Discovery and biocatalytic characterization of opine dehydrogenases by metagenome mining

Enzymatic processes play an increasing role in synthetic organic chemistry which requires the access to a broad and diverse set of enzymes. Metagenome mining is a valuable and efficient way to discover novel enzymes with unique properties for biotechnological applications. Here, we report the discovery and biocatalytic characterization of six novel metagenomic opine dehydrogenases from a hot spring environment (mODHs) (EC 1.5.1.X). These enzymes catalyze the asymmetric reductive amination between an amino acid and a keto acid resulting in opines which have defined biochemical roles and represent promising building blocks for pharmaceutical applications. The newly identified enzymes exhibit unique substrate specificity and higher thermostability compared to known examples. The feature that they preferably utilize negatively charged polar amino acids is so far unprecedented for opine dehydrogenases. We have identified two spatially correlated positions in their active sites that govern this substrate specificity and demonstrated a switch of substrate preference by site-directed mutagenesis. While they still suffer from a relatively narrow substrate scope, their enhanced thermostability and the orthogonality of their substrate preference make them a valuable addition to the toolbox of enzymes for reductive aminations. Importantly, enzymatic reductive aminations with highly polar amines are very rare in the literature. Thus, the preparative-scale enzymatic production, purification, and characterization of three highly functionalized chiral secondary amines lend a special significance to our work in filling this gap. KEY POINTS: • Six new opine dehydrogenases have been discovered from a hot spring metagenome • The newly identified enzymes display a unique substrate scope • Substrate specificity is governed by two correlated active-site residues.

Opine dehydrogenases, an underexplored enzyme family for the enzymatic synthesis of chiral amines

Opines and opine-type chemicals are valuable natural products with diverse biochemical roles, and potential synthetic building blocks of bioactive compounds. Their synthesis involves reductive amination of ketoacids with amino acids. This transformation has high synthetic potential in producing enantiopure secondary amines. Nature has evolved opine dehydrogenases for this chemistry. To date, only one enzyme has been used as biocatalyst, however, analysis of the available sequence space suggests more enzymes to be exploited in synthetic organic chemistry. This review summarizes the current knowledge of this underexplored enzyme class, highlights key molecular, structural, and catalytic features with the aim to provide a comprehensive general description of opine dehydrogenases, thereby supporting future enzyme discovery and protein engineering studies.

Visible-Light-Induced Synthesis of Branched Ethers via Multicomponent Reactions

The Spin-Center Shift (SCS) elimination is a specific way for the generation of radicals with relevance in synthetic and biochemical pathways. The combination of SCS-mediated radical chemistry and atom-transfer radical addition (ATRA) offers new directions in diversity-oriented chemical synthesis. Herein, we report a photoredox three-component reaction of α-acyloxy-N-heterocycles as radical precursors, styrene derivatives as radical trapping agents, and alcohols as nucleophilic quenchers. The novel radical-polar crossover reaction provides access to a diverse set of branched ethers possessing high structural complexity. The utility of the transformation was also demonstrated by the synthesis of a complex drug derivative and it was easily scalable to the multigram level. The scope and limitations were also explored and a plausible mechanism was proposed.

Design and application of a bi-functional redox biocatalyst through covalent co-immobilization of ene-reductase and glucose dehydrogenase

An immobilized bi-functional redox biocatalyst was designed for the asymmetric reduction of alkenes by nicotinamide-dependent ene-reductases. The biocatalyst, which consists of co-immobilized ene-reductase and glucose dehydrogenase, was implemented in biotransformations in the presence of glucose as source of reducing equivalents and catalytic amounts of the cofactor. Enzyme co-immobilization employing glutaraldehyde activated Relizyme HA403/M as support material was performed directly from the crude cell-free extract obtained after protein overexpression in E. coli and cell lysis, avoiding enzyme purification steps. The resulting optimum catalyst showed excellent level of activity and stereoselectivity in asymmetric reduction reactions using either OYE3 from Saccharomyces cerevisiae or NCR from Zymomonas mobilis in the presence of organic cosolvents in up to 20 vol%. The bi-functional redox biocatalyst, which demonstrated remarkable reusability over several cycles, was applied in preparative-scale synthesis at 50 mM substrate concentration and provided access to three industrially relevant chiral compounds in high enantiopurity (ee up to 97 %) and in up to 42 % isolated yield. The present method highlights the potential of (co-)immobilization of ene-reductases, notorious for their poor scalability, and complements the few existing methods available for increasing productivity in asymmetric bioreduction reactions.

Preparative-Scale Enzymatic Synthesis of rac-Glycerol-1-phosphate from Crude Glycerol Using Acid Phosphatases and Phosphate

Glycerol is a byproduct of biodiesel production and is generated in large amounts, which has resulted in an increased interest in its valorization. In addition to its use as an energy source directly, the chemical modification of glycerol may result in value-added derivatives. Herein, acid phosphatases employed in the synthetic mode were evaluated for the enzymatic phosphorylation of glycerol. Nonspecific acid phosphatases could tolerate glycerol concentrations up to 80 wt % and pyrophosphate concentrations up to 20 wt % and led to product titers up to 167 g L-1 in a kinetic approach. In the complementary thermodynamic approach, phytases were able to condense glycerol and inorganic monophosphate directly. This unexpected behavior enabled the simple and cost-effective production of rac-glycerol-1-phosphate from crude glycerol obtained from a biodiesel plant. A preparative-scale synthesis on a 100 mL-scale resulted in the production of 16.6 g of rac-glycerol-1-phosphate with a reasonable purity (≈75 %).

Smart Nanoparticles for Selective Immobilization of Acid Phosphatases

An easy to use method combining the selectivity of metal chelate affinity binding with strong covalent linking was developed for immobilization of non-specific acid phosphatases bearing a His-tag from crude cell lysate. Silica nanoparticles were grafted with aminopropyl functions which were partially transformed further with EDTA dianhydride to chelators. The heterofunctionalized nanoparticles charged with Ni2+ as the most appropriate metal ion were applied as support. First, the His-tagged phosphatases were selectively bound to the metal-chelate functions of the support. Then, the enzyme-charged silica nanoparticles were further stabilized by forming a covalent linkage between nucleophilic moieties at the enzyme surface and free amino groups of the support using neopentylglycol diglycidylether as the most effective bifunctional linking agent. The phosphatase biocatalysts obtained by this method exhibited better phosphate transfer activity with a range of alcohols and PPi as phosphate donor in aqueous medium applying batch and continuous-flow modes than the ones immobilized on conventional supports. Furthermore, this novel strategy opens up novel possibility for efficient immobilization of other His-tagged recombinant enzymes.

Evaluation of Natural and Synthetic Phosphate Donors for the Improved Enzymatic Synthesis of Phosphate Monoesters

Undesired product hydrolysis along with large amounts of waste in form of inorganic monophosphate by-product are the main obstacles associated with the use of pyrophosphate in the phosphatase-catalyzed synthesis of phosphate monoesters on large scale. In order to overcome both limitations, we screened a broad range of natural and synthetic organic phosphate donors with several enzymes on a broad variety of hydroxyl-compounds. Among them, acetyl phosphate delivered stable product levels and high phospho-transfer efficiency at the lower functional pH-limit, which translated into excellent productivity. The protocol is generally applicable to acid phosphatases and compatible with a range of diverse substrates. Preparative-scale transformations using acetyl phosphate synthesized from cheap starting materials yielded multiple grams of various sugar phosphates with up to 433 g L-1 h-1 space-time yield and 75% reduction of barium phosphate waste.

Exploiting Acid Phosphatases in the Synthesis of Phosphorylated Monoalcohols and Diols

A set of phosphatases was evaluated for their potential to catalyze the regio‐ and stereoselective phosphorylation of alcohols using a high‐energy inorganic phosphate donor, such as di‐, tri‐ and polyphosphate. Parameters such as type and amount of phosphate donor and pH of the reaction were investigated in order to minimize the thermodynamically favored hydrolysis of the phosphate donor and the formed phosphate ester. Diols were monophosphorylated with high selectivities. This biocatalytic phosphorylation method provides selectively activated and/or protected synthetic intermediates for further chemical and/or enzymatic transformations and is applicable to a large scale (6.86 g) in a flow setup with immobilized phosphatase.

Bioreduction and disproportionation of cyclohex-2-enone catalyzed by ene-reductase OYE-1 in 'micro-aqueous' organic solvents

The bioreduction and disproportionation of cyclohex-2-enone catalyzed by Old Yellow Enzyme 1 was investigated in presence of organic (co)solvents. Whereas the NADH-dependent bioreduction activity strongly decreased at elevated co-solvent concentrations due to the insolubility of the nicotinamide-cofactor, the NADH-free disproportionation was significantly improved in water-immiscible organic co-solvents at 90 % (v/v) with near-quantitative conversion. This positive effect was attributed to removal of the inhibiting co-product, phenol, from the enzyme's active site. The best co-solvents show high lipophilicity (logP) and a high potential to solubilize phenol (Kphenol). As a predictive parameter, the ratio of logP/Kphenol should be preferably ≥100.

A substrate-driven approach to determine reactivities of α,β-unsaturated carboxylic esters towards asymmetric bioreduction

The degree of C=C bond activation in the asymmetric bioreduction of α,β-unsaturated carboxylic esters by ene-reductases was studied, and general recommendations to render these "borderline-substrates" more reactive towards enzymatic reduction are proposed. The concept of "supported substrate activation" was developed. In general, an additional α-halogenated substituent proved to be beneficial for enzymatic activity, whereas β-alkyl or β-aryl substituents were detrimental for the reactivity of nonhalogenated substrates, and α-cyano groups showed little effect. The alcohol moiety of the ester functionality was found to have a strong influence on the reaction rate. Overall, activities were determined by both steric and electronic effects.

Asymmetric bioreduction of activated alkenes to industrially relevant optically active compounds

Ene-reductases from the ‘Old Yellow Enzyme’ family of flavoproteins catalyze the asymmetric reduction of various α,β-unsaturated compounds at the expense of a nicotinamide cofactor. They have been applied to the synthesis of valuable enantiopure products, including chiral building blocks with broad industrial applications, terpenoids, amino acid derivatives and fragrances. The combination of these highly stereoselective biocatalysts with a cofactor recycling system has allowed the development of cost-effective methods for the generation of optically active molecules, which is strengthened by the availability of stereo-complementary enzyme homologues.

Peritoneo-venous shunt implantation as a therapy for chylous ascites

We report a case of primary chylous ascites in a progressively poor condition with proteinlosing enteropathy, pleural and pericardiac effusion. Therapy: Denver peritoneo- venous shunt was implanted. Three years later the patient became septic and the shunt was removed. Clinical course: Soon after implantation the patient’s condition improved, and in two months he lost 51 kg. After three years fibrinous occlusion of the shunt and sepsis developed. The occlusion could be dissolved with streptokinase. Removing the shunt cured the patient’s septic state. The chylous ascites did not recur. Conclusion: In emergency cases implantation of a peritoneo-venous shunt leads to a rapid relief of the state. Usually this is a palliative treatment, but in this case it gave a final result. Fibrin can deposit on or occlude the drain which can be solved without further complications. If the patient is septic the drain must be removed and the septic state ends.

[New possibilities and results in pediatric thoracic surgery]

In the last 5 years 104 thoracic operations were performed on children and adolescents in the 3rd Surgical Clinic of the Semmelweis University of Budapest and in the Heim Pál Children's Hospital by the authors. The number of the lung interventions was 47 and 57 operations were performed on the mediastinum, thoracic wall, pleura, trachea and on the diaphragm. Distribution ot the lung operations: 21 developmental malformations, 14 bronchiectasia, 9 tumours and 3 others. All the 16 mediastinal operations were performed because of tumours. The others were: 27 thoracic wall interventions, 11 pleural, and 3 other operations. All the operations were performed by the same surgeon. Authors introduced new techniques in the pediatric thoracic surgery in their country as the laser technique and the Video Assisted Thoracic Surgery. One reoperation was performed because of postoperative bleeding. No patients died in the postoperative period.

Identification of acute intermittent porphyria carriers by molecular biologic methods

Acute intermittent porphyria (AIP) is an autosomal dominant disease caused by mutations of the gene coding for hydroxymethylbilane synthase. Acute attack of AIP is a potentially life-threatening condition precipitated by certain drugs, alcohol, fasting and stress. Biochemical diagnosis before the manifestation of the symptoms is problematic, and genetic screening is required to identify asymptotic carriers. The aim of this study was to establish a fast, reproducible and reliable genetic method to detect mutations causing AIP. Exon 10 of one healthy individual and 12 AIP patients was studied using a recently developed method, temporal temperature gradient electrophoresis (TTGE). Mutation of exon 10 was detected using TTGE in one patient, DNA sequence analysis confirmed the presence of a heterozygous point mutation causing substitution of the arginine in position 173 of the gene with glutamine. These results were also confirmed using restriction enzyme analysis, and this method and TTGE identified a child of this patient as an asymptotic carrier of AIP.

Gastrojejunal interposition for esophageal replacement

The main considerations in replacing the esophagus are to avoid postoperative necrosis of all or part of the graft, leakage or stenosis of the anastomoses, and complications related to acid-peptic or alkaline reflux. A 5-year-old boy, after two unsuccessful thoracic operations for atresia and then stenosis of the esophagus, underwent resection of the esophagus because of duodeno-gastroesophageal reflux. The continuity of the alimentary tract was restored by gastrojejunal interposition. We recommend this method of reconstruction when the esophago-gastrostoma is created in the chest, and the possibility of alkaline reflux must be considered.

[Aortopexy as a surgical treatment method in infantile congenital tracheo-chondrohypoplasia and tracheomalacia]

Congenital tracheal chondrohypoplasia and tracheomalacia have come to present a significant challenge in recent paediatric surgery. Authors shortly describe their pathology, pathophysiology, symptoms, diagnostic evaluation and outline the most frequency surgical procedures for them. The first three aortopexies in the authors' departments are described in this paper, with particular attention paid to the clinical problems of congenital tracheal chondrohypoplasia and tracheomalacia and the role of this method in surgical treatment, respectively. Aortopexy is quick, easy to carry out and atraumatic for the growing trachea. Authors want to draw attention to the fact that not every form of congenital tracheal chondrohypoplasia and tracheomalacia may be treated by conservative therapy, and that surgical treatment is recommended not only for life threatening forms, but for other severe forms as well.

[Hepato-lobectomy for carcinoid tumor of the hepatic duct]

Authors performed successful right hepatolobectomy in a 13-year-old girl because of a carcinoid tumor of the right hepatic duct, resulting in obstructive jaundice. The patient remains well 2 years after surgical treatment, complaint- and symptom free. This is the first hepatolobectomy in the Hungarian--and the second one in the world literature performed for carcinoid of the extrahepatic bile ducts.

[Embolization of arteriovenous angiomas in infancy and childhood]

The authors report on 35 children in whom embolisation of arteriovenous angiomas was performed. Embolisation alone caused the tumor to disappear in 50% of the cases. Permanent success was achieved in all cases by means of embolisation and steroid treatment or by embolisation and exstirpation of the tumor.

The effect of inorganic pyrophosphate (PPi) anions on the in vitro collagen fibril formation

This paper presents a study on the effect of sodium salt of pyrophosphate (Na-PPi) in different concentrations (1-10 mMol/l) on the in vitro collagen fibril formation. Collagen was mixture of type I and III collagen dissolved in neutral buffer containing sodium chloride of 1.0 mol/l. Na-PPi added to collagen delayed the fibrillary precipitation of collagen from the solution. Electronmicroscopically the fibrils are of native collagen type, but they are thicker and show a pattern of fine substriation. Calcium and other divalent cations showed differences in their influence on the effect of Na-PPi. This experimental model was developed in order to interpret the supposed role of PPi in the inhibition of apatite crystal formations as well as the fibrogenic effect of its calcium salt in PPi-arthropathy. The effect of PPi on the collagen fibril formation should also be taken into consideration.