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We have previously reported the kinetic properties of maize starch synthase I (SSI), SSIIa and SSIIb using glycogen and amylopectin as primer and have determined that these SS enzymes can be distinguished based on their kinetic properties. However, it is not clear how starch synthesis is initiated and what the native primer for SS in vivo is. Therefore, in this study, we determined whether maize SS can catalyze the de novo synthesis of α-glucan and studied how maize SS utilizes potential native primers such as maltooligosaccharides (MOS) and developing native starch granules. Using purified recombinant maize SSI, we provide evidence that maize SSI cannot catalyze the de novo synthesis of α-glucan and the “unprimed activity” may be due to the presence of primers in ADPG. However, maize SSI, SSIIa and SSIIb, can use maltose as a primer, although much less efficiently than MOS with a DP of 3 or greater. All MOSs tested can be elongated by SSI, SSIIa and SSIIb with more than one glucose residues. For the first time we describe here that purified maize SS can further elongate the chains of native starch granule isolated from developing maize waxy endosperm (18 days after pollination). Compared with their kinetic properties in the presence of soluble amylopectin and glycogen, differences in the Vmax and optimum temperature of these three enzymes were ob served in the presence of native starch granules. The differences between SSI, SSIIa and SSIIb in their primer preferences using MOS and native starch granule may provide some clues as to the functions of different SS in starch synthesis.

Understanding Catalytic Properties and Functions of Maize Starch Synthase Isozymes