Catalytic activities of glycogenin additional to autocatalytic self-glucosylation

Miriam D. Alonso, Joseph Lomako, Wieslawa M. Lomako, William J. Whelan

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Abstract

Glycogenin is the autocatalytic, self-glucosylating protein that initiates glycogen synthesis in muscle and other tissues. We have sequenced the cDNA for rabbit muscle glycogenin and expressed and purified the protein in high yield as well as two mutant proteins in which Phe or Thr replaces Tyr-194, the site of glucosylation. While the wild-type protein can self-glucosylate, the mutants cannot, but all three utilize alternative acceptors by intermolecular glucose transfer for which the mutants have altered specificity. Tyr-194 is therefore not essential for the catalytic activity of glycogenin. All three proteins also hydrolyze UDP-glucose to glucose at rates comparable with the rate of self-glucosylation. The hydrolysis is competitive with glucose transfer to p-nitrophenyl α-maltoside. Self-glucosylation, glucosylation of other acceptors, and hydrolysis all appear to be catalyzed by the same active center. In the absence of peptidase inhibitors, the homogenous recombinant proteins of Mr 37,000 break down to equally active species having Mr 32,000. The kinetics of self-glucosylation catalyzed by the wild-type enzyme suggest that the reaction could be intermolecular rather than, as previously reported, intramolecular. The wild-type recombinant enzyme and native muscle glycogenin, which is phosphorylated, are inhibited quite differently by ATP at physiological concentration.

Original languageEnglish
Pages (from-to)15315-15319
Number of pages5
JournalJournal of Biological Chemistry
Volume270
Issue number25
StatePublished - Jun 23 1995

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ASJC Scopus subject areas

  • Biochemistry

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