Hexokinase 4 (HXK4) catalyze the reaction of plastid alpha-D-glucose and ATP to form alpha-D-glucose 6-phosphate and ADP. Ten rice hexokinase isozymes have been identified in complementation studies with hexokinase-deficient mutant yeast strains. One of these, HXK4, has been localized to plastids (Cho et al. 2006).