Hexokinase II of Pea Seeds

Abstract

A 2nd hexokinase (EC 2.7.1.1) was obtained from pea seed (P. sativum L. cv. Progress No. 9) extracts. The enzyme, termed hexokinase II, had a high affinity (Km 48 .mu.M) for glucose and a relatively low affinity (Km, 10 mM) for fructose. The Km for MgATP was 86 .mu.M. Mg2+ was required for activity, but excess Mg2+ was inhibitory. MgADP inhibited hexokinase II. The addition of salts of monovalent cations increased hexokinase II activity. Al3+ was a strong inhibitor of the enzyme at pH 6.6 but not at the optimum pH (8.2). Citrate and 3-phosphoglycerate activated pea seed hexokinase II at pH 6.6, probably by coordinating with Al present as a contaminant in commercial ATP. The properties of hexokinase II are compared with those of the other 3 hexose kinases obtained from pea seed extracts. The possible role of these enzymes in plant carbohydrate metabolism is discussed.