Microbial starch debranching enzymes: Developments and applications

Starch debranching enzymes (SDBEs) hydrolyze the alpha-1,6 glycosidic bonds in polysaccharides such as starch, amylopectin, pullulan and glycogen. SDBEs are also important enzymes for the preparation of sugar syrup, resistant starch and cyclodextrin. As the synergistic catalysis of SDBEs and other starch-acting hydrolases can effectively improve the raw material utilization and production efficiency during starch processing steps such as saccharification and modification, they have attracted substantial research interest in the past decades. The substrate specificities of the two major members of SDBEs, pullulanases and isoamylases, are quite different. Pullulanases generally require at least two alpha-1,4 linked glucose units existing on both sugar chains linked by the alpha-1,6 bond, while isoamylases require at least three units of alpha-1,4 linked glucose. SDBEs mainly belong to glycoside hydrolase (GH) family 13 and 57. Except for GH57 type II pullulanse, GH13 pullulanases and isoamylases share plenty of similarities in sequence and structure of the core catalytic domains. However, the N terminal domains, which might be one of the determinants contributing to the substrate binding of SDBEs, are distinct in different enzymes. In order to overcome the current defects of SDBEs in catalytic efficiency, thermostability and expression level, great efforts have been made to develop effective enzyme engineering and fermentation strategies. Herein, the diverse biochemical properties and distinct features in the sequence and structure of pullulanase and isoamylase from different sources are summarized. Up-to-date developments in the enzyme engineering, heterologous production and industrial applications of SDBEs is also reviewed. Finally, research perspective which could help understanding and broadening the applications of SDBEs are provided.

Automated summary

Starch debranching enzymes (SDBEs) play a crucial role in hydrolyzing alpha-1,6 glycosidic bonds in polysaccharides and are essential for various industrial processes. While the substrate specificities of pullulanases and isoamylases differ, both enzymes belong to glycoside hydrolase families and have similarities in their core catalytic domains. Efforts have been made to improve the catalytic efficiency, thermostability, and expression level of SDBEs through enzyme engineering and fermentation strategies. Researchers have also summarized the diverse biochemical properties and features of pullulanase and isoamylase from different sources, aiming to broaden the applications of SDBEs in various industries.