Analysis of Purified Maize Starch Synthases IIa and IIb: SS Isoforms Can Be Distinguished Based on Their Kinetic Properties

Since starch synthases IIa (SSIIa) and SSIIb have not been purified from plant tissue, their structure–function relationships have not been well characterized. Therefore, we have expressed these SS genes inEscherichia coli,purified them to apparent homogeneity, and studied their kinetic properties. In addition, the N-terminally truncated forms of these enzymes were studied in an attempt to understand the function of the diverse N-terminal sequences in SS. Our results show that, like SSI, the N-terminal extensions of SSIIa and SSIIb are not essential for catalytic activity and no extensive changes in their kinetic properties are observed upon their N-terminal truncation. Each isoform of SS can be distinguished based on its kinetic properties. Maize SSI and maize SSIIb exhibit higherVmaxwith glycogen as a primer, while the converse is true for SSIIa. However, the specific activity of SSIIb is at least two- to threefold higher than that for either SSI or SSIIa. Although SSIIb exhibits the highest maximal velocity of the isoforms compared, its apparent affinity for primer is twofold lower than the affinity of SSI and SSIIa for primer. Perhaps these differences in primer affinity, primer preference, and maximal velocities all contribute in some way to the different structure(s) of starch during its synthesis. Expression and purification of maize SS has now provided us a useful tool to address the role(s) of SS in starch synthesis and starch structure.