Structural dynamics of the transaminase active site revealed by the crystal structure of a co-factor free omega-transaminase from Vibrio fluvialis JS17

Expanding limits of artificial enzymes: unprecedented catalysis by an oxidase nanozyme in activating a structural protein for covalent crosslinking and conferring remarkable proteolytic resistance

Nature has endowed us with some complex enzymes capable of utilizing proteins as their substrates to generate functional proteins through post-translational modification. However, nanozymes' interplay with proteins as substrates is scarce, with their chemistry predominantly established using only small molecule substrates, featuring a significant gap in this area. Due to the huge prospects of nanozymes in biotechnological and therapeutic interventions, studies establishing the unexplored roles of nanozymes in the biological environments and their interplay beyond small molecule substrates warrant immediate attention. In this study, we unveil the unprecedented role of a Mn-based oxidase nanozyme (MnN) in activating a structural protein, collagen, and covalently crosslinking its tyrosine residues with only a trace amount of tannic acid (TA) without compromising its triple-helical structural integrity. While therapeutic applications demand materials prepared from collagen, the current chemical and physical crosslinking of collagen often presents significant challenges such as toxicity, denaturation, or high costs. MnN lucidly accomplishes crosslinking interplay at its 101 facets using oxygen as a co-substrate under mild conditions. This process takes advantage of MnN being active at mild acidic pH where collagen preferentially exists as a soluble triple helix (monomeric form), exposing functionalities and enhancing the crosslinking degree. Importantly, this reaction also confers 100% resistance to collagenase attack on the collagen tendon-derived biological material. The catalyzed TA-tyrosine linkage in the telopeptide region of collagen probably impedes the initial recognition step of collagenase, providing robust protection against its degradative action. Our study not only expands the repertoire of nanozymes' substrates beyond the existing library of small molecules but also establishes a significant step toward designing a gold standard for collagen crosslinking. With biomedical applications demanding biomaterials derived from protein scaffolds with preserved structural integrity, our investigation bridges the gap between nanozymes' chemistry and crosslinking proteins, opening exciting prospects for biomaterial development.

Rational design of transaminases based on comparative analysis of catalytically active and distance-free modes of the high-energy intermediate state

Bioproduction of chiral amines is limited by low transaminase (TA) activity on nonnatural substrates, leading to the need for protein engineering. To address the challenge of quickly and precisely identifying the engineering targets, a strategy was proposed based on analyzing the mode changes in the high-energy intermediate state (H-state) of the substrate-enzyme complex during catalysis. By substituting the residues with minimal structural changes in catalytically active mode (A-mode) and distance-free mode (F-mode) of the H-state complex with more conserved ones to stabilize it, a TA mutant M5(T295C/L387A/V436A) with 121.9-fold higher activity for synthesizing the (S)-Rivastigmine precursor (S)-1-(3-methoxyphenyl)ethylamine was created. The applicability of this strategy was also validated by engineering another TA for 1.52-fold higher activity and >99% selectivity toward (R)-3-amino-1-butanol biopreparation. The much higher stereoselectivity of the mutant compared with the wild type (28.3%) demonstrated that this strategy is not only advantageous in engineering enzyme activity but also applicable for modulating stereoselectivity.

Crystal Structure and Functional Characterization of Acetylornithine Aminotransferase from Corynebacterium glutamicum

The amino acids l-arginine and l-ornithine are widely used in animal feed and as health supplements and pharmaceutical compounds. In arginine biosynthesis, acetylornithine aminotransferase (AcOAT) uses pyridoxal-5'-phosphate (PLP) as a cofactor for amino group transfer. Here, we determined the crystal structures of the apo and PLP complex forms of AcOAT from Corynebacterium glutamicum (CgAcOAT). Our structural observations revealed that CgAcOAT undergoes an order-to-disorder conformational change upon binding with PLP. Additionally, we observed that unlike other AcOATs, CgAcOAT exists as a tetramer. Subsequently, we identified the key residues involved in PLP and substrate binding based on structural analysis and site-directed mutagenesis. This study might provide structural insights on CgAcOAT, which can be utilized for the development of improved l-arginine production enzymes.

Biocatalytic synthesis of chiral five-membered carbasugars intermediates utilizing CV2025 ω-transaminase from Chromobacterium violaceum

(S)-4-(Hydroxymethyl)cyclopent-2-enone is a key intermediate in the synthesis of chiral five-membered carbasugars, which can be used to synthesize a large number of pharmacologically relevant carbocyclic nucleosides. Herein, CV2025 ω-transaminase from Chromobacterium violaceum was selected based on substrate similarity to convert ((1S,4R)-4-aminocyclopent-2-enyl)methanol to (S)-4-(hydroxymethyl)cyclopent-2-enone. The enzyme was successfully cloned, expressed in Escherichia coli, purified and characterized. We show that it has R configuration preference in contrast with the conventional S preference. The highest activity was obtained below 60 °C and at pH 7.5. Cations Ca²⁺ and K⁺ enhanced activity by 21% and 13%, respectively. The conversion rate reached 72.4% within 60 min at 50 °C, pH 7.5, using 0.5 mM pyridoxal-5'-phosphate, 0.6 μM CV2025, and 10 mM substrate. The present study provides a promising strategy for preparing five-membered carbasugars economically and efficiently.

Comparison of Four Immobilization Methods for Different Transaminases

Biocatalytic syntheses often require unfavorable conditions, which can adversely affect enzyme stability. Consequently, improving the stability of biocatalysts is needed, and this is often achieved by immobilization. In this study, we aimed to compare the stability of soluble and immobilized transaminases from different species. A cysteine in a consensus sequence was converted to a single aldehyde by the formylglycine-generating enzyme for directed single-point attachment to amine beads. This immobilization was compared to cross-linked enzyme aggregates (CLEAs) and multipoint attachments to glutaraldehyde-functionalized amine- and epoxy-beads. Subsequently, the reactivity and stability (i.e., thermal, storage, and solvent stability) of all soluble and immobilized transaminases were analyzed and compared under different conditions. The effect of immobilization was highly dependent on the type of enzyme, the immobilization strategy, and the application itself, with no superior immobilization technique identified. Immobilization of HAGA-beads often resulted in the highest activities of up to 62 U/g beads, and amine beads were best for the hexameric transaminase from Luminiphilus syltensis. Furthermore, the immobilization of transaminases enabled its reusability for at least 10 cycles, while maintaining full or high activity. Upscaled kinetic resolutions (partially performed in a SpinChemTM reactor) resulted in a high conversion, maintained enantioselectivity, and high product yields, demonstrating their applicability.

Enzymatic synthesis of benzylisoquinoline alkaloids using a parallel cascade strategy and tyrosinase variants

Benzylisoquinoline alkaloid derived pharmaceuticals are widely applied in modern medicines. Recent studies on the microbial production of benzylisoquinolines have highlighted key biological syntheses towards these natural products. Routes to non-natural benzylisoquinolines have been less explored, particularly halogenated compounds which are more challenging. Here, we show the use of a tyrosinase, tyrosine decarboxylase, transaminase, and norcoclaurine synthase which are combined in a parallel cascade design, in order to generate halogenated benzylisoquinoline alkaloids in high enantiomeric excess. Notably, mutagenesis studies are applied to generate tyrosinase mutants, which enhance the acceptance of halogenated tyrosines for use in the biocatalytic cascades developed.

Comparative Proteomic Analysis for a Putative Pyridoxal Phosphate-Dependent Aminotransferase Required for Virulence in Acidovorax citrulli

Acidovorax citrulli (Ac) is the causative agent of bacterial fruit blotch disease in watermelon. Since resistant cultivars have not yet been developed, the virulence factors/mechanisms of Ac need to be characterized. This study reports the functions of a putative pyridoxal phosphate-dependent aminotransferase (PpdaAc) that transfers amino groups to its substrates and uses pyridoxal phosphate as a coenzyme. It was observed that a ppdaAc knockout mutant had a significantly reduced virulence in watermelon when introduced via germinated-seed inoculation as well as leaf infiltration. Comparative proteomic analysis predicted the cellular mechanisms related to PpdaAc. Apart from causing virulence, the PpdaAc may have significant roles in energy production, cell membrane, motility, chemotaxis, post-translational modifications, and iron-related mechanisms. Therefore, it is postulated that PpdaAc may possess pleiotropic effects. These results provide new insights into the functions of a previously unidentified PpdaAc in Ac.

Chemical and physical Chitosan modification for designing enzymatic industrial biocatalysts: How to choose the best strategy?

Chitosan is one of the most abundant natural polymer worldwide, and due to its inherent characteristics, its use in industrial processes has been extensively explored. Because it is biodegradable, biocompatible, non-toxic, hydrophilic, cheap, and has good physical-chemical stability, it is seen as an excellent alternative for the replacement of synthetic materials in the search for more sustainable production methodologies. Thus being, a possible biotechnological application of Chitosan is as a direct support for enzyme immobilization. However, its applicability is quite specific, and to overcome this issue, alternative pretreatments are required, such as chemical and physical modifications to its structure, enabling its use in a wider array of applications. This review aims to present the topic in detail, by exploring and discussing methods of employment of Chitosan in enzymatic immobilization processes with various enzymes, presenting its advantages and disadvantages, as well as listing possible chemical modifications and combinations with other compounds for formulating an ideal support for this purpose. First, we will present Chitosan emphasizing its characteristics that allow its use as enzyme support. Furthermore, we will discuss possible physicochemical modifications that can be made to Chitosan, mentioning the improvements obtained in each process. These discussions will enable a comprehensive comparison between, and an informed choice of, the best technologies concerning enzyme immobilization and the application conditions of the biocatalyst.

Enhancing PLP-Binding Capacity of Class-III ω-Transaminase by Single Residue Substitution

Transaminases are pyridoxal-5′-phosphate (PLP) binding enzymes, broadly studied for their potential industrial application. Their affinity for PLP has been related to their performance and operational stability and while significant differences in PLP requirements have been reported, the environment of the PLP-binding pocket is highly conserved. In this study, thorough analysis of the residue interaction network of three homologous transaminases Halomonas elongata (HeTA), Chromobacterium violaceum (CvTA), and Pseudomonas fluorescens (PfTA) revealed a single residue difference in their PLP binding pocket: an asparagine at position 120 in HeTA. N120 is suitably positioned to interact with an aspartic acid known to protonate the PLP pyridinium nitrogen, while the equivalent position is occupied by a valine in the other two enzymes. Three different mutants were constructed (HeTA-N120V, CvTA-V124N, and PfTA-V129N) and functionally analyzed. Notably, in HeTA and CvTA, the asparagine variants, consistently exhibited a higher thermal stability and a significant decrease in the dissociation constant (K_d) for PLP, confirming the important role of N120 in PLP binding. Moreover, the reaction intermediate pyridoxamine-5′-phosphate (PMP) was released more slowly into the bulk, indicating that the mutation also enhances their PMP binding capacity. The crystal structure of PfTA, elucidated in this work, revealed a tetrameric arrangement with the PLP binding sites near the subunit interface. In this case, the V129N mutation had a negligible effect on PLP-binding, but it reduced its temperature stability possibly destabilizing the quaternary structure.

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.

Structural basis of substrate recognition by a novel thermostable (S)-enantioselective ω-transaminase from Thermomicrobium roseum

Transaminases catalyze the reversible transfer reaction of an amino group between a primary amine and an α-keto acid, utilizing pyridoxal 5'-phosphate as a cofactor. ω-transaminases (ωTAs) recognize an amino group linked to a non-α carbon of amine substrates. Recently, a novel (S)-enantioselective ωTA from Thermomicrobium roseum (Tr-ωTA) was identified and its enzymatic activity reported. However, the detailed mechanism of (S)-enantioselective substrate recognition remained unclear. In this study, we determined the crystal structure of Tr-ωTA at 1.8 Å resolution to elucidate the mechanism underlying Tr-ωTA substrate (S)-enantioselectivity. A structural analysis of Tr-ωTA along with molecular docking simulations revealed that two pockets at the active site tightly restrict the size and orientation of functional groups of substrate candidates. Based on the structural information and docking simulation results, we propose a comprehensive catalytic mechanism of Tr-ωTA. The present study thus provides structural and functional insights into the (S)-enantioselectivity of Tr-ωTA.

Crystal structure of the apo form of a β-transaminase from Mesorhizobium sp. strain LUK

Pyridoxal 5'-phosphate (PLP)-dependent β-transaminases (βTAs) reversibly catalyze transamination reactions by recognizing amino groups linked to the β-carbon atoms of their substrates. Although several βTA structures have been determined as holo forms containing PLP, little is known about the effect of PLP on the conversion of the apo structure to the holo structure. We determined the crystal structure of the apo form of a βTA from Mesorhizobium sp. strain LUK at 2.2 Å resolution to elucidate how PLP affects the βTA structure. The structure revealed three major disordered regions near the active site. Structural comparison with the holo form also showed that the disordered regions in the apo form are ordered and partially adopt secondary structures in the holo form. These findings suggest that PLP incorporation into the active site contributes to the structural stability of the active site architecture, thereby forming the complete active site. Our results provide novel structural insights into the role of PLP in terms of active site formation.