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Metabolism and Enzymology

Characterization of Pea Chloroplast D-Enzyme (4--D-Glucanotransferase) ¹

Department of Agronomy, 1575 Linden Drive, University of Wisconsin, Madison, Wisconsin 53706-1597

Pea (Pisum sativum L.) chloroplast D-enzyme (4--D-glucanotransferase, EC 2.4. 1.25) was purified greater than 750-fold and partially characterized. It is a dimer with a subunit M_r of ca. 50,000. Optimal activity is between pH 7.5 and 8.0 with maltotriose as substrate and the enzyme's K_m for maltotriose is 3.3 millimolar. Chloroplast D-enzyme converts maltotriose to maltopentaose and glucose via the exchange of -1,4-glycosidic linkages. Maltotriose acts either as a donor or acceptor of a maltosyl group. The enzyme has highest activity with maltotriose as substrate. As initial substrate degree of polymerization is increased to maltoheptaose, D-enzyme activity drops to zero at 10 millimolar substrate concentrations and by 70% at 1 millimolar concentrations. The enzyme cannot use maltose as a substrate. Glucose was found to be a suitable acceptor substrate for this D-enzyme. Addition of glucose to incubation mixtures, or production of glucose by D-enzyme, prevents the synthesis of maltodextrins larger than maltopentaose. Removal of glucose produced by D-enzyme activity with maltotriose as substrate resulted in the synthesis of maltopentaose and maltodextrins with sufficient degrees of polymerization to be suitable substrates for pea chloroplast starch phosphorylase. The possible role of D-enzyme in pea chloroplast starch metabolism is discussed.

¹ This material is based upon work supported by the Cooperative State Research Service, U.S. Department of Agriculture under Agreement 87-CRCR-1-2324 and by the College of Agricultural and Life Sciences, University of Wisconsin-Madison.

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