Calorimetric and fluorescence studies of protein propert

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ClpA is an ATP dependent chaperone that forms a hexameric ring that alone has an unfoldase activity in vitro. Two of these rings flank ClpP tetradecamer forming ClpAP protease. In this complex ClpA assists the proteolytic core by binding, unfolding and translocating substrates to the proteolytic chamber. Thermodynamic parameters of ClpA-peptide interactions have been determined by isothermal titration calorimetry (ITC) using a known 11-amino acid substrate ANDENYALAA (SsrA) and 3betaSsrA. Dansylated SsrA (DNS-SsrA) has been used in fluorescence titrations to obtain the association constant and binding stoichiometry by an independent method. The DNS-SsrA has been found also to be competitively displaced by larger substrates of ClpA such as RepA or 3betaSsrA but not by the nonbinding peptide AANDENYALDD (DD-SsrA). Competition studies with DNS-SsrA has been used to locate specific binding sequences of known ClpA substrates. The Glucose/Galactose Binding Protein (GGBP) from Escherichia coli is a periplasmic protein that serves as a high-affinity receptor for the active transport and hemotaxis towards both sugar ligands. Tight binding of sugars by the binding site located in the cleft between the two domains of the GGBP is achieved by the formation of hydrogen bonds with the ligand. Glucose and galactose binding induces a hinge motion between the two domains of the GGBP protein. GGBP become an important model for reagentless glucose sensor development due to the opportunity to measure sugar concentration by monitoring induced protein conformational change. We have investigated the effect of glucose binding on the thermal stability of the GGBP protein from E. coli by differential scanning calorimetry (DSC), circular dichroism (CD) and intrinsic Trp fluorescence. All three techniques reveal strong stabilizing effects of 10 mM D-glucose on the unfolding of this two-domain protein.