Calorimetric and dielectrpspectroscopic studies of energy-transfer mechanism of within a protein molecule

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The I-type of glucosyltransferase of Streptococcus sobrinus was used to study the of energy-transfer mechanism of within a protein molecule. Protein samples of whole GTF molecule(GTF), its catalytic domain (GS) and glucanbinding domain (DXB) were subjected to differential scanning calorimetry and dielectric and CD spectroscopies. DSC results indicated the enthalpy change of thermal denaturation of GTF{ΔH(GTF)} <ΔH(GS)+ ΔH(DXB). In addition, the T_m (mid-temperature of thermal denaturatin) of GS fused with DXB (within GTF) was lower than that of un-fused GS, whereas T_m of DXB fused with GS (within GTF) was higher than that of un-fused DXB. These results indicate that fusion of GS and DXB may destabilize the former and stabilize the latter. However, CD-spectroscopy indicated that the fusion did not affect the secondary and higher-order structures of either domains. In accordance with this result, dielectroscopy did not show any significant hydration change accompanying the domain fusion.