QCM-D Monitoring of Binding-Induced Conformational Change of Calmodulin

Understanding conformational changes are important when studying a protein such as calmodulin (CaM), which activates various target enzymes and regulates numerous physiological functions. CaM is a highly flexible protein that can transitorily adopt various conformations. A quartz crystal microbalance with dissipation (QCM-D) sensor was used to study binding-induced conformational changes of surface-immobilized CaM. Structural changes of CaM were evaluated using the Voigt’s viscoelastic model with frequency (ΔF) and dissipation change (ΔD). When Apo-CaM layer was incubated in 0.1 mM Ca2+ solution, the layer decreased by approximately 0.56 nm, due to the release of coupled water molecules and conformational change. The application of CaM itself also caused a significantly more compact layer, supporting previous findings that CaM dimerization forms a collapsed structure that exposes a hydrophobic tunnel. The binding characteristics of CaM with peptides derived from proteins in a signal transduction pathway also demonstrated diverse biophysical properties of the CaM complexes. Each peptide showed a unique ΔF/ΔD pattern indicating versatility of CaM configuration to favorably adjust to each target molecule. The study demonstrates that the QCM-D sensor is capable of simultaneously studying binding affinity and plasticity of protein configuration for target binding. The CaM data obtained on hydrated protein layer thickness is complementary to configuration measurements of a single CaM molecule. QCM-D Monitoring of Binding-Induced Conformational Change of Calmodulin