Fourier-transform infrared spectroscopic studies on the coordination of the side-chain COO- groups to Ca2+ in equine lysozyme.

The research investigates the interaction between calcium ions (Ca2+) and the Aspartic acid (Asp) side chains in the calcium-binding region of a horse protein, lysozyme, using Fourier-transform infrared spectroscopy. It shows that these interactions have specific spectral signatures and that these findings provide further evidence of a similarity between calcium-binding to lysozyme and another protein, alpha-lactalbumin.

Investigation technique

• The researchers used Fourier-transform infrared (FT-IR) spectroscopy, a technique that provides qualitative and quantitative data about how molecules absorb different frequencies of infrared light. These absorption patterns can be related to the types of chemical bonds and structures in the sample, thus offering clues about the interactions occurring.

Key Findings

• Upon binding with Ca2+, the infrared spectrum of the equine lysozyme displayed increased intensity at the band regions around 1595 cm-1 and 1578 cm-1, corresponding to the COO antisymmetric stretches. Additionally, there was a decrease in intensity at around 1388 cm-1 and gains at about 1423 cm-1 and 1403 cm-1, representing COO- symmetric stretches.
• The aspartic acid residues (Asp85, Asp90, and Asp91) in the Ca2+-binding site interact with the Ca2+ ions in a specific manner referred to as “pseudo-bridging” mode. In this mode, the Ca2+ ions attach to one of the two oxygen atoms of the COO- group, and at the same time, a water molecule forms a hydrogen bond with the other oxygen.

Comparison with other proteins

• The research also included a comparison of effects of Ca2+ binding on the main-chain conformation of equine lysozyme to those of bovine alpha-lactalbumin and hen egg-white lysozyme.
• This comparison revealed a high similarity between how Ca2+ binds to equine lysozyme and alpha-lactalbumin, which is another milk protein. These findings suggest potential shared functional elements between these two proteins at the structural level.