Fructokinase (Fraction IV) of Pea Seeds

A fructokinase (EC 2.7.1.4) was obtained from pea (Pisum sativum L.) seeds. This enzyme, termed fructokinase (fraction IV), was specific for fructose as substrate and had little activity with glucose or mannose. Excess fructose inhibited the enzyme at the optimum pH (8.2) but not at pH 6.6. MgATP was inhibitory at pH 6.6. The apparent Michaelis-Menten constants at pH 8.2 were 0.057 mm for fructose and 0.10 mm for MgATP. Mg2+ ions were essential for activity; Mn2+ could partially replace Mg2+. Fructokinase (fraction IV) had a requirement for K+ ions which could be substantially replaced by Rb+ or NH4+ but not by Na+. The enzyme was inhibited by MgADP. The possible significance of fructokinase (fraction IV) in plant carbohydrate metabolism is discussed.