Proton-Magnetic-Resonance Spectroscopic Study of the Histidine Residues of Bovine alpha-Lactalbumin

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Abstract
A study of the three histidine residues of bovine alpha-lactalbumin has been made using proton magnetic resonance (PMR) spectroscopy in order to obtain information on their environments in the protein and thereby to test in part the previously proposed structure. PMR titration curves are obtained for the H-4 resonances using difference spectroscopy and for the H-2 resonances and the 1-H-2-H exchange rates of the H-2 protons have been measured. The assignment of resonances to particular histidine residues is achieved by utilising their selective reaction with iodoacetate in conjunction with a PMR study of the carboxymethylation of alpha-N-acetyl-L-histidine. The H-2 and H-4 resonances labelled 1, 2 and 3 starting from the downfield end of the spectrum are assigned to histidine residues 107, 68 and 32 respectively. Their apparent pK values at low ionic strength and 20 degrees C are 5.78, 6.49 and 6.51 respectively. The experimental results on two histidine residues are consistent with the predictions of the proposed structure, which indicate that histidine-68 is an external residue and histidine-32 is partially buried and in the vicinity of aromatic residues. The experimental data on histidine 107 can also be rationalised with less certainty in terms of the proposed structure, which indicates a partially buried residue that may be involved in hydrogen bonding.