isoamylase

Isoamylase is an enzyme that catalyzes the hydrolysis of α-1,6-glycosidic bonds in branched polysaccharides, most notably amylopectin and related starch degradation intermediates such as limit dextrins. It is a debranching enzyme that helps remodel branched glucans by removing glucose units at branch points, yielding more linear chains that can be further processed by other starch-metabolizing enzymes. Isoamylase is commonly designated as EC 3.2.1.68.

In higher plants, isoamylase functions in plastids during starch biosynthesis. Plant isoamylases exist as multiple isoforms, typically encoded by ISA1 and ISA2 genes, that assemble into active complexes. By debranching branched chains, isoamylase cooperates with starch synthases and branching enzymes to enable proper packing and crystallization of amylopectin within starch granules. Loss of isoamylase activity in cereals and dicots results in the accumulation of phytoglycogen and altered starch properties, often producing kernels with a sticky or translucent appearance.

Isoamylases are found in plants, bacteria, and fungi. In bacteria they participate in carbohydrate metabolism and can act on branched glucans formed by α-1,6 linkages. The enzyme often requires calcium for activity and preferentially hydrolyzes α-1,6 bonds; it has limited activity on α-1,4 linkages.

Applications of isoamylase and related debranching enzymes include industrial starch processing, where they help modify starch structure during saccharification and production of glucose syrups. They also feature in research on starch biosynthesis and the genetic control of starch properties in crops.