The formation of glycogenin (Gn)-bound a-1,4-oligoglucan primer
is required for the de novo biosynthesis of glycogen (G). Dimeric
Gn, as it exists in solution and in the enzyme crystals, was
considered to be the molecular form which synthesizes the primer by
intersubunit glucosylation mechanism. We have described however
that monomeric Gn is also able to catalyze its intramolecular
glucopolymerization. In considering which Gn form actually primes
G biosynthesis, this might be determined by the polymerization
degree (pd) of the Tyr-linked oligoglucan the Gn form can produce,
having the size required by glycogen synthase and branching
enzyme for further elongation and branching. A pd of 12 was
reported for the oligoglucan produced by Gn dimer; however the
auto-glucopolymerization extent capacity of the monomer was
unknown. Now we determined the glucopolymerization degree of
fully autoglucosylated monomeric and homodimeric Gn and of
heterodimers formed by mixing 1) a Gn mutant lacking its tyrosine
acceptor with a mutant containing the tyrosine acceptor but lacking
glucosylation activity, and 2) the wild type enzyme with a mutant
which lacked both, glucosylating activity and tyrosine acceptor. The
results show that besides the intersubunit glucosylation of Gn dimer,
the intramolecular glucosylation of Gn monomer can produce the
oligo-glucopolymer primer for G biosynthesis.
