An ultra-stable glucanotransferase-cum-exoamylase from the hyperthermophile archaeon Thermococcus onnurineus

Structural engineering and truncation of α-amylase from the hyperthermophilic archaeon Methanocaldococcus jannaschii

Alpha amylases catalyse the hydrolysis of α-1, 4-glycosidic bonds in starch, yielding glucose, maltose, dextrin, and short oligosaccharides, vital to various industrial processes. Structural and functional insights on α-amylase from Methanocaldococcus jannaschii were computationally explored to evaluate a catalytic domain and its fusion with a small ubiquitin-like modifier (SUMO). The recombinant proteins' production, characterization, ligand binding studies, and structural analysis of the cloned amylase native full gene (MjAFG), catalytic domain (MjAD) and fusion enzymes (S-MjAD) were thoroughly analysed in this comparative study. The MjAD and S-MjAD showed 2-fold and 2.5-fold higher specific activities (μmol min-1 mg -1) than MjAFG at 95 °C at pH 6.0. Molecular modelling and MD simulation results showed that the removal of the extra loop (178 residues) at the C-terminal of the catalytic domain exposed the binding and catalytic residues near its active site, which was buried in the MjAFG enzyme. The temperature ramping and secondary structure analysis of MjAFG, MjAD and S-MjAD through CD spectrometry showed no notable alterations in the secondary structures but verified the correct folding of MjA variants. The chimeric fusion of amylases with thermostable α-glucosidases makes it a potential candidate for the starch degrading processes.

Acceptor dependent catalytic properties of GH57 4-α-glucanotransferase from Pyrococcus sp. ST04

The 4-α-glucanotransferase (4-α-GTase or amylomaltase) is an essential enzyme in maltodextrin metabolism. Generally, most bacterial 4-α-GTase is classified into glycoside hydrolase (GH) family 77. However, hyperthermophiles have unique 4-α-GTases belonging to GH family 57. These enzymes are the main amylolytic protein in hyperthermophiles, but their mode of action in maltooligosaccharide utilization is poorly understood. In the present study, we investigated the catalytic properties of 4-α-GTase from the hyperthermophile Pyrococcus sp. ST04 (PSGT) in the presence of maltooligosaccharides of various lengths. Unlike 4-α-GTases in GH family 77, GH family 57 PSGT produced maltotriose in the early stage of reaction and preferred maltose and maltotriose over glucose as the acceptor. The kinetic analysis showed that maltotriose had the lowest KM value, which increased amylose degradation activity by 18.3-fold. Structural models of PSGT based on molecular dynamic simulation revealed two aromatic amino acids interacting with the substrate at the +2 and +3 binding sites, and the mutational study demonstrated they play a critical role in maltotriose binding. These results clarify the mode of action in carbohydrate utilization and explain acceptor binding mechanism of GH57 family 4-α-GTases in hyperthermophilic archaea.

Amyrel, a novel glucose-forming α-amylase from Drosophila with 4-α-glucanotransferase activity by disproportionation and hydrolysis of maltooligosaccharides

The α-amylase paralogue Amyrel present in true flies (Diptera Muscomorpha) has been classified as a glycoside hydrolase in CAZy family GH13 on the basis of its primary structure. Here we report that, in fact, Amyrel is currently unique amongst Animals as it possesses both the hydrolytic α-amylase activity (EC 3.2.1.1) and a 4-α-glucanotransferase (EC 2.4.1.25) transglycosylation activity. Amyrel reacts specifically on α-(1-4) glycosidic bonds of starch and related polymers but produces a complex mixture of maltooligosaccharides, in sharp contrast with canonical animal α-amylases. With model maltooligosaccharides G2 (maltose) to G7, the Amyrel reaction starts by a disproportionation leading to Gn-1 and Gn + 1 products, which become themselves substrates for new disproportionation cycles. As a result, all detectable odd- and even-numbered maltooligosaccharides at least up to G12 were observed. However, hydrolysis of these products proceeds simultaneously, as shown by p-nitrophenyl-tagged oligosaccharides and microcalorimetry, and upon prolonged reaction, glucose is the major end product followed by maltose. The main structural determinant of these atypical activities was found to be a Gly-His-Gly-Ala deletion in the so-called flexible loop bordering the active site. Indeed, engineering this deletion in pig pancreatic and D. melanogaster α-amylases results in reaction patterns similar to those of Amyrel. It is proposed that this deletion provides more freedom to the substrate for subsites occupancy and allows a less constrained action pattern resulting in versatile activities at the active site.

New groups of protein homologues in the α-amylase family GH57 closely related to α-glucan branching enzymes and 4-α-glucanotransferases

The glycoside hydrolase family GH57 is known as the second α-amylase family. Its main characteristics are as follows: (i) employing the retaining reaction mechanism; (ii) adopting the (β/α)₇-barrel (the incomplete TIM-barrel) with succeeding bundle of α-helices as the catalytic domain; (iii) sharing the five conserved sequence regions (CSRs) exhibiting the sequence fingerprints of the individual enzyme specificities; and (iv) using the catalytic machinery consisting of glutamic acid (the catalytic nucleophile) and aspartic acid (the proton donor) positioned at strands β4 (CSR-3) and β7 (CSR-4) of the (β/α)₇-barrel domain, respectively. Several years ago, a group of hypothetical proteins closely related to the specificity of α-amylase was revealed, the so-called α-amylase-like homologues, the members of which lack either one or even both catalytic residues. The novelty of the present study lies in delivering two additional groups of the "like" proteins that are homologues of α-glucan-branching enzyme (GBE) and 4-α-glucanotransferase (4AGT) specificities. Based on a recently published in silico analysis of more than 1600 family GH57 sequences, 13 GBE-like and 18 4AGT-like proteins from unique sources were collected and analyzed in a detail with respect to their taxonomical origin, sequence and structural features as well as evolutionary relationships. This in silico study could accelerate the efforts leading to experimental revealing the real function of the enzymes-like proteins in the α-amylase family GH57.