Abstract
All of the α-glucan phosphorylases so far purified from diverse origins have similar molecular and catalytic properties, whereas they differ in regulatory properties and glucan specificities. The activity of the rabbit muscle enzyme is regulated by phosphorylation-dephosphorylation and activated by AMP. On the other hand, the potato and Escherichia coli enzymes exist only in the active form, and are unaffected by the nucleotide. To elucidate the structural bases for these differences, we have determined the complete amino acid sequence of potato phosphorylase and compared it with those of the rabbit muscle and E. coli enzymes. The monomer of the potato enzyme 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 these enzymes 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 the 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 structural comparison among the three enzymes.
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Fukui, T., Nakano, K. Structural comparison of phosphorylases from different sources. J Protein Chem 6, 47–53 (1987). https://doi.org/10.1007/BF00248826
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DOI: https://doi.org/10.1007/BF00248826