THERMAL-CHARACTERISTICS OF SOLUBLE STARCH SYNTHASE FROM WHEAT ENDOSPERM

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 V-max and the K-m varied with temperature. Values for V-max were higher at 45 degrees C compared with 25 degrees C. However, the K-m values for both substates wer also higher at 45 degrees C than at 25 degrees C indicating that the affinity of the enzyme for its substrates was reduced at the higher temperature. Over the temperature range 15-45 degrees C, the K-m for amylopectin was minimal at 20 degrees C, and rose exponentially between 25 and 45 degrees C. Kinetic analyses indicated that the reaction was sequential and that the substrates could bind to the enzyme in either order. At 25 degrees C the binding of one substrate to the enzyme increased the affinity of the complex for the second substrate but at 45 degrees C 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.