Biocontrolled formal inversion or retention of L-α-amino acids to enantiopure (R)- or (S)-hydroxyacids

One-pot enantioselective synthesis of chiral phenyllactic acids by combining stereocomplementary d- and l-lactate dehydrogenases with multi-enzyme expression fine-tuning

Chiral phenyllactic acid (PLA) is a new type of antiseptic agent and a valuable precursor for active ingredients in pharmaceuticals and agrochemicals. In this study, we designed a multi-enzyme cascade that combined stereocomplementary d- and l-lactate dehydrogenases with threonine aldolase, phenylserine dehydratase, and formate dehydrogenase for the one-pot conversion of achiral glycine and benzaldehyde to synthesize d-PLA and l-PLA. To overcome the imbalance of multi-enzymes in a single cell, two enzyme modules, overexpressing four enzymes, were assembled in Escherichia coli cells to construct whole-cell catalysis systems (WCCSs). Furthermore, by optimizing reaction conditions and components, recombinant E. coli (WCCS 26) was able to produce 100 mM d-PLA with >99 % ee using a fed-batch strategy, while E. coli (WCCS 60) produced 47.2 mM l-PLA with >99 % ee. This study presents a sustainable and efficient method for synthesizing chiral PLAs from food-grade achiral starting materials.

Glyoxalase-based toolbox for the enantioselective synthesis of α-hydroxy carboxylic acids

We report highly enantioselective synthesis of L-α-hydroxy carboxylic acids (L-αHCAs) via enzymatic intramolecular Cannizzaro reaction of (hetero)aryl glyoxals in the presence of glutathione-independent human glyoxalase DJ-1. Combined with the optimized synthesis of D-αHCAs using glyoxalases I and II, this approach offers a general, scalable and operationally simple access to both enantiomers of α-hydroxy acids in moderate to excellent yields with uniformly high enantioselectivity.

A Novel Strategy for Whole-Cell Biotransformation Enabling Simultaneous l-Phenyllactic Acid Production and Coenzyme Regeneration

l-Phenyllactic acid (l-PLA) is a small molecular organic acid that exhibits a powerful capacity for inhibition against foodborne pathogens. In this work, we developed a new cost-effective and environmentally friendly process for the biosynthesis of l-PLA. This strategy designed a novel whole-cell biotransformation system employing two heterologous enzymes, namely, phenylalanine dehydrogenase (PheDH) and l-hydroxyisocaproate dehydrogenase (l-HicDH). The novelty of this strategy lies in the first-time utilization of these two enzymes, which not only enables cascade catalysis for the production of l-PLA but also facilitates the regeneration of the coenzymes (NAD+/NADH) using only two enzymes rather than introducing more heterologous enzymes to the system. Consequently, this strategy can effectively simplify the biosynthesis process of l-PLA and minimize production costs. The initial l-PLA yield using this process achieved 2.53 ± 0.07 g/L. Furthermore, through meticulous optimization of the parameters for inducible enzyme expression and l-PLA biosynthesis, the l-PLA yield was successfully increased to 4.68 ± 0.04 g/L with a yield rate of 64.54 ± 0.29%. Moreover, this novel strategy is versatile in the biosynthesis of other organic acids, which can be achieved by easily modulating the combinations of substrates and enzymes.

Rational Design of Artificial Biofilms as Sustainable Supports for Whole-Cell Catalysis Through Integrating Extra- and Intracellular Catalysis

Biofilms are promising candidates for sustainable bioprocessing applications. This work presents a rational design of biofilm catalysts by integrating extra- and intracellular catalysis systems with optimized substrate channeling to realize efficient multistep biosynthesis. An assembly of four enzymes in a "three-in-one" structure was achieved by rationally placing the enzymes on curli nanofibers, the cell surface, and inside cells. The catalytic efficiency of the biofilm catalysts was over 2.8 folds higher than that of the control whole-cell catalysis when the substrate benzaldehyde was fed at 100 mm. The highest yield of d-phenyllactic acid catalyzed by biofilm catalysts under optimized conditions was 102.19 mm, also much higher than that of the control catalysis test (52.29 mm). The results demonstrate that engineered biofilms are greatly promising in integrating extra- and intracellular catalysis, illustrating great potentials of rational design in constructing biofilm catalysts as sustainable supports for whole-cell catalysis.

Expression and activity of heterologous hydroxyisocaproate dehydrogenases in Synechocystis sp. PCC 6803 ΔhoxYH

Exploiting light to drive redox reactions is currently a hot topic since light is considered as an environmentally friendly source of energy. Consequently, cyanobacteria, which can use light e.g., for generating NADPH, are in the focus of research. Previously, it has been shown that various heterologous redox enzymes could be expressed in these microorganisms. Here we demonstrated the successful inducer-free expression of α-keto-acid dehydrogenases (L-HicDH and D-HicDH) from Lactobacillus confusus DSM 20196 and Lactobacillus paracasei DSM 20008 in Synechocystis sp. PCC 6803 ΔhoxYH mutant using replicative plasmids. While the L-HicDH showed poor activity limited by the amount of expressed enzyme, the D-HicDH was applied both in vivo and in vitro, transforming the selected α-keto acids to the corresponding optically pure (R)-α-hydroxy acids (ee >99%) in up to 53% and 90% conversion, respectively.

Chemoenzymatic Production of Enantiocomplementary 2-Substituted 3-Hydroxycarboxylic Acids from L-α-Amino Acids

A two-enzyme cascade reaction plus in situ oxidative decarboxylation for the transformation of readily available canonical and non-canonical L-α-amino acids into 2-substituted 3-hydroxy-carboxylic acid derivatives is described. The biocatalytic cascade consisted of an oxidative deamination of L-α-amino acids by an L-α-amino acid deaminase from Cosenzaea myxofaciens, rendering 2-oxoacid intermediates, with an ensuing aldol addition reaction to formaldehyde, catalyzed by metal-dependent (R)- or (S)-selective carboligases namely 2-oxo-3-deoxy-l-rhamnonate aldolase (YfaU) and ketopantoate hydroxymethyltransferase (KPHMT), respectively, furnishing 3-substituted 4-hydroxy-2-oxoacids. The overall substrate conversion was optimized by balancing biocatalyst loading and amino acid and formaldehyde concentrations, yielding 36-98% aldol adduct formation and 91- 98% ee for each enantiomer. Subsequent in situ follow-up chemistry via hydrogen peroxide-driven oxidative decarboxylation afforded the corresponding 2-substituted 3-hydroxycarboxylic acid derivatives.

P450Jα : A New, Robust and α-Selective Fatty Acid Hydroxylase Displaying Unexpected 1-Alkene Formation

Oxyfunctionalization of fatty acids (FAs) is a key step in the design of novel synthetic pathways for biobased/biodegradable polymers, surfactants and fuels. Here, we show the isolation and characterization of a robust FA α-hydroxylase (P450_Jα ) which catalyses the selective conversion of a broad range of FAs (C6:0-C16:0) and oleic acid (C18:1) with H₂ O₂ as oxidant. Under optimized reaction conditions P450_Jα yields α-hydroxy acids all with >95 % regioselectivity, high specific activity (up to 15.2 U mg^-1 ) and efficient coupling of oxidant to product (up to 85 %). Lauric acid (C12:0) turned out to be an excellent substrate with respect to productivity (TON=394 min^-1 ). On preparative scale, conversion of C12:0 reached 83 % (0.9 g L^-1 ) when supplementing H₂ O₂ in fed-batch mode. Under similar conditions P450_Jα allowed further the first biocatalytic α-hydroxylation of oleic acid (88 % conversion on 100 mL scale) at high selectivity and in good yields (1.1 g L^-1 ; 79 % isolated yield). Unexpectedly, P450_Jα displayed also 1-alkene formation from shorter chain FAs (≤C10:0) showing that oxidative decarboxylation is more widely distributed across this enzyme family than reported previously.

Recent advances in biocatalytic derivatization of L-tyrosine

L-Tyrosine is an aromatic, polar, non-essential amino acid that contains a highly reactive α-amino, α-carboxyl, and phenolic hydroxyl group. Derivatization of these functional groups can produce chemicals, such as L-3,4-dihydroxyphenylalanine, tyramine, 4-hydroxyphenylpyruvic acid, and benzylisoquinoline alkaloids, which are widely employed in the pharmaceutical, food, and cosmetics industries. In this review, we summarize typical L-tyrosine derivatizations catalyzed by enzymatic biocatalysts, as well as the strategies and challenges associated with their production processes. Finally, we discuss future perspectives pertaining to the enzymatic production of L-tyrosine derivatives.Key points• Summary of recent advances in enzyme-catalyzed L-tyrosine derivatization.• Highlights of relevant strategies involved in L-tyrosine derivatives biosynthesis.• Future perspectives on industrial applications of L-tyrosine derivatization.

Overview on Multienzymatic Cascades for the Production of Non-canonical α-Amino Acids

The 22 genetically encoded amino acids (AAs) present in proteins (the 20 standard AAs together with selenocysteine and pyrrolysine), are commonly referred as proteinogenic AAs in the literature due to their appearance in ribosome-synthetized polypeptides. Beyond the borders of this key set of compounds, the rest of AAs are generally named imprecisely as non-proteinogenic AAs, even when they can also appear in polypeptide chains as a result of post-transductional machinery. Besides their importance as metabolites in life, many of D-α- and L-α-"non-canonical" amino acids (NcAAs) are of interest in the biotechnological and biomedical fields. They have found numerous applications in the discovery of new medicines and antibiotics, drug synthesis, cosmetic, and nutritional compounds, or in the improvement of protein and peptide pharmaceuticals. In addition to the numerous studies dealing with the asymmetric synthesis of NcAAs, many different enzymatic pathways have been reported in the literature allowing for the biosynthesis of NcAAs. Due to the huge heterogeneity of this group of molecules, this review is devoted to provide an overview on different established multienzymatic cascades for the production of non-canonical D-α- and L-α-AAs, supplying neophyte and experienced professionals in this field with different illustrative examples in the literature. Whereas the discovery of new or newly designed enzymes is of great interest, dusting off previous enzymatic methodologies by a "back and to the future" strategy might accelerate the implementation of new or improved multienzymatic cascades.

Crystal structure of (R,S)-2-hy-droxy-4-(methyl-sulfan-yl)butanoic acid

The title compound, a major animal feed supplement, abbreviated as HMTBA and alternatively called dl-me-thio-nine hy-droxy analogue, C5H10O3S, (I), was isolated in pure anhydrous monomeric form. The melting point is 302.5 K and the compound crystallizes in the monoclinic space group P21/c, with two conformationally non-equivalent mol-ecules [(I A ) and (I B )] in the asymmetric unit. The crystal structure is formed by alternating polar and non-polar layers running along the bc plane and features an extensive hydrogen-bonding network within the polar layers. The Hirshfeld surface analysis revealed a significant contribution of non-polar H⋯H and H⋯S inter-actions to the packing forces for both mol-ecules.

Active Expression of Membrane-Bound L-Amino Acid Deaminase from Proteus mirabilis in Recombinant Escherichia coli by Fusion with Maltose-Binding Protein for Enhanced Catalytic Performance

L-amino acid deaminases (LAADs) are membrane flavoenzymes that catalyze the deamination of neutral and aromatic L-amino acids to α-keto acids and ammonia. LAADs can be used to develop many important biotechnological applications. However, the transmembrane α-helix of LAADs restricts its soluble active expression and purification from a heterologous host, such as Escherichia coli. Herein, through fusion with the maltose-binding protein (MBP) tag, the recombinant E. coli BL21 (DE3)/pET-21b-MBP-PmLAAD was constructed and the LAAD from Proteus mirabilis (PmLAAD) was actively expressed as a soluble protein. After purification, the purified MBP-PmLAAD was obtained. Then, the catalytic activity of the MBP-PmLAAD fusion protein was determined and compared with the non-fused PmLAAD. After fusion with the MBP-tag, the catalytic efficiency of the MBP-PmLAAD cell lysate was much higher than that of the membrane-bound PmLAAD whole cells. The soluble MBP-PmLAAD cell lysate catalyzed the conversion of 100 mM L-phenylalanine (L-Phe) to phenylpyruvic acid (PPA) with a 100% yield in 6 h. Therefore, the fusion of the MBP-tag not only improved the soluble expression of the PmLAAD membrane-bound protein, but also increased its catalytic performance.

Asymmetric assembly of high-value α-functionalized organic acids using a biocatalytic chiral-group-resetting process

The preparation of α-functionalized organic acids can be greatly simplified by adopting a protocol involving the catalytic assembly of achiral building blocks. However, the enzymatic assembly of small amino acids and aldehydes to form numerous α-functionalized organic acids is highly desired and remains a significant challenge. Herein, we report an artificially designed chiral-group-resetting biocatalytic process, which uses simple achiral glycine and aldehydes to synthesize stereodefined α-functionalized organic acids. This cascade biocatalysis comprises a basic module and three different extender modules and operates in a modular assembly manner. The engineered Escherichia coli catalysts, which contained different module(s), provide access to α-keto acids, α-hydroxy acids, and α-amino acids with excellent conversion and enantioselectivities. Therefore, this biocatalytic process provides an attractive strategy for the conversion of low-cost achiral starting materials to high-value α-functionalized organic acids.

Alpha-functionalized organic acids are building blocks of many bioactive compounds. Here, the authors developed a toolbox-like, modular set of enzymes that reset chiral groups, turning achiral glycine and simple aldehydes into stereodefined α-keto acids, α-hydroxy acids, and α-amino acids.

Bioproduction of Benzylamine from Renewable Feedstocks via a Nine-Step Artificial Enzyme Cascade and Engineered Metabolic Pathways

Production of chemicals from renewable feedstocks has been an important task for sustainable chemical industry. Although microbial fermentation has been widely employed to produce many biochemicals, it is still very challenging to access non-natural chemicals. Two methods (biotransformation and fermentation) have been developed for the first bio-derived synthesis of benzylamine, a commodity non-natural amine with broad applications. Firstly, a nine-step artificial enzyme cascade was designed by biocatalytic retrosynthetic analysis and engineered in recombinant E. coli LZ243. Biotransformation of l-phenylalanine (60 mm) with the E. coli cells produced benzylamine (42 mm) in 70 % conversion. Importantly, the cascade biotransformation was scaled up to 100 mL and benzylamine was successfully isolated in 57 % yield. Secondly, an artificial biosynthesis pathway to benzylamine from glucose was developed by combining the nine-step cascade with an enhanced l-phenylalanine synthesis pathway in cells. Fermentation with E. coli LZ249 gave benzylamine in 4.3 mm concentration from glucose. In addition, one-pot syntheses of several useful benzylamines from the easily available styrenes were achieved, representing a new type of alkene transformation by formal oxidative cleavage and reductive amination.

Isolation and Synthesis of Misszrtine A: A Novel Indole Alkaloid From Marine Sponge-Associated Aspergillus sp. SCSIO XWS03F03

A novel indole alkaloid, misszrtine A (1), was isolated from marine sponge-derived fungus Aspergillus sp. SCSIO XWS03F03. The planar structure of 1 was assigned by analysis of spectroscopic data, the absolute configuration of which was unambiguously determined by total synthesis. Compound 1 represents the first example of N-isopentenyl tryptophan methyl ester with a phenylpropanoic amide arm, which exhibited a potent antagonistic activity on HL60 (IC₅₀ = 3.1 μM) and LNCaP (IC₅₀ = 4.9 μM) cell lines. Bioactivity evaluation reveals that functional group on indole nitrogen of 1 has a great effect on its cytotoxity, which provides a mean to probe the structure-activity relationships of 1.

Artificial Biocatalytic Linear Cascades to Access Hydroxy Acids, Lactones, and α- and β-Amino Acids

α-, β-, and ω-Hydroxy acids, amino acids, and lactones represent common building blocks and intermediates for various target molecules. This review summarizes artificial cascades published during the last 10 years leading to these products. Renewables as well as compounds originating from fossil resources have been employed as starting material. The review provides an inspiration for new cascade designs and may be the basis to design variations of these cascades starting either from alternative substrates or extending them to even more sophisticated products.

Enantioselective biocatalytic formal α-amination of hexanoic acid to l-norleucine

A three-step one-pot biocatalytic cascade enabled the enantioselective formal α-amination of hexanoic acid to l-norleucine in >97% ee.

Biorefining of protein waste for production of sustainable fuels and chemicals

To mitigate the climate change caused by CO₂ emission, the global incentive to the low-carbon alternatives as replacement of fossil fuel-derived products continuously expands the need for renewable feedstock. There will be accompanied by the generation of enormous protein waste as a result. The economical viability of the biorefinery platform can be realized once the surplus protein waste is recycled in a circular economy scenario. In this context, the present review focuses on the current development of biotechnology with the emphasis on biotransformation and metabolic engineering to refine protein-derived amino acids for production of fuels and chemicals. Its scope starts with the explosion of potential feedstock sources rich in protein waste. The availability of techniques is applied for purification and hydrolysis of various feedstock proteins to amino acids. Useful lessons are leaned from the microbial catabolism of amino acids and lay a foundation for the development of the protein-based biotechnology. At last, the future perspective of the biorefinery scheme based on protein waste is discussed associated with remarks on possible solutions to overcome the technical bottlenecks.

Poly(α-hydroxy acid)s and poly(α-hydroxy acid-co-α-amino acid)s derived from amino acid

Polyesters derived from the α-hydroxy acids, lactic acid, and glycolic acid, are the most common biodegradable polymers in clinical use. These polymers have been tailored for a range of applications that require a physical material possessing. The physical and mechanical properties of these polymers fit the specific application and also safely biodegrade. These polymers are hydrophobic and do not possess functional side groups. This does not allow hydrophilic or hydrophobic manipulation, conjugation of active agents along the polymer chain, etc. These manipulations have partly been achieved by block copolymerization with, for example, poly(ethylene glycol), to obtain an amphiphilic copolymer. The objective of this review is to survey PLA functional copolymers in which functional α-hydroxy acids derived from amino acids are introduced along the polymer chain, allowing endless manipulation of PLA. Biodegradable functional polyesters are one of the most versatile biomaterials available to biomedical scientists. Amino acids with their variable side chains are ideal candidates for synthesizing such structural as well as stereochemically diverse polymers. They render control over functionalization, conjugation, crosslinking, stimulus responsiveness, and tunable mechanical/thermal properties. Functionalized amino acid derived polyesters are widely used, mainly due to advancement in ring opening polymerization (primarily O-carboxyanhydride mediated). The reaction proceeds under milder conditions and yields high molecular weight polymers. We reviewed on advances in the synthetic methodologies for poly-α-hydroxy esters derived from amino acids with appropriate recent examples.

Cascade Biocatalysis for Sustainable Asymmetric Synthesis: From Biobased l-Phenylalanine to High-Value Chiral Chemicals

Sustainable synthesis of useful and valuable chiral fine chemicals from renewable feedstocks is highly desirable but remains challenging. Reported herein is a designed and engineered set of unique non-natural biocatalytic cascades to achieve the asymmetric synthesis of chiral epoxide, diols, hydroxy acid, and amino acid in high yield and with excellent ee values from the easily available biobased l-phenylalanine. Each of the cascades was efficiently performed in one pot by using the cells of a single recombinant strain over-expressing 4-10 different enzymes. The cascade biocatalysis approach is promising for upgrading biobased bulk chemicals to high-value chiral chemicals. In addition, combining the non-natural enzyme cascades with the natural metabolic pathway of the host strain enabled the fermentative production of the chiral fine chemicals from glucose.

Structure-Function Relationships in l-Amino Acid Deaminase, a Flavoprotein Belonging to a Novel Class of Biotechnologically Relevant Enzymes

l-Amino acid deaminase from Proteus myxofaciens (PmaLAAD) is a membrane flavoenzyme that catalyzes the deamination of neutral and aromatic l-amino acids into α-keto acids and ammonia. PmaLAAD does not use dioxygen to re-oxidize reduced FADH2 and thus does not produce hydrogen peroxide; instead, it uses a cytochrome b-like protein as an electron acceptor. Although the overall fold of this enzyme resembles that of known amine or amino acid oxidases, it shows the following specific structural features: an additional novel α+β subdomain placed close to the putative transmembrane α-helix and to the active-site entrance; an FAD isoalloxazine ring exposed to solvent; and a large and accessible active site suitable to bind large hydrophobic substrates. In addition, PmaLAAD requires substrate-induced conformational changes of part of the active site, particularly in Arg-316 and Phe-318, to achieve the correct geometry for catalysis. These studies are expected to pave the way for rationally improving the versatility of this flavoenzyme, which is critical for biocatalysis of enantiomerically pure amino acids.

A synthetic biology approach for the transformation of l-α-amino acids to the corresponding enantiopure (R)- or (S)-α-hydroxy acids

Combinatorial assembly and variation of promoters on a single expression plasmid allowed the balance of the catalytic steps of a three enzyme (l-AAD, HIC, FDH) cascade in E. coli. The designer cell catalyst quantitatively transformed l-amino acids to the corresponding optically pure (R)- and (S)-α-hydroxy acids at up to 200 mM substrate concentration.

Oxidative Decarboxylation of Short-Chain Fatty Acids to 1-Alkenes

The enzymatic oxidative decarboxylation of linear short-chain fatty acids (C4:0-C9:0) employing the P450 monooxygenase OleT, O2 as the oxidant, and NAD(P)H as the electron donor gave the corresponding terminal C3 to C8  alkenes with product titers of up to 0.93 g L(-1) and TTNs of >2000. Key to this process was the construction of an efficient electron-transfer chain employing putidaredoxin CamAB in combination with NAD(P)H recycling at the expense of glucose, formate, or phosphite. This system allows for the biocatalytic production of industrially important 1-alkenes, such as propene and 1-octene, from renewable resources for the first time.

Cascade catalysis – strategies and challenges en route to preparative synthetic biology

In this feature article recent progress and future perspectives of cascade catalysis combining bio/bio or bio/chemo catalysts are presented.