Impact of Starch Binding Domain Fusion on Activities and Starch Product Structure of 4-α-Glucanotransferase

Bioinformatics and functional selection of GH77 4-α-glucanotransferases for potato starch modification

4-α-glucanotransferases (4αGTs, EC 2.4.1.25) from glycoside hydrolase family 77 (GH77) catalyze chain elongation of starch amylopectin chains and can be utilized to structurally modify starch to tailor its gelation properties. The potential relationship between the structural design of 4αGTs and functional starch modification is unknown. Here, family GH77 was mined in silico for enzyme candidates based on sub-grouping guided by Conserved Unique Peptide Patterns (CUPP) bioinformatics categorization. From + 12,000 protein sequences a representative set of 27 4αGTs, representing four different domain architectures, different bacterial origins and diverse CUPP groups, was selected for heterologous expression and further study. Most of the enzymes catalyzed starch modification, but their efficacies varied substantially. Five of the 4αGTs were characterized in detail, and their action was compared to that of the industrial benchmark enzyme, Tt4αGT (CUPP 77_1.2), from Thermus thermophilus. Reaction optima of the five 4αGTs ranged from ∼40-60 °C and pH 7.3-9.0. Several were stable for a minimum 4 h at 70 °C. Domain architecture type A proteins, consisting only of a catalytic domain, had high thermal stability and high starch modification ability. All five novel 4αGTs (and Tt4αGT) induced enhanced gelling of potato starch. One, At4αGT from Azospirillum thermophilum (CUPP 77_2.4), displayed distinct starch modifying abilities, whereas T24αGT from Thermus sp. 2.9 (CUPP 77_1.2) modified the starch similarly to Tt4αGT, but slightly more effectively. T24αGT and At4αGT are thus interesting candidates for industrial starch modification. A model is proposed to explain the link between the 4αGT induced molecular modifications and macroscopic starch gelation.

Sequential starch modification by branching enzyme and 4-α-glucanotransferase improves retention of curcumin in starch-alginate beads

A new super-branched amylopectin with longer exterior chains was produced from normal maize starch by modification with branching enzyme followed by 4-α-glucanotransferase, and applied for co-entrapment of a curcumin-loaded emulsion in alginate beads. The network structure of the gel beads was obtained with Ca2+-cross-linked alginate and a modest load of retrograded starch. The dual enzyme modified starch contained more and longer α-1,6-linked branch chains than single enzyme modified and unmodified starches and showed superior resistance to digestive enzymes. Alginate beads with or without starch were of similar size (1.69-1.74 mm), but curcumin retention was improved 1.4-2.8-fold in the presence of different starches. Thus, subjecting the curcumin-loaded beads to in vitro simulated gastrointestinal digestion resulted in retention of 70, 43 and 22 % of the curcumin entrapped in the presence of modified, unmodified, or no starch, respectively. Molecular docking provided support for curcumin interacting with starch via hydrogen bonding, hydrophobic contacts and π-π stacking. The study highlights the potential of utilizing low concentration of dual-enzyme modified starch with alginate to create a versatile vehicle for controlled release and targeted delivery of bioactive compounds.

Functional Roles of N-Terminal Domains in Pullulanase from Human Gut Lactobacillus acidophilus

Pullulanases are multidomain α-glucan debranching enzymes with one or more N-terminal domains (NTDs) including carbohydrate-binding modules (CBMs) and domains of unknown function (DUFs). To elucidate the roles of NTDs in Lactobacillus acidophilus NCFM pullulanase (LaPul), two truncated variants, Δ41-LaPul (lacking CBM41) and Δ(41+DUFs)-LaPul (lacking CBM41 and two DUFs), were produced recombinantly. LaPul recognized 1.3- and 2.2-fold more enzyme attack-sites on starch granules compared to Δ41-LaPul and Δ(41+DUFs)-LaPul, respectively, as measured by interfacial kinetics. Δ41-LaPul displayed markedly lower affinity for starch granules and β-cyclodextrin (10- and >21-fold, respectively) in comparison to LaPul, showing substrate binding mainly stems from CBM41. Δ(41+DUFs)-LaPul exhibited a 12 °C lower melting temperature than LaPul and Δ41-LaPul, indicating that the DUFs are critical for LaPul stability. Notably, Δ41-LaPul exhibited a 14-fold higher turnover number (kcat) and 9-fold higher Michaelis constant (KM) compared to LaPul, while Δ(41+DUFs)-LaPul's values were close to those of LaPul, possibly due to the exposure of aromatic by truncation.

Novel Design of an α-Amylase with an N-Terminal CBM20 in Aspergillus niger Improves Binding and Processing of a Broad Range of Starches

In the starch processing industry including the food and pharmaceutical industries, α-amylase is an important enzyme that hydrolyses the α-1,4 glycosidic bonds in starch, producing shorter maltooligosaccharides. In plants, starch molecules are organised in granules that are very compact and rigid. The level of starch granule rigidity affects resistance towards enzymatic hydrolysis, resulting in inefficient starch degradation by industrially available α-amylases. In an approach to enhance starch hydrolysis, the domain architecture of a Glycoside Hydrolase (GH) family 13 α-amylase from Aspergillus niger was engineered. In all fungal GH13 α-amylases that carry a carbohydrate binding domain (CBM), these modules are of the CBM20 family and are located at the C-terminus of the α-amylase domain. To explore the role of the domain order, a new GH13 gene encoding an N-terminal CBM20 domain was designed and found to be fully functional. The starch binding capacity and enzymatic activity of N-terminal CBM20 α-amylase was found to be superior to that of native GH13 without CBM20. Based on the kinetic parameters, the engineered N-terminal CBM20 variant displayed surpassing activity rates compared to the C-terminal CBM20 version for the degradation on a wide range of starches, including the more resistant raw potato starch for which it exhibits a two-fold higher Vmax underscoring the potential of domain engineering for these carbohydrate active enzymes.