Open-to-closed transition in apo maltose-binding protein observed by paramagnetic NMR

It is well known that large-scale rearrangements of domains of a protein can play an important role in ligand binding and recognition, catalysis, and regulation. Though X-ray crystal structures can provide a static picture of the apo (usually open) and holo (usually closed) states of a protein, it is not usually clear whether the apo state exists as a single species where the closed state is energetically inaccessible and interdomain rearrangement is induced by ligand or substrate binding or whether the predominantly open form already co-exists in rapid equilibrium with a minor closed species. In this paper, the authors obtained paramagnetic relaxation enhancement data that indicate that there is a rapidly exchanging mixture of a predominantly open form of the maltose-binding protein and a minor (partially-closed) form of the protein. Ensemble simulated annealing refinement was used to determine an ensemble average structure of the minor apo species and demonstrate that it is distinct from the sugar-bound state.