The amino acid requirements for binding of Ca+ in alpha-lactalbumin have been studied. Comparison of the sequences of Ca2+-binding region of alpha-lactalbumin with the corresponding regions in the Ca2+ and non-Ca2+ c-type lysozymes suggest that in addition to the residues which have been identified by the X-ray crystal structure analysis as liganded to Ca2+ there are neighboring residues which also contribute to the binding of calcium ion. Computer modelling methods are being used to address the question """"""""why alpha-lactalbumin does not bind any sugar while as its homologous protein c-type lysozyme with which it has both sequence and structural homology does bind and hydrolyse oligosaccharides""""""""? The three dimensional structure of alpha-lactalbumin and lysozyme are very similar but the two proteins have different functions. Lysozyme catalyses the hydrolysis of a beta(1-4)glycosidic linkage in polysaccharides, while as alpha-lactalbumin does not bind sugar by itself but does interact with the enzyme betal-4galactosyltransferase, modifying its substrate specificity in a way which promotes the transfer of galactose to glucose resulting in a beta(1-4) linkage. Using computer modelling methods we have identified the side chain of amino acid residues in alpha-lactalbumin that block the entry of mono-saccharides into the C, B and D sites, equivalent to the one identified as sugar binding sites in the lysozyme. The side chain alterations in the protein that will allow binding of the sugar have been predicted and are being tested by the site directed mutagenesis of alpha-lactalbumin.
