Three peptides containing 6-pyridoxyllysine have been isolated from the tryptic digest of the allosteric fructose-1,6-bisphosphate-dependent pyruvate kinase from Escherichia coli, which had been almost completely inactivated with pyridoxal 5'-phosphate. The labelled peptides have been sequenced. The comparison of their sequences with the primary structure of the cat muscle pyruvate kinase allowed to state that peptide I fits the region spanning residues 423-438 (53% identity), peptide II corresponds to residues 442-457 (44% identity) and peptide III encompasses residues 342-368 (70% identity). These findings are discussed in connection with our previous results on the involvement of the three peptides in the catalytic and regulatory properties of the enzyme (Valentini, G., Speranza, M.L., Iadarola, P., Ferri, G. & Malcovati, M. (1988) Biol. Chem. Hoppe-Seyler 369, 1219-1226) and in connection with their location in the three-dimensional structure of the cat muscle pyruvate kinase (Muirhead, H., Clayden, D.A., Lorimer, C.G., Fothergill-Gilmore, L.A., Schiltz, E. & Schmitt, W. (1986) EMBO J. 5, 475-481).
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