Abstract
Glucosephosphate isomerase ( d-glucose-6-phosphate ketol-isomerase, EC 5.3.1.9) of pea was isolated in the crystalline state in 14.5% yield from pea extract after 1500-fold purification. The molecular weight of the enzyme was estimated at 110 000 by ultracentrifugal analysis. The K m and molecular activity of the enzyme were determined as 2.7·10 −4 M ( Glc-6-P ) and 165 000 in Tris-HCl buffer (pH 8.5) at 37°. The isomerization reaction was equilibrated in 65% Glc-6-P and 35% Fru-6-P at pH 7.5 at 37°. 6-Phosphogluconate competitively inhibited the reaction and its K i was 1.3·10 −5 M. The pH optimum of the reaction was found to be 8.5 in Tris-HCl buffer and 9.5 in glycine-NaCl-NaOH buffer. The maximal reaction rate was observed at 50° and the activation energy was calculated as 8100 cal. Sulfhydryl groups are not thought to be involved in its active center because the reaction was not inhibited by p- chloromercuribenzoate and monoiodoacetate. The enzyme was able to isomerize glucose into fructose in the presence of arsenate.
Published Version
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