Iron and citrate interactions with hen egg white lysozyme

The binding of ions to proteins is of great interest in biological science (catalytic function, structural stability) and a good understanding of this relationship is needed for the control of the structure and functionality of proteins. The interaction of hen egg white lysozyme with calcium, magnesium, iron, citrate, phosphate and sulphate are studied by using the Scatchard representation or ESI–MS for iron-binding studies. This is the first time that anions have been considered and the results indicate that citrate interacts with lysozyme through electrostatic bonds. A pH of 6 favours citrate interaction to lysozyme, whereas no interaction appears at pH 9. ESI–MS studies also highlight that iron co-ordinates lysozyme, inducing the release of three protons, including one arising from an amine lateral chain.