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Pullulanase is an effective starch debranching enzyme widely used in starch saccharification and modification. In this work, the biochemical characteristics and potential application of a new type I pullulanase from Priestia koreensis HL12 (HL12Pul) were evaluated and reported for the first time. Through in-depth evolutionary analysis, HL12Pul was classified as type I pullulanase belonging to glycoside hydrolase family 13, subfamily 14 (GH13_14). HL12Pul comprises multi-domains architecture, including two carbohydrate-binding domains, CBM68 and CBM48, at the N-terminus, the TIM barrel structure of glycoside hydrolase family 13 (GH13) and C-domain. Based on sequence analysis and experimental cleavage profile, HL12Pul specifically hydrolyzes only α-1,6 glycosidic linkage-rich substrates. The enzyme optimally works at 40 °C, pH 6.0, with the maximum specific activity of 181.14 ± 3.55 U/mg protein and catalytic efficiency (kcat/Km) of 49.39 mL/mg·s toward pullulan. In addition, HL12Pul worked in synergy with raw starch-degrading α-amylase, promoting raw cassava starch hydrolysis and increasing the sugar yield by 2.9-fold in comparison to the α-amylase alone in a short reaction time. Furthermore, HL12Pul effectively produces type III-resistant starch (RSIII) from cassava starch with a production yield of 70%. These indicate that HL12Pul has the potential as a biocatalyst for starch saccharification and modification.

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Exploring new raw starch-hydrolyzing -amylases and understanding their biochemical characteristics are important for the utilization of starch-rich materials in bio-industry. In this work, the biochemical characteristics of a novel raw starch-degrading -amylase (HL11 Amy) from Roseateles terrae HL11 was firstly reported. Evolutionary analysis revealed that HL11Amy was classified into glycoside hydrolase family 13 subfamily 32 (GH13_32). It contains four protein domains consisting of domain A, domain B, domain C and carbohydrate-binding module 20 (CMB20). The enzyme optimally worked at 50 °C, pH 4.0 with a specific activity of 6270 U/mg protein and 1030 raw starch-degrading (RSD) U/mg protein against soluble starch. Remarkably, HL11Amy exhibited activity toward both raw and gelatinized forms of various substrates, with the highest catalytic efficiency (kcat/Km) on starch from rice, followed by potato and cassava, respectively. HL11Amy effectively hydrolyzed cassava pulp (CP) hydrolysis, with a reducing sugar yield of 736 and 183 mg/g starch from gelatinized and raw CP, equivalent to 72% and 18% conversion based on starch content in the substrate, respectively. These demonstrated that HL11Amy represents a promising raw starch-degrading enzyme with potential applications in starch modification and cassava pulp saccharification.

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