New insight into structure/function relationships in plant alpha-amylase family GH13 members
2010
Seo, E.S., Technical Univ. of Denmark, Lyngby | Andersen, J.M. | Nielsen, M.M. | Vester-Christensen, M.B. | Christiansen, C. | Jensen, J.M. | Motyan, J.A. | Glaring, M.A. | Blennow, A. | Kandra, L. | Gyemant, G. | Janecek, S. | Haser, R. | Aghajari, N. | Hachem, M.A. | Svensson, B.
Two carbohydrate binding surface sites (SBSs) on barley alpha-amylase 1 (AMY1) of glycoside hydrolase family 13 (GH13) displayed synergy in interactions with starch granules, thus being pivotal for hydrolysis of supramolecular substrates. Mutational analysis showed that SBS1 is more critical for the conversion of starch granules, while SBS2 has higher affinity than SBS1 for beta-cyclodextrin (beta-CD). Noticeably, the binding preference of beta-CD to SBS2 differed distinctly from that of maltooligosaccharides to the catalytic nucleophile mutant D180A AMYI. Binding energy maps at subsites -8 through +4 of the active site indicated remarkably elevated affinity due to the Y380A mutation at SBS2. The high-yield AMY2 expression variant A42P, made it possible to show that Tyr378-corresponding to Tyr380 in AMY1-has a role in interactions with starch granules, but not in beta-CD binding. Besides SBSs, dedicated starch binding domains (SBDs) mediate binding to starch granules. SBDs are currently categorised into 9 carbohydrate binding module (CBM) families. A novel CBM20 subfamily encountered in regulatory enzymes possesses characteristically low affinity for beta-CD. Although alpha-amylase is essential for starch mobilisation in germinating barley seeds, efficient degradation requires the concerted action of alpha-amylase, beta-amylase, limit dextrinase (LD) and possibly alpha-glucosidase. Limit dextrinase (LD) is encoded by a single gene and represents the sole debranching activity during germination. Recent expression of functional LD in Pichia pastoris makes biochemical and biophysical characterisation of this GH13 enzyme possible. An endogenous limit dextrinase inhibitor was cloned and produced recombinantly and demonstrated to have sub-nanomolar affinity for LD.
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