N.M.R. studies of myelin basic protein. VI. Proton spectra in aqueous solutions of proteins from mammalian and avian species

Abstract

Myelin basic proteins from human, cow, pig, rabbit and chicken central nervous systems were studied in aqueous solution by proton n.m.r. at 400 MHz. Species comparisons and other techniques led to the assignment of resonances of 23 specific amino acid residues in the primary structure. Resonances from side chains of polar amino acids adjacent to aspartic residues could be assigned by anomalous effects of pH on the chemical shifts. The pK values of histidine side chains all fall in the range 6.0-6.9, and four specific histidine residues were assigned. The conformation of the protein in aqueous solution is that of an extended non-random polypeptide chain with regions of localized structure. Nuclear Overhauser difference spectra showed that a reverse turn similar to that previously suggested for an encephalitogenic nonapeptide isolated from the protein (guinea pig determinant) occurs also in the protein itself, thus supporting the concept of special low-energy conformations responsible for biological activity. Upfield chemical shifts of some side chain methyl groups from the central region of the primary structure suggest ring-current shifts due to higher-order structuring.