Starch Branching Enzyme II from Maize Endosperm

ADP-Glc pyrophosphorylase (EC 2.7.7.27), starch synthases (EC 2.4.1.21), and SBEs (EC 2.4.1.18) are the key enzymes in the pathway of plant starch biosynthesis. ADP-Glc pyrophosphorylase and starch synthase catalyze the formation of ADP-Glc, the substrate for starch synthesis, and the polymerization of Glc into the amylose and amylopectin fractions of the starch granule, respectively. SBEs catalyze branch point formation by the cleavage and reattachment of a-1,4-linked Glc chains to a-1,6 branch points in the growing starch molecule (Borovsky et al., 1979; Boyer, 1985). Branching enzymes are proposed to interact with starch synthases in formation of amylopectin (Boyer and Preiss, 1981). Multiple forms of SBE have been identified in maize (Zea mays L.) endosperm (Boyer and Preiss, 1978, 1981). Three forms of SBE, I, Ila, and Ilb, from developing maize endosperm have been characterized by their leve1 of branching activity on amylose and amylopectin and by kinetic and immunological properties (Boyer and Preiss, 1978; Fisher and Boyer, 1983; Singh and Preiss, 1985). Distinct differences have been shown between SBE I and IIa or IIb by the above parameters, but only small differences exist between SBE IIa and IIb. Genetic evidence suggests that IIa and IIb are prod-ucts