two types of motion: wobbling of tryptophan side-chains relative to the protein backbone, and orientational fluctuations of entire domains. After binding of a ligand, domain motions are no longer detectable and the fluctuations of some of the tryptophan side-chains become rather restricted. This transformation into a more rigid state is observed upon binding of both ligands, maltose and the larger beta-cyclodextrin. The fluctuations of tryptophan side-chains in direct contact with the ligand, however, are affected in a slightly different way by the two ligands.