Table I.

Kinetic parameters of SEX4 acting on prephosphorylated A- and B-type MDcryst (STA)

Vmax and S0.5 values were determined according to Hanes (1932). A- and B-type MDcryst were prephosphorylated using StGWD and [β-33P]ATP for 60 min. Subsequently, varying amounts of both prelabeled water-insoluble MD were incubated with AtSEX4 (25 ng). At 1 min intervals, aliquots of the reaction mixtures were heat inactivated and the [33P]orthophosphate was quantified by TLC and phosphor imaging (n = 3; ±sd). G6P, Glc 6-P.

AllomorphDegree of PhosphorylationS0.5VmaxVmax/S0.5
nmol G6P/mmol Glcmgnmol min−1 mg−1 SEX4nmol min−1 mg−1 SEX4/mg MD
A type0.562.4 ± 0.130 ± 1.112.5
B type0.523.6 ± 0.249 ± 2.113.6