Safety evaluation of amylomaltase from Thermus aquaticus

Production of Large-Ring Cyclodextrins by Amylomaltases

Amylomaltase is a well-known glucan transferase that can produce large ring cyclodextrins (LR-CDs) or so-called cycloamyloses via cyclization reaction. Amylomaltases have been found in several microorganisms and their optimum temperatures are generally around 60–70 °C for thermostable amylomaltases and 30–45 °C for the enzymes from mesophilic bacteria and plants. The optimum pHs for mesophilic amylomaltases are around pH 6.0–7.0, while the thermostable amylomaltases are generally active at more acidic conditions. Size of LR-CDs depends on the source of amylomaltases and the reaction conditions including pH, temperature, incubation time, and substrate. For example, in the case of amylomaltase from Corynebacterium glutamicum, LR-CD productions at alkaline pH or at a long incubation time favored products with a low degree of polymerization. In this review, we explore the synthesis of LR-CDs by amylomaltases, structural information of amylomaltases, as well as current applications of LR-CDs and amylomaltases.

Heterologous expression of 4α-glucanotransferase: overproduction and properties for industrial applications

4α-Glucanotransferase (4α-GTase) is unique in its ability to form cyclic oligosaccharides, some of which are of industrial importance. Generally, low amount of enzymes is produced by or isolated from their natural sources: animals, plants, and microorganisms. Heterologous expressions of these enzymes, in an attempt to increase their production for applicable uses, have been widely studied since 1980s; however, the expressions are mostly performed in the prokaryotic bacteria, mostly Escherichia coli. Site-directed mutagenesis has added more value to these expressed enzymes to display the desired properties beneficial for their applications. The search for further suitable properties for food application leads to an extended research in expression by another group of host organism, the generally-recognized as safe host including the Bacillus and the eukaryotic yeast systems. Herein, our review focuses on two types of 4α-GTase: the cyclodextrin glycosyltransferase and amylomaltase. The updated studies on the general structure and properties of the two enzymes with emphasis on heterologous expression, mutagenesis for property improvement, and their industrial applications are provided.

Amylomaltases in Extremophilic Microorganisms

Amylomaltases (4-α-glucanotransferases, E.C. 2.4.1.25) are enzymes which can perform a double-step catalytic process, resulting in a transglycosylation reaction. They hydrolyse glucosidic bonds of α-1,4'-d-glucans and transfer the glucan portion with the newly available anomeric carbon to the 4'-position of an α-1,4'-d-glucan acceptor. The intramolecular reaction produces a cyclic α-1,4'-glucan. Amylomaltases can be found only in prokaryotes, where they are involved in glycogen degradation and maltose metabolism. These enzymes are being studied for possible biotechnological applications, such as the production of (i) sugar substitutes; (ii) cycloamyloses (molecules larger than cyclodextrins), which could potentially be useful as carriers and encapsulating agents for hydrophobic molecules and also as effective protein chaperons; and (iii) thermoreversible starch gels, which could be used as non-animal gelatin substitutes. Extremophilic prokaryotes have been investigated for the identification of amylomaltases to be used in the starch modifying processes, which require high temperatures or extreme conditions. The aim of this article is to present an updated overview of studies on amylomaltases from extremophilic Bacteria and Archaea, including data about their distribution, activity, potential industrial application and structure.

In silico analysis of family GH77 with focus on amylomaltases from borreliae and disproportionating enzymes DPE2 from plants and bacteria

The CAZy glycoside hydrolase (GH) family GH77 is a monospecific family containing 4-α-glucanotransferases that if from prokaryotes are known as amylomaltases and if from plants including algae are known as disproportionating enzymes (DPE). The family GH77 is a member of the α-amylase clan GH-H. The main difference discriminating a GH77 4-α-glucanotransferase from the main GH13 α-amylase family members is the lack of domain C succeeding the catalytic (β/α)8-barrel. Of more than 2400 GH77 members, bacterial amylomaltases clearly dominate with more than 2300 sequences; the rest being approximately equally represented by Archaea and Eucarya. The main goal of the present study was to deliver a detailed bioinformatics study of family GH77 (416 collected sequences) focused on amylomaltases from borreliae (containing unique sequence substitutions in functionally important positions) and plant DPE2 representatives (possessing an insert of ~140 residues between catalytic nucleophile and proton donor). The in silico analysis reveals that within the genus of Borrelia a gradual evolutionary transition from typical bacterial Thermus-like amylomaltases may exist to family-GH77 amylomaltase versions that currently possess progressively mutated the most important and otherwise invariantly conserved positions. With regard to plant DPE2, a large group of bacterial amylomaltases represented by the amylomaltase from Escherichia coli with a longer N-terminus was identified as a probable intermediary connection between Thermus-like and DPE2-like (existing also among bacteria) family GH77 members. The presented results concerning both groups, i.e. amylomaltases from borreliae and plant DPE2 representatives (with their bacterial counterpart), may thus indicate the direction for future experimental studies.

Overproduction of a thermostable 4-α-glucanotransferase by codon optimization at N-terminus region

BACKGROUND

4-α-Glucanotransferases are useful enzymes to modify starch owing to their transglycosylation activity. In this study, codon optimizations were conducted to overproduce a thermostable 4-α-glucanotransferase from Thermus thermophilus (TTαGT).

RESULTS

Two variants, termed TTαGT-P4CCG and TTαGT-mut6, were constructed, which have the optimized codon at the first rare codon and optimized codons at all six chosen rare codons at the N-terminus of TTαGT, respectively. In the Escherichia coli system, the expression of both optimized genes was enhanced by about 100-fold relative to that of the original gene, whereas all six mutated codons contributed to the overall enhancement of TTαGT production in Bacillus subtilis. On the basis of the αGTase activity of the crude cell extracts, relative activities of 1:2.9:5.8 were determined for TTαGT, TTαGT-P4CCG and TTαGT-mut6, respectively, in B. subtilis. In addition, the activity of TTαGT-mut6 from B. subtilis grown without antibiotics was as much as that with the antibiotics. Finally, after heat treatment, the specific activity of TTαGT-mut6 from B. subtilis was 1.5-fold greater than that from E. coli.

CONCLUSION

The codon-optimized TTαGT that was produced in a GRAS microorganism, B. subtilis, without the selection antibiotics is potentially useful in the food industry as a food-grade enzyme.

Impact of food processing on the safety assessment for proteins introduced into biotechnology-derived soybean and corn crops

The food safety assessment of new agricultural crop varieties developed through biotechnology includes evaluation of the proteins introduced to impart desired traits. Safety assessments can include dietary risk assessments similar to those performed for chemicals intentionally, or inadvertently added to foods. For chemicals, it is assumed they are not degraded during processing of the crop into food fractions. For introduced proteins, the situation can be different. Proteins are highly dependent on physical forces in their environment to maintain appropriate three-dimensional structure that supports functional activity. Food crops such as corn and soy are not consumed raw but are extensively processed into various food fractions. During processing, proteins in corn and soy are subjected to harsh environmental conditions that drastically change the physical forces leading to denaturation and loss of protein function. These conditions include thermal processing, changes in pH, reducing agents, mechanical shearing etc. Studies have shown that processing of introduced proteins such as enzymes that impart herbicide tolerance or proteins that control insect pests leads to a complete loss of functional activity. Thus, dietary exposure to functionally active proteins in processed food products can be negligible and below levels of any safety concerns.