Properties of conserved amino acid residues in tandem homologous protein domains. Proton NMR studies of the histidines of chicken ovomucoid

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Abstract
Peaks corresponding to the C.epsilon. protons of the 4 histidine residues (positions 58, 111, 123, and 182) of chicken ovomucoid were assigned in 1H NMR spectra (360 or 470 MHz) of the native single-chain protein and of fragments of the protein corresponding to its 3 homologous structural domains. Comparison of the 1H NMR pH titration behavior of these histidine residues and the deuterium exchange rates of their C.epsilon.-H positions show the following: The chemical shift properties of histidine residues 58, 123 and 182 differ despite the fact that the 3 residues are located in homologous positions in the 3 tandem domains. The properties of 3 of the 4 histidine residues (58, 111 and 123) do not change appreciably when the domains in which they are located are isolated, indicating that their environments are similar in both the fragment and the native protein. The properties of the fourth histidine (182) differ in the isolated domain and in the native protein. The observed properties of the histidine residues stem primarily from intradomain interactions that remain constant in isolated domains rather than from interactions with neighboring domains; an interdomain interaction is required to explain the behavior of only histidine-182. The chemical shift of histidine-111 is affected by the titration of the side chain of aspartate-98 with pHmid 2.6 in native ovomucoid but not in isolated second domain; the chemical shift of histidine-182 is perturbed by the titration of the carboxyl group of the C-terminal cysteine-186 with pHmid 2.4 in native ovomucoid and pHmid 2.6 in isolated third domain.