The limit dextrinases from ungerminated oats (Avena sativa L.) and ungerminated rice (Oryza sativa L.)

Crystal structure of barley limit dextrinase-limit dextrinase inhibitor (LD-LDI) complex reveals insights into mechanism and diversity of cereal type inhibitors

Molecular details underlying regulation of starch mobilization in cereal seed endosperm remain unknown despite the paramount role of this process in plant growth. The structure of the complex between the starch debranching enzyme barley limit dextrinase (LD), hydrolyzing α-1,6-glucosidic linkages, and its endogenous inhibitor (LDI) was solved at 2.7 Å. The structure reveals an entirely new and unexpected binding mode of LDI as compared with previously solved complex structures of related cereal type family inhibitors (CTIs) bound to glycoside hydrolases but is structurally analogous to binding of dual specificity CTIs to proteases. Site-directed mutagenesis establishes that a hydrophobic cluster flanked by ionic interactions in the protein-protein interface is vital for the picomolar affinity of LDI to LD as assessed by analysis of binding by using surface plasmon resonance and also supported by LDI inhibition of the enzyme activity. A phylogenetic analysis identified four LDI-like proteins in cereals among the 45 sequences from monocot databases that could be classified as unique CTI sequences. The unprecedented binding mechanism shown here for LDI has likely evolved in cereals from a need for effective inhibition of debranching enzymes having characteristic open active site architecture. The findings give a mechanistic rationale for the potency of LD activity regulation and provide a molecular understanding of the debranching events associated with optimal starch mobilization and utilization during germination. This study unveils a hitherto not recognized structural basis for the features endowing diversity to CTIs.

Purification and characterisation of limit dextrinase from Pisum sativum L

A limit dextrinase has been purified 2,700-fold from ungerminated pass by affinity chromatography. The enzyme hydrolyses (1 yield 6)-alpha-d-glucosidic linkages in alpha-limit dextrins containing at least one alpha-1(1 in alpha 4)-linked d-glucose residue on either side of the susceptible linkage. The limit dextrinase also hydrolyses the polysaccharides amylopectin, amylopectin beta-limit dextrin, glycogen beta-limit dextrin, and pullulan, but has no activity towards glycogen.

The limit dextrinase from malted sorghum (Sorghum vulgare)

A limit dextrinase, free from contaminating carbohydrases, has been purified from malted sorghum flour. The enzyme readily hydrolysed alpha-limit dextrins having maltosyl or maltotriosyl side-chains, pullulan, and amylopectin beta-limit dextrin. Glycogen beta-limit dextrin and amylopectin were more slowly hydrolysed, the detection of the hydrolysis of amylopectin being dependent on enzyme concentration. No significant debranching of glycogen could be detected.