The complete amino acid sequence of potato alpha-glucan phosphorylase

The complete amino acid sequence of potato alpha-glucan phosphorylase has been determined. The monomer contains 916 amino acids with a molecular weight of 103,916. About one-fourth of the amino-terminal threonine is blocked by an acetyl group. Sequence comparison among phosphorylases from potato tuber, rabbit muscle, and Escherichia coli reveals the presence of a characteristic 78-residue insertion in the middle of the polypeptide chain of the potato enzyme. Except for the large inserted portion, 51 and 40% of the amino acids in the potato enzyme are identical with the rabbit muscle and E. coli enzymes, respectively. The regions relevant to the regulation of activity are completely different among the three enzymes, whereas those involved in the catalytic reaction are well conserved. The potato enzyme sequence is consistent with the tertiary structure of the rabbit muscle enzyme. The 78-residue insertion is located at the junction of the amino- and carboxyl-terminal domains on the molecular surface near the glycogen storage site. This insertion could account for the substrate discrimination of the potato enzyme. The molecular evolution of phosphorylase is discussed based on the presence of the large insertion of the potato enzyme.