Two modes of ligand binding in maltose-binding protein of Escherichia coli. Correlation with the structure of ligands and the structure of binding protein.
Ligands that are transported by the maltose transport system of Escherichia coli must first bind to the periplasmic maltose-binding protein (MBP). However, binding of a ligand does not always lead to its transport. As reported earlier, reduced or oxidized maltodextrins bind tightly to MBP but are not transported; some mutant MBPs, such as MalE254, bind maltodextrins tightly but cannot produce their transport. In this study, UV differential spectroscopy and fluorescence emission spectroscopy were used to study the modes by which various ligands bind to MBP. Maltose binding produced a red shift in the fluorescence emission spectrum of wild type MBP and a sharp hypochromatic trend below 265 nm in its UV spectrum (R mode (for red)). On the other hand, binding of reduced, oxidized, or cyclic maltodextrins produced a pronounced blue shift in the fluorescence emission spectrum of wild type MBP and a peak at about 250 nm in its UV difference spectrum (B mode (for blue). Binding of reducing maltodextrins to wild type MBP produced spectral changes that seemed to be a mixture of predominantly R mode binding and some B mode binding, whereas their binding to mutant MBP MalE254 produced changes indicative of pure B mode binding. Thus, the ligands that are bound exclusively via the B mode to either the wild type or MalE254 MBP are not transported.