Essential dextrin structure as donor substrate for 4-α-glucanotransferase in glycogen debranching enzyme.

This article focuses on the substrate specificity of the glycogen debranching enzyme (GDE), particularly its 4-α-glucanotransferase catalytic site, which is essential for glycogen degradation. Using novel fluorogenic biantennary dextrins, the study demonstrates that both the main- and side-branches of the donor substrate must be recognized simultaneously by 4-α-glucanotransferase for full enzymatic activity, challenging previous assumptions that only the side-branch was critical. The research also reveals that disproportionation activities toward nonbranching maltooligosaccharides are minimal and likely abnormal, suggesting that traditional assays using such substrates may inaccurately assess enzyme function and mutations causing glycogen storage disease type III. These findings provide a refined basis for more accurate evaluation of GDE activity and its mutations relevant to this inherited metabolic disorder.