Determination and Evaluation of Secondary Structure Content Derived from Calcium-Induced Conformational Changes in Wild-Type and Mutant Mnemiopsin 2 by Synchrotron-Based Fourier-transform Infrared Spectroscopy

Mnemiopsin 2 from a luminous ctenophore with two functional EF-hand motifs is a calcium-regulated photoprotein that is responsible for emitting a bright blue bioluminescence upon reacting with coelenterazine and calcium ions in Mnemiopsis leidyi. Synchrotron radiation-based Fourier-transform infrared (SR-FTIR) spectroscopy was applied to analyze the distribution of secondary structures, the conformational changes resulting from calcium binding and the structural stabilities in wild-type mnemiopsin 2, as well as its mutant type that possesses three EF-hand motifs. The distribution of secondary structures of these proteins indicates that mutant apo-mnemiopsin 2 has a more stable secondary structure than the wild-type. Analyses of the SR-FTIR spectra revealed that the conformational changes at the secondary structures of both mnemiopsin 2 depend on the calcium concentrations, such that the most noticeable changes in structures of wild-type and mutant mnemiopsin 2 occur at optimum concentrations 6 and 2 mM of calcium chloride, respectively. The addition of calcium to both proteins increases the proportions of their secondary structures in the amide I and II regions. The major amide I bands in the IR spectra of both mnemiopsin‑calcium complexes shift towards smaller wavenumbers, whereas their main amide II bands are identified at larger wavenumbers.