| A study of protein-carbohydrate interactions in the binding of group A Streptococcus (GAS) MBP and maltodextrins is presented. First, we determined that GAS MBP binds linear maltodextrins up to at least seven glucose molecules long. The thermodynamics and fluorescence changes induced by GAS MBP-maltodextrin binding were essentially opposite those reported for E. coli MBP. Moreover, unlike E. coli MBP, GAS MBP exhibited no specific binding of maltose or cyclic maltodextrins. To understand the structural basis of the binding thermodynamics, we determined the high-resolution structures of complexes of GAS MBP with maltotriose/maltotetraose using protein X-ray crystallography. Detailed structural analyses revealed that GAS MBP binds maltotriose and maltotetraose in a closed form like E. coli MBP, but with different sub binding sites other than those of E. coli MBP. There are numerous hydrogen bonds and hydrophobic interactions between GAS MBP and maltodextrins, which is consistent with the thermodynamics study.;Furthermore, we studied two binding modes between maltotriitol, a reducing maltodextrin, and a chimera of E. coli MBP with the green fluorescence protein (GFP) in vitro. Fluorescence emissions from the chimera indicate that maltotriitol binding induces the two domains of the maltose-binding protein to close at high, but not low, temperatures. Based on a simple thermodynamic argument, the temperature-dependence of the maltotriitol-induced conformational change allowed us to conclude that the energy cost to close these domains is about 6 kcal/mole. |
