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Bovine α-lactalbumin and hen egg-white lysozyme have closely similar amino acid sequences. A model of α-lactalbumin has been constructed on the basis of the main chain conformation established for lysozyme. The side chain interactions of lysozyme are listed (Table 2) and the consequences of the side chain replacements in α-lactalbumin examined. Changes in internal side chains are generally interrelated in a convincing manner, suggesting that the model is largely correct, but there are some regions where it has not been possible to deduce the conformation unequivocally. Glu 35, which acts as a proton donor in lysozyme, is absent in α-lactalbumin, in which a neighbouring histidine residue may assume a similar function. The surface cleft, which is the site of substrate binding in lysozyme, is shorter in α-lactalbumin. While this would be consistent with α-lactalbumin having a mono- or disaccharide as …