Screening and Comparative Characterization of Microorganisms from Iranian Soil Samples Showing ω-Transaminase Activity toward a Plethora of Substrates

Synthesis of (S)- and (R)-β-Tyrosine by Redesigned Phenylalanine Aminomutase

Phenylalanine aminomutase from Taxus chinensis (TchPAM) is employed in the biosynthesis of the widely used antitumor drug paclitaxel. TchPAM has received substantial attention due to its strict enantioselectivity towards (R)-β-phenylalanine, in contrast to the bacterial enzymes classified as EC 5.4.3.11 which are (S)-selective for this substrate. However, the understanding of the isomerization mechanism of the reorientation and rearrangement reactions in TchPAM might support and promote further research on expanding the scope of the substrate and thus the establishment of large-scale production of potential synthesis for drug development. Upon conservation analysis, computational simulation, and mutagenesis experiments, we report a mutant from TchPAM, which can catalyze the amination reaction of trans-p-hydroxycinnamic acid to (R)- and (S)-β-tyrosine. We propose a mechanism for the function of the highly conserved residues L179, N458, and Q459 in the active site of TchPAM. This work highlights the importance of the hydrophobic residues in the active site, including the residues L104, L108, and I431, for maintaining the strict enantioselectivity of TchPAM, and the importance of these residues for substrate specificity and activation by altering the substrate binding position or varying the location of neighboring residues. Furthermore, an explanation of (R)-selectivity in TchPAM is proposed based on the mutagenesis study of these hydrophobic residues. In summary, these studies support the future exploitation of the rational engineering of corresponding enzymes with MIO moiety (3,5-dihydro-5-methylidene-4H-imidazole-4-one) such as ammonia lyases and aminomutases of aromatic amino acids.

Screening a Strain of Klebsiella sp. O852 and the Optimization of Fermentation Conditions for Trans-Dihydrocarvone Production

Flavors and fragrances have high commercial value in the food, cosmetic, chemical and pharmaceutical industries. It is interesting to investigate the isolation and characterization of new microorganisms with the ability to produce flavor compounds. In this study, a new strain of Klebsiella sp. O852 (accession number CCTCC M2020509) was isolated from decayed navel orange (Citrus sinensis (L.) Osbeck), which was proved to be capable of converting limonene to trans-dihydrocarvone. Besides, the optimization of various reaction parameters to enhance the trans-dihydrocarvone production in shake flask was performed for Klebsiella sp. O852. The results showed that the yield of trans-dihydrocarvone reached up to 1 058 mg/L when Klebsiella sp. O852 was incubated using LB-M medium for 4 h at 36 °C and 150 rpm, and the biotransformation process was monitored for 36 h after adding 1680 mg/L limonene/ethanol (final ethanol concentration of 0.8% (v/v)). The content of trans-dihydrocarvone increased 16 times after optimization. This study provided a basis and reference for producing trans-dihydrocarvone by biotransformation.

Growth optimization and identification of an ω-transaminase by a novel native PAGE activity staining method in a Bacillus sp. strain BaH isolated from Iranian soil

ω-Transaminases’ (ω-TAs) importance for synthesizing chiral amines led to the development of different methods to quickly identify and characterize new sources of these enzymes. Here we describe the optimization of growth and induction of such an enzyme in a wild type strain of Bacillus sp. strain BaH (IBRC-M 11337) isolated from Iranian soil in shaking flasks by the response surface methodology (RSM). Optimum conditions were set in a multiplexed bench-top bioreactor system (Sixfors). ω-TA activity of obtained biomass was checked by an innovative efficient colorimetric assay for localizing ω-TAs in crude extracts on acrylamide gel by using ortho-xylylenediamine (OXD) as amino donor. The application of the established OXD assay is thereby expanded from high-throughput activity screenings and colony-based screenings of heterologously expressed mutants to a direct identification of ω-TAs in wild-type strains: This assay can be used to detect the protein band of the respective enzyme in crude extracts of novel isolates by visual inspection of native PAGEs without any upstream protein purification, thus enabling subsequent further investigations of a newly discovered enzyme directly from the crude extract.

Editorial Catalysts: Special Issue on Novel Enzyme and Whole-Cell Biocatalysts

n/a

Counterbalance of Stability and Activity Observed for Thermostable Transaminase from Thermobaculum terrenum in the Presence of Organic Solvents

Pyridoxal-5’-phosphate-dependent transaminases catalyze stereoselective amination of organic compounds and are highly important for industrial applications. Catalysis by transaminases often requires organic solvents to increase the solubility of reactants. However, natural transaminases are prone to inactivation in the presence of water-miscible organic solvents. Here, we present the solvent tolerant thermostable transaminase from Thermobaculum terrenum (TaTT) that catalyzes transamination between L-leucine and alpha-ketoglutarate with an optimum at 75 °C and increases the activity ~1.8-fold upon addition of 15% dimethyl sulfoxide or 15% methanol at high but suboptimal temperature, 50 °C. The enhancement of the activity correlates with a decrease in the thermal denaturation midpoint temperature. The blue-shift of tryptophan fluorescence suggested that solvent molecules penetrate the hydration shell of the enzyme. Analysis of hydrogen bonds in the TaTT dimer revealed a high number of salt bridges and surface hydrogen bonds formed by backbone atoms. The latter are sensitive to the presence of organic solvents; they rearrange, conferring the relaxation of some constraints inherent to a thermostable enzyme at low temperatures. Our data support the idea that the counterbalance of stability and activity is crucial for the catalysis under given conditions; the obtained results may be useful for fine-tuning biocatalyst efficiency.

Transaminases for industrial biocatalysis: novel enzyme discovery

Transaminases (TAms) are important enzymes for the production of chiral amines for the pharmaceutical and fine chemical industries. Novel TAms for use in these industries have been discovered using a range of approaches, including activity-guided methods and homologous sequence searches from cultured microorganisms to searches using key motifs and metagenomic mining of environmental DNA libraries. This mini-review focuses on the methods used for TAm discovery over the past two decades, analyzing the changing trends in the field and highlighting the advantages and drawbacks of the respective approaches used. This review will also discuss the role of protein engineering in the development of novel TAms and explore possible directions for future TAm discovery for application in industrial biocatalysis. KEY POINTS: • The past two decades of TAm enzyme discovery approaches are explored. • TAm sequences are phylogenetically analyzed and compared to other discovery methods. • Benefits and drawbacks of discovery approaches for novel biocatalysts are discussed. • The role of protein engineering and future discovery directions is highlighted.