Table I.

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

V_max and S_0.5 values were determined according to Hanes (1932). A- and B-type MD_cryst 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.

Allomorph	Degree of Phosphorylation	S0.5	Vmax	Vmax/S0.5
	nmol G6P/mmol Glc	mg	nmol min−1 mg−1 SEX4	nmol min−1 mg−1 SEX4/mg MD
A type	0.56	2.4 ± 0.1	30 ± 1.1	12.5
B type	0.52	3.6 ± 0.2	49 ± 2.1	13.6