Structural basis for the difference of the regulatory properties between potato and rabbit muscle phosphrylases. The NH2-terminal sequence of the potato enzyme.

Abstract

alpha-Glucan phosphorylases (EC 2.4.1.1) from potato tuber and rabbit muscle are similar in some of the structural and kinetic properties, despite the difference in the regulation of enzyme activity. The NH2-terminal region in the rabbit muscle enzyme is important for both allosteric and covalent controls. In this paper, the NH2-terminal 104-residue sequence of the potato enzyme was determined to compare it with that of the rabbit muscle enzyme. Edman degradations of the whole protein revealed the terminal sequence of Thr-Leu-X-Glu-X-X-His-His-. Using this sequence as a marker, the NH2-terminal 81-residue peptide was isolated from the CNBr-treated protein and sequenced. The sequence of this peptide was extended further by the 23-residue sequence of the other two CNBr peptides by considering the sequence similarity between the potato and rabbit muscle enzymes. The combined results of the present and previous studies (Nakano, K., Wakabayashi, S., Hase, T., Matsubara, H. and Fukui, T. (1978) J. Biochem. (Tokyo) 83, 1085-1094 and Nakano, K., Fukui, T., and Matsubara, H. (1980) J. Biochem. (Tokyo) 87, 919-927) indicate that the potato and rabbit muscle enzymes are highly homologous except for the NH2-terminal 33-residue region which is very different. The potato enzyme has 2 more residues attached to the NH2-terminal region. The remarkable dissimilarity in the structure of this region provides a basis for the difference of the regulatory properties between the two enzymes. It is conceivable that phosphorylases existed originally as a large catalytically active molecule, and that the regulatory mechanism was then formed within the molecule during the course of evolution.