Thermal characteristics of soluble starch synthase from wheat endosperm

Abstract

The aim of the work reported in this paper was to characterise the thermal responses of soluble starch synthase (SSS) extracted from the endosperm of the developing wheat grain. Using partially purified preparations of SSS, the reaction obeyed Michaelis-Menten kinetics with both substrates amylopectin and ADPglucose. Both the Vmax and the Km varied with temperature. Values for Vmax were higher at 45ºC compared with 25ºC. However, the Km values for both substrates were also higher at 45ºC than at 25ºC indicating that the affinity of the enzyme for its substrates was reduced at the higher temperature. Over the temperature range 15-45ºC, the Km for arnylopectin was minimal at 20ºC, and rose exponentially between 25 and 45ºC. Kinetic analyses indicated that the reaction was sequential and that the substrates could bind to the enzyme in either order. At 25ºC the binding of one substrate to the enzyme increased the affinity of the complex for the second substrate but at 45ºC these effects were abolished. These thermal characteristics of SSS could explain certain important features of the temperature responses of starch deposition in the wheat grain in vivo.