Functional Identification of Calcium Binding Residues in Bovine α-Lactalbumin

The functional role of previously identified calcium binding residues in α-lactalbumin (α-LA) was investigated by site-directed mutagenesis. Mutation of D82 to alanine did not effect the binding affinity for calcium, the protein structure, or its function in the lactose synthase assay, suggesting that this aspartate side chain is not essential for calcium binding or structural stabilization. In contrast, mutation of either D87 or D88 to alanine completely eliminated the strong calcium binding and altered α-LA as shown by several spectroscopically derived properties such as near- and far-UV CD and intrinsic fluorescence studies. These latter two mutants displayed significantly reduced abilities to stimulate lactose synthase activity (