Sucrose Synthase of Soybean Nodules

Abstract

Sucrose synthase (UDPglucose: D-fructose 2-.alpha.-D-glucosyl transferase, EC 2.4.1.13) has been purified from the plant cytosolic fraction of soybean (G. max L. Merr cv Williams) nodules. The native enzyme had a MW of 400,000. The subunit MW was 90,000 and a tetrameric structure is proposed for soybean nodule sucrose synthase. Optimum activity in the sucrose cleavage and synthesis directions was at pH 6 and pH 9.5, respectively, and the enzyme displayed typical Michaelis-Menten kinetics. Soybean nodule sucrose synthase had a high affinity for UDP (Km, 5 .mu.M) and a relatively low affinity for ADP (apparent Km, 0.13 mM) and CDP (apparent Km 1.1 mM). The Km for sucrose was 31 mM. In the synthesis direction, UDPglucose (Km, 0.012 mM) was a more effective glucosyl donor than ADPglucose (Km, 1.6 mM) and the Km for fructose was 3.7 mM. Divalent cations stimulated activity in both the cleavage and synthesis directions and the enzyme was sensitive to inhibition by heavy metals.