Characterization of amyloplastic phosphohexose isomerase from immature wheat (Triticum aestivum L.) endosperm.
Phosphohexose isomerase from amyloplasts of immature wheat endosperm was purified 133-fold. The enzyme had a molecular weight of 130 kDa and maximum activity at pH 8.6. It showed normal hyperbolic kinetics for both fructose-6-P and glucose-6-P with Km of 0.12 mM and 0.44 mM, respectively. pH had a great influence on Km for fructose-6-P. Using glucose-6-P as the substrate, the equilibrium was reached at 23% fructose-6-P and 77% glucose-6-P and an equilibrium constant of about 3.0. The delta F calculated from the apparent equilibrium constant was +742 cal.mol-1. The activation energy calculated from the Arrhenius plot was 7450 cal.mol-1. None of the sulphydryl reagents at 2.5 mM concentration inactivated the enzyme. The enzyme was competitively inhibited by 6-phosphogluconate, ribose-5-P and ribulose-5-P with Ki values of 0.18, 0.14, and 0.13 mM, respectively. The probable role of the enzyme in starch biosynthesis in amyloplasts is discussed.
Sangwan
R S
RS
Department of Chemistry and Biochemistry, Haryana Agricultural University, Hisar.
Singh
R
R
D005956:Glucose-6-Phosphate Isomerase / Q000302:isolation & purification* / Q000378:metabolism; D007700:Kinetics; D013347:Subcellular Fractions / Q000201:enzymology; D014908:Triticum / Q000201:enzymology*
D005956:Glucose-6-Phosphate Isomerase