J. Biol. Chem., Vol. 267, Issue 1, 210-217, Jan, 1992
Dual anomeric specificity and dual anomerase activity of phosphoglucoisomerase quantified by two-dimensional phase-sensitive 13C EXSY NMR
R Willem, M Biesemans, K Hallenga, G Lippens, F Malaisse-Lagae and WJ Malaisse
Free Universities of Brussels, Belgium.
The reversible conversion between D-glucose 6-phosphate and D-fructose
6-phosphate catalyzed by yeast phosphoglucoisomerase was studied by phase
sensitive two-dimensional 13C-[1H] EXSY NMR spectroscopy at 150.869 and
125.759 MHz, using 13C-enriched substrates in the C2 position of the
D-hexose 6-phosphates. The shape of the build-up curves of the cross-peaks
associated with the 13C2 resonances of the alpha- and beta-anomers of both
D-[2-13C]glucose 6-phosphate and D-[2- 13C]fructose 6-phosphate reveals
that phosphoglucoisomerase selectively catalyzes the reversible conversion
between alpha-D-[2-13C]glucose 6- phosphate and beta-D-[2-13C]fructose
6-phosphate. Quantitative analysis of the build-up curves by three
different methods allowed us to conclude that phosphoglucoisomerase not
only selectively channels the latter isomerization but also considerably
accelerates the anomerization of both D-hexose 6-phosphates. The rate
constants of anomerization were indeed much higher in the presence than in
the absence of enzyme. The major finding in the present study consists in
the anomeric specificity of phosphoglucoisomerase toward the beta- anomer
of D-fructose 6-phosphate both as a substrate and a product, contrary to
previous proposals. This finding supports recent evidence suggesting the
direct channelling of beta-D-fructose 6-phosphate from
phosphoglucoisomerase to phosphofructokinase.
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