Measurement of Conformational Changes in the Structure of Transglutaminase on Binding Calcium Ions Using Optical Evanescent Dual Polarisation Interferometry

The conformational changes occurring when the protein transglutaminase binds calcium ions have been studied using the optical evanescent technique of dual polarization interferometry (DPI) implemented via a dual slab waveguide structure. Immobilized transglutaminase layers of 4-5 nm in thickness were obtained, which when challenged with calcium ions underwent a contraction of approximately 0.5 nm (depending on the concentration of calcium) and an increase in refractive index of approximately 1 x 10-2. The affinity constant for the calcium binding was found to be in the range of 0.95 +/- 0.2 mM. The results reported are in good agreement with those found in the literature obtained by other techniques. It has also been shown that the structural changes occurring during the binding event are considerably larger than the mass changes that take place; thus, DPI offers a potentially valuable method to study real-time structural changes occurring to proteins when they bind metal ions.