A Novel highly thermostable branched-chain amino acid aminotransferase from the crenarchaeon Vulcanisaeta moutnovskia

Research progress on branched-chain amino acid aminotransferases

Branched-chain amino acid aminotransferases, widely present in natural organisms, catalyze bidirectional amino transfer between branched-chain amino acids and branched-chain α-ketoacids in cells. Branched-chain amino acid aminotransferases play an important role in the metabolism of branched-chain amino acids. In this paper, the interspecific evolution and biological characteristics of branched-chain amino acid aminotransferases are introduced, the related research of branched-chain amino acid aminotransferases in animals, plants, microorganisms and humans is summarized and the molecular mechanism of branched-chain amino acid aminotransferase is analyzed. It has been found that branched-chain amino acid metabolism disorders are closely related to various diseases in humans and animals and plants, such as diabetes, cardiovascular diseases, brain diseases, neurological diseases and cancer. In particular, branched-chain amino acid aminotransferases play an important role in the development of various tumors. Branched-chain amino acid aminotransferases have been used as potential targets for various cancers. This article reviews the research on branched-chain amino acid aminotransferases, aiming to provide a reference for clinical research on targeted therapy for various diseases and different cancers.

Metagenomic Type IV Aminotransferases Active toward (R)-Methylbenzylamine

Aminotransferases (ATs) are pyridoxal 5′-phosphate-dependent enzymes that catalyze the reversible transfer of an amino group from an amino donor to a keto substrate. ATs are promising biocatalysts that are replacing traditional chemical routes for the production of chiral amines. In this study, an in silico-screening of a metagenomic library isolated from the Curonian Lagoon identified 11 full-length fold type IV aminotransferases that were successfully expressed and used for substrate profiling. Three of them (AT-872, AT-1132, and AT-4421) were active toward (R)-methylbenzylamine. Purified proteins showed activity with L- and D-amino acids and various aromatic compounds such as (R)-1-aminotetraline. AT-872 and AT-1132 exhibited thermostability and retained about 55% and 80% of their activities, respectively, even after 24 h of incubation at 50 °C. Active site modeling revealed that AT-872 and AT-4421 have an unusual active site environment similar to the AT of Haliscomenobacter hydrossis, while AT-1132 appeared to be structurally related to the AT from thermophilic archaea Geoglobus acetivorans. Thus, we have identified and characterized PLP fold type IV ATs that were active toward both amino acids and a variety of (R)-amines.

A Puzzling Protein from Variovorax paradoxus Has a PLP Fold Type IV Transaminase Structure and Binds PLP without Catalytic Lysine

Effective biocatalysts for the synthesis of optically pure amines from keto precursors are highly required in organic synthesis. Transaminases are a large group of PLP-dependent enzymes, which can be utilized for production of chiral amines or amino acids. The bioinformatic approach previously made to search for promising transaminases with unusual characteristics surprisingly revealed mysterious genes in some Gram-negative bacteria, which products were annotated as aminotransferases, but they lacked the key catalytic lysine residue required for covalent binding of the PLP-cofactor. To address the question of which products these genes encode, we obtained the first structure of such a type of protein from the bacterium Variovorax paradoxus (VP5454) and provided its comprehensive analysis. We demonstrated that VP5454 has a typical aminotransferase fold and architecture of the active site, where substitution of the catalytic lysine with asparagine was observed. Despite that no covalent adduct can be formed between PLP and asparagine residue, using X-ray analysis and molecular dynamic (MD) simulation, we demonstrated that VP5454 is able to bind the PLP molecule in the transaminase in a specific manner, with PLP coordinated via its phosphate moiety. Taking into account a number of sequences homologous to VP5454 with a substituted catalytic lysine found in the genomes of various bacteria, we speculate that the proteins encoded by these sequences may have hidden functional roles.

Probing the role of the residues in the active site of the transaminase from Thermobaculum terrenum

Creating biocatalysts for (R)-selective amination effectively is highly desirable in organic synthesis. Despite noticeable progress in the engineering of (R)-amine activity in pyridoxal-5'-phosphate-dependent transaminases of fold type IV, the specialization of the activity is still an intuitive task, as there is poor understanding of sequence-structure-function relationships. In this study, we analyzed this relationship in transaminase from Thermobaculum terrenum, distinguished by expanded substrate specificity and activity in reactions with L-amino acids and (R)-(+)-1-phenylethylamine using α-ketoglutarate and pyruvate as amino acceptors. We performed site-directed mutagenesis to create a panel of the enzyme variants, which differ in the active site residues from the parent enzyme to a putative transaminase specific to (R)-primary amines. The variants were examined in the overall transamination reactions and half-reaction with (R)-(+)-1-phenylethylamine. A structural analysis of the most prominent variants revealed a spatial reorganization in the active sites, which caused changes in activity. Although the specialization to (R)-amine transaminase was not implemented, we succeeded in understanding the role of the particular active site residues in expanding substrate specificity of the enzyme. We showed that the specificity for (R)-(+)-1-phenylethylamine in transaminase from T. terrenum arises without sacrificing the specificity for L-amino acids and α-ketoglutarate and in consensus with it.

Structural insight into the substrate specificity of PLP fold type IV transaminases

Pyridoxal-5'-phosphate-dependent transaminases of fold type IV (class IV) are promising enzymes for (R)-selective amination of organic compounds. Transaminases of fold type IV exhibit either strict (R)-selectivity or (S)-selectivity that is implemented within geometrically similar active sites of different amino acid compositions. Based on substrate specificity, class IV comprises three large families of transaminases: (S)-selective branched-chain L-amino acid aminotransferases and (R)-selective D-amino acid aminotransferases and (R)-amine:pyruvate transaminases. In this review, we aim to analyze the substrate profiles and correlations between the substrate specificity and organization of the active site in transaminases from these structurally related families. New transaminases with an expanded substrate specificity are also discussed. An analysis of the structural features of substrate binding and comparisons of structural determinants of chiral discrimination between members of the class IV transaminases could be helpful in identifying new biocatalytically relevant enzymes as well as rational protein engineering.

Effects of His-tag on Catalytic Activity and Enantioselectivity of Recombinant Transaminases

Recombinant proteins were often expressed with His-tag to simplify the purification process. Among them, transaminase was mostly expressed with fusion tags and widely used in the production of numerous amino moieties. However, the existence of the His-tag has been reported to affect various properties of different recombinant enzymes, while the effect on transaminase was rarely studied. In this paper, we investigated the effect of His-tag on transaminase based on the various activities of 4-aminobutyrate-2-oxoglutarate transaminase (GabT) when it was expressed in vector pETDuet-1. We found that His-tag did not affect the enantioselectivity, but decreased the catalytic activity to different extents according to its existence and location. Native GabT maintained the highest catalytic activity; GabT with C-terminal His-tag showed slightly lower activity than native GabT but about 2.2-fold higher than GabT with N-terminal His-tag. Besides, other fusion tags like T7-tag and S-tag inserted between N-His-tag and GabT can relieve the decreasing effect of His-tag on GabT activity. Furthermore, whole cell catalytic activity of several transaminases was improved by deleting the N-terminal His-tag. This study provided a strategy for the efficient expression of recombinant transaminase with improved catalytic activity and might attract attention to the effect of His-tag on other enzymatic properties.

Biochemical and structural characterization of a highly active branched-chain amino acid aminotransferase from Pseudomonas sp. for efficient biosynthesis of chiral amino acids

Aminotransferases (ATs) are important biocatalysts for the synthesis of chiral amines because of their capability of introducing amino group into ketones or keto acids as well as their high enantioselectivity, high regioselectivity. Among all ATs, branched-chain amino acid aminotransferase (BCAT) can use branched-chain amino acids (BCAAs) as substrate, including _L-valine, _L-leucine, and _L-isoleucine, with α-ketoglutarate to form the corresponding α-keto acids and _L-glutamate. Alternatively, BCATs have been used for the biosynthesis of unnatural amino acids, such as _L-tert-leucine and _L-norvaline. In the present study, the BCAT from Pseudomonas sp. (PsBCAT) was cloned and expressed in Escherichia coli for biochemical and structural analyses. The optimal reaction temperature and pH of PsBCAT were 40 °C and 8.5, respectively. PsBCAT exhibited a comparatively broader substrate spectrum and showed remarkably high activity with bulked aliphatic _L-amino acids (k_cat up to 220 s^-1). Additionally, PsBCAT had activities with aromatic _L-amino acids, _L-histidine, _L-lysine, and _L-threonine. This substrate promiscuity is unique for the BCAT family and could prove useful in industrial applications. To analyze the catalytic mechanism of PsBCAT with the broad substrate spectrum, the crystal structure of PsBCAT was also determined. Based on the determined crystal structure, we found some differences in the organization of the substrate binding cavity, which may influence the substrate specificity of the enzyme. Finally, conjugated with the ornithine aminotransferase (OrnAT) to shift the reaction equilibrium towards the product formation, the coupled system was applied to the asymmetric synthesis of _L-tert-leucine and _L-norvaline. In summary, the structural and functional characteristics of PsBCAT were analyzed in detail, and this information will be conducive to industrial production of enantiopure chiral amino acids by aminotransferase.

Structural Consideration of the Working Mechanism of Fold Type I Transaminases From Eubacteria: Overt and Covert Movement

Transaminases (TAs) reversibly catalyze the transfer reaction of an amino group between an amino group donor and an amino group acceptor, using pyridoxal 5′-phosphate (PLP) as a cofactor. TAs are categorized according to the amino group position of the donor substrate and respective TAs recognize their own specific substrates. Over the past decade, a number of TA structures have been determined by X-ray crystallography. On the basis of the structural information, the detailed mechanism of substrate recognition by TAs has also been elucidated. In this review, fold type I TAs are addressed intensively. Comparative studies on structural differences between the apo and holo forms of fold type I TAs have demonstrated that regions containing the active site exhibit structural plasticity in the apo form, facilitating PLP insertion into the active site. In addition, given that TAs recognize two different kinds of substrates, they possess dual substrate specificity. It is known that spatial rearrangements of active site residues occur upon binding of the substrates. Intriguingly, positively charged residues are predominantly distributed at the active site cavity. The electric field generated by such charge distributions may attract negatively charged molecules, such as PLP and amino group acceptors, into the active site. Indeed, TAs show remarkable dynamics in diverse aspects. In this review, we describe the comprehensive working mechanism of fold type I TAs, with a focus on conformational changes.

Thermophiles and the applications of their enzymes as new biocatalysts

Ecological and efficient alternatives to industrial processes have sparked interest for using microorganisms and enzymes as biocatalysts. One of the difficulties is finding candidates capable of resisting the harsh conditions in which industrial processes usually take place. Extremophiles, microorganisms naturally found in "extreme" ecological niches, produce robust enzymes for bioprocesses and product development. Thermophiles like Geobacillus, Alyciclobacillus, Anoxybacillus, Pyrococcus and Thermoccocus are some of the extremophiles containing enzymes showing special promise for biocatalysis. Glutamate dehydrogenase used in food processes, laccases and xylanases in pulp and paper processes, nitrilases and transaminases for pharmaceutical drug synthesis and lipases present in detergents, are examples of the increasing use of enzymes for biocatalytic synthesis from thermophilic microorganisms. Some of these enzymes from thermophiles have been expressed as recombinant enzymes and are already in the market. Here we will review recent discoveries of thermophilic enzymes and their current and potential applications in industry.

Functional characterization of PLP fold type IV transaminase with a mixed type of activity from Haliangium ochraceum

Pyridoxal-5'-phosphate (PLP)-dependent transaminases are industrially important enzymes catalyzing the stereoselective amination of ketones and keto acids. Transaminases of PLP fold type IV are characterized by (R)- or (S)-stereoselective transfer of amino groups, depending on the substrate profile of the enzyme. PLP fold type IV transaminases include branched-chain amino acid transaminases (BCATs), D-amino acid transaminases and (R)-amine:pyruvate transaminases. Recently, transaminases with a mixed type of activity were identified and characterized. Here, we report biochemical and structural characterization of a transaminase from myxobacterium Haliangium ochraceum (Hoch3033), which is active towards keto analogs of branched-chain amino acids (specific substrates for BCATs) and (R)-(+)-α-methylbenzylamine (specific substrate for (R)-amine:pyruvate transaminases). The enzyme is characterized by an alkaline pH optimum (pH 10.0-10.5) and a tolerance to high salt concentrations (up to 2 M NaCl). The structure of Hoch3033 was determined at 2.35 Å resolution. The overall fold of the enzyme was similar to those of known enzymes of PLP fold type IV. The mixed type of activity of Hoch3033 was implemented within the BCAT-like active site. However, in the active site of Hoch3033, we observed substitutions of specificity-determining residues that are important for substrate binding in canonical BCATs. We suggest that these changes result in the loss of activity towards α-ketoglutarate and increase the affinity towards (R)-(+)-α-methylbenzylamine. These results complement our knowledge of the catalytic diversity of transaminases and indicate the need for further research to understand the structural basis of substrate specificity in these enzymes.

Biochemical and structural insights into PLP fold type IV transaminase from Thermobaculum terrenum

The high catalytic efficiency of enzymes under reaction conditions is one of the main goals in biocatalysis. Despite the dramatic progress in the development of more efficient biocatalysts by protein design, the search for natural enzymes with useful properties remains a promising strategy. The pyridoxal 5'-phosphate (PLP)-dependent transaminases represent a group of industrially important enzymes due to their ability to stereoselectively transfer amino groups between diverse substrates; however, the complex mechanism of substrate recognition and conversion makes the design of transaminases a challenging task. Here we report a detailed structural and kinetic study of thermostable transaminase from the bacterium Thermobaculum terrenum (TaTT) using the methods of enzyme kinetics, X-ray crystallography and molecular modeling. TaTT can convert L-branched-chain and L-aromatic amino acids as well as (R)-(+)-1-phenylethylamine at a high rate and with high enantioselectivity. The structures of TaTT in complex with the cofactor pyridoxal 5'-phosphate covalently bound to enzyme and in complex with its reduced form, pyridoxamine 5'-phosphate, were determined at resolutions of 2.19 Å and 1.5 Å, and deposited in the Protein Data Bank as entries 6GKR and 6Q8E, respectively. TaTT is a fold type IV PLP-dependent enzyme. In terms of structural similarity, the enzyme is close to known branched-chain amino acid aminotransferases, but differences in characteristic sequence motifs in the active site were observed in TaTT compared to canonical branched-chain amino acid aminotransferases, which can explain the improved binding of aromatic amino acids and (R)-(+)-1-phenylethylamine. This study has shown for the first time that high substrate specificity towards both various l-amino acids and (R)-primary amines can be implemented within one pyridoxal 5'-phosphate-dependent active site of fold type IV. These results complement our knowledge of the catalytic diversity of transaminases and indicate the need for further biochemical and bioinformatic studies to understand the sequence-structure-function relationship in these enzymes.

Properties of Bacterial and Archaeal Branched-Chain Amino Acid Aminotransferases

Branched-chain amino acid aminotransferases (BCATs) catalyze reversible stereoselective transamination of branched-chain amino acids (BCAAs) L-leucine, L-isoleucine, and L-valine. BCATs are the key enzymes of BCAA metabolism in all organisms. The catalysis proceeds through the ping-pong mechanism with the assistance of the cofactor pyridoxal 5'-phosphate (PLP). BCATs differ from other (S)-selective transaminases (TAs) in 3D-structure and organization of the PLP-binding domain. Unlike other (S)-selective TAs, BCATs belong to the PLP fold type IV and are characterized by the proton transfer on the re-face of PLP, in contrast to the si-specificity of proton transfer in fold type I (S)-selective TAs. Moreover, BCATs are the only (S)-selective enzymes within fold type IV TAs. Dual substrate recognition in BCATs is implemented via the "lock and key" mechanism without side-chain rearrangements of the active site residues. Another feature of the active site organization in BCATs is the binding of the substrate α-COOH group on the P-side of the active site near the PLP phosphate group. Close localization of two charged groups seems to increase the effectiveness of external aldimine formation in BCAT catalysis. In this review, the structure-function features and the substrate specificity of bacterial and archaeal BCATs are analyzed. These BCATs differ from eukaryotic ones in the wide substrate specificity, optimal temperature, and reactivity toward pyruvate as the second substrate. The prospects of biotechnological application of BCATs in stereoselective synthesis are discussed.

The ω-transaminase engineering database (oTAED): A navigation tool in protein sequence and structure space

The ω-Transaminase Engineering Database (oTAED) was established as a publicly accessible resource on sequences and structures of the biotechnologically relevant ω-transaminases (ω-TAs) from Fold types I and IV. The oTAED integrates sequence and structure data, provides a classification based on fold type and sequence similarity, and applies a standard numbering scheme to identify equivalent positions in homologous proteins. The oTAED includes 67 210 proteins (114 655 sequences) which are divided into 169 homologous families based on global sequence similarity. The 44 and 39 highly conserved positions which were identified in Fold type I and IV, respectively, include the known catalytic residues and a large fraction of glycines and prolines in loop regions, which might have a role in protein folding and stability. However, for most of the conserved positions the function is still unknown. Literature information on positions that mediate substrate specificity and stereoselectivity was systematically examined. The standard numbering schemes revealed that many positions which have been described in different enzymes are structurally equivalent. For some positions, multiple functional roles have been suggested based on experimental data in different enzymes. The proposed standard numbering schemes for Fold type I and IV ω-TAs assist with analysis of literature data, facilitate annotation of ω-TAs, support prediction of promising mutation sites, and enable navigation in ω-TA sequence space. Thus, it is a useful tool for enzyme engineering and the selection of novel ω-TA candidates with desired biochemical properties.

Identification of branched-chain amino acid aminotransferases active towards (R)-(+)-1-phenylethylamine among PLP fold type IV transaminases

New class IV transaminases with activity towards L-Leu, which is typical of branched-chain amino acid aminotransferases (BCAT), and with activity towards (R)-(+)-1-phenylethylamine ((R)-PEA), which is typical of (R)-selective (R)-amine:pyruvate transaminases, were identified by bioinformatics analysis, obtained in recombinant form, and analyzed. The values of catalytic activities in the reaction with L-Leu and (R)-PEA are comparable to those measured for characteristic transaminases with the corresponding specificity. Earlier, (R)-selective class IV transaminases were found to be active, apart from (R)-PEA, only with some other (R)-primary amines and D-amino acids. Sequences encoding new transaminases with mixed type of activity were found by searching for changes in the conserved motifs of sequences of BCAT by different bioinformatics tools.

Enzymatic asymmetric synthesis of chiral amino acids

This review summarizes the progress achieved in the enzymatic asymmetric synthesis of chiral amino acids from prochiral substrates.