FOURIER-TRANSFORM INFRARED SPECTROSCOPIC STUDIES ON THE COORDINATION OF THE SIDE-CHAIN COO- GROUPS TO CA2+ IN EQUINE LYSOZYME

Interactions between Ca2+ and the Asp side chains in the Ca2+-binding site of equine lysozyme were investigated by Fourier-transform infrare d (FT-IR) spectroscopy. In the spectrum of equine lysozyme, the intens ities of the bands at about 1595 cm(-1) and 1578 cm(-1) in the region of the COO- antisymmetric stretches increased upon Ca2+ binding. In th e region of the COO- symmetric stretches, the loss of intensity at abo ut 1388 cm(-1) and gains of intensities at about 1423 cm(-1) and 1403 cm(-1) were observed due to Ca2+ binding to equine lysozyme. The spect ral changes for equine lysozyme indicate that the COO- groups of Asp85 , Asp90 and Asp91 in the Ca2+-binding site coordinate to Ca2+ in the p seudo-bridging mode, where divalent metal cation is bound to one of th e two oxygens in the COO- group and a water molecule is hydrogen bonde d to the other oxygen, The results presented here provide further evid ence for a high degree of similarity between Ca2+-binding lysozyme and alpha-lactalbumin. The effects of Ca on the main-chain conformation o f equine lysozyme were compared with those of bovine alpha-lactalbumin and hen egg-white lysozyme.