Superior thermostability and divalent cation sensitivity of isoamylase CMI294C from Cyanidioschyzon merolae.

A storage polysaccharide in the red alga Cyanidioschyzon merolae is semi-amylopectin, a glucan with properties intermediate between noncrystalline glycogen and semicrystalline amylopectin. The debranching enzyme isoamylase plays a crucial role in determining the semicrystalline nature of glucans. In amylopectin-storing organisms, isoamylases consist of the isozymes ISA1, ISA2, and ISA3, with the former two primarily responsible for semicrystallinity. While the semicrystallinity of C. merolae semi-amylopectin is weaker than that of amylopectin, it retains a semicrystalline structure. Based on a previous analysis of isoamylase-deficient strains of C. merolae, the isoform CMI294C is the main contributor to glucan synthesis. Although the biochemical properties of isoamylases involved in amylopectin synthesis have been characterized, those of isoamylases involved in semi-amylopectin synthesis remain largely unknown. Here, we performed a detailed biochemical analysis of CMI294C to gain insights of isoamylases in semi-amylopectin synthesis. Similar to isoamylases in amylopectin-synthesizing organisms, CMI294C hydrolyzes amylopectin more efficiently than glycogen. However, unlike typical isoamylases, CMI294C is uniquely more active against pullulan than against glycogen; and it is strongly inhibited by Zn²⁺. Our results indicate that CMI294C can be potentially used for industrial maltose production due to its enzymatic properties. Overall, our findings provide molecular insights into the isoamylase in glucan structure modulation and enhance our understanding of glucan metabolism in C. merolae.