The Molten Globule State of Maltose-Binding Protein: Structural and Thermodynamic Characterization by EPR Spectroscopy and Isothermal Titration Calorimetry

Employing site-directed spin labeling (SDSL), the structure of maltose-binding protein (MBP) had previously been studied in the native state by electron paramagnetic resonance (EPR) spectroscopy. Several spin-labeled double cysteine mutants were distributed all over the structure of this cysteine-free protein and revealed distance information between the nitroxide residues from double electron–electron resonance (DEER). The results were in good agreement with the known X-ray structure. We have now extended these studies to the molten globule (MG) state, a folding intermediate, which can be stabilized around pH 3 and that is characterized by secondary but hardly any tertiary structure. Instead of clearly defined distance features as found in the native state, several additional characteristics indicate that the MG structure of MBP contains different polypeptide chain and domain orientations. MBP is also known to bind its substrate maltose even in MG state although with lower affinity. Additionally, we have now created new mutants allowing for spin labeling at or near the active site. Our data confirm an already preformed ligand site structure in the MG explaining its substrate binding capability and thus most probably serving as a nucleation center for the final native structure.