Amino Acid Sequence around the Pyridoxal 5′-Phosphate Binding Site in Potato Phosphorylase1

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Abstract
The amino acid sequence around the pyridoxal 5′-phosphate binding site in potato phosphorylase was determined in order to compare it with those in phosphorylases from other sources having different regulatory properties. The potato enzyme was reduced by NaBH4 in the presence of urea, carboxymethylated, and digested with chymotrypsin and trypsin. Pyridoxyl peptides were isolated by the differential procedure using paper electrophoresis or DEAE-cellulose column chromatography. In Edman degradation of these peptides, pyridoxyllysine was identified as the phenylthiohydantoin derivative of pyridoxyllysine using a combination of thin-layer chromatography and the Pauli reaction. The sequence around pyridoxyllysine, comprising 57 amino acid residues, was determined except for a region with 6 amino acid residues. The pyridoxal 5′-phosphate binding site in potato phosphorylase showed a high homology with those of the rabbit muscle and yeast enzymes. This finding suggests that the cofactor should be directly related to the essential process of phosphorylase action.