Kakefuda G, Duke S H
Department of Agronomy, 1575 Linden Drive, University of Wisconsin, Madison, Wisconsin 53706-1597.
Plant Physiol. 1989 Sep;91(1):136-43. doi: 10.1104/pp.91.1.136.
Pea (Pisum sativum L.) chloroplast D-enzyme (4-alpha-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 alpha-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.
豌豆（Pisum sativum L.）叶绿体D-酶（4-α-D-葡聚糖转移酶，EC 2.4.1.25）被纯化了750倍以上，并进行了部分特性鉴定。它是一种二聚体，亚基的相对分子质量约为50,000。以麦芽三糖为底物时，最佳活性在pH 7.5至8.0之间，该酶对麦芽三糖的米氏常数为3.3毫摩尔。叶绿体D-酶通过α-1,4-糖苷键的交换将麦芽三糖转化为麦芽五糖和葡萄糖。麦芽三糖既可以作为麦芽糖基的供体，也可以作为受体。该酶以麦芽三糖为底物时活性最高。随着初始底物聚合度增加到麦芽七糖，在底物浓度为10毫摩尔时D-酶活性降至零，在1毫摩尔浓度时活性下降70%。该酶不能以麦芽糖为底物。发现葡萄糖是这种D-酶合适的受体底物。向孵育混合物中添加葡萄糖，或由D-酶产生葡萄糖，可防止合成大于麦芽五糖的麦芽糊精。去除以麦芽三糖为底物时D-酶活性产生的葡萄糖，会导致合成麦芽五糖和聚合度足够高的麦芽糊精，这些麦芽糊精是豌豆叶绿体淀粉磷酸化酶的合适底物。文中讨论了D-酶在豌豆叶绿体淀粉代谢中的可能作用。
