Exploitation of the alcohol dehydrogenase-acetone NADP-regeneration system for the enzymatic preparative-scale production of 12-ketochenodeoxycholic acid

An insight into primary biliary cholangitis and its recent advances in treatment: semi-synthetic analogs to combat ursodeoxycholic-acid resistance

INTRODUCTION

Primary biliary cholangitis (PBC) is a chronic cholestatic liver disease which on progression causes cirrhosis; various studies also suggested that several diseases can co-exist in patients. In existing depiction of disease PBC, apart from entire use of ursodeoxycholic acid (UDCA), several patients need to step forward to liver-transplantation or death due to resistance or non-responder with UDCA monotherapy.

AREAS COVERED

To overcome this non-respondent treatment, novel bile acid semi-synthetic analogs have been identified which shows their potency against for farnesoid X receptor and transmembrane G protein-coupled receptor-5 which are identified as target for many developing analogs which have desirable pharmacokinetic profiles.

EXPERT OPINION

A range of studies suggests that adding semisynthetic analogs in therapeutic regime improves liver biochemistries in patients with suboptimal response to UDCA. Thus, the aspire of this review is to abridge and compare therapeutic value and current markets affirm of various bile acids semi-synthetic analogs which certainly are having promising effects in PBC monotherapy or in pooled treatment with UDCA for PBC.

Biocatalytic Oxidation of Alcohols

Enzymatic methods for the oxidation of alcohols are critically reviewed. Dehydrogenases and oxidases are the most prominent biocatalysts, enabling the selective oxidation of primary alcohols into aldehydes or acids. In the case of secondary alcohols, region and/or enantioselective oxidation is possible. In this contribution, we outline the current state-of-the-art and discuss current limitations and promising solutions.

Laccase Did It again: A Scalable and Clean Regeneration System for NAD+ and Its Application in the Synthesis of 12-oxo-Hydroxysteroids

The specific oxidation of 12α-OH group of hydroxysteroids is required for the preparation of cheno- and ursodeoxycholic acid (CDCA and UDCA, respectively). The C12 oxidation of hydroxysteroids into their 12-oxo derivatives can selectively be performed by employing 12α-hydroxysteroid dehydrogenases. These enzymes use NAD(P)+ as an electron acceptor, which has to be re-oxidized in a so-called “regeneration system”. Recently, the enzyme NAD(P)H oxidase (NOX) was applied for the regeneration of NAD+ in the enzymatic preparation of 12-oxo-CDCA from cholic acid (CA), which allows air to be used as an oxidant. However, the NOX system suffers from low activity and low stability. Moreover, the substrate loading is limited to 10 mM. In this study, the laccase/mediator system was investigated as a possible alternative to NOX, employing air as an oxidant. The laccase/mediator system shows higher productivity and scalability than the NOX system. This was proven with a preparative biotransformation of 20 g of CA into 12-oxo-CDCA (92% isolated yield) by employing a substrate loading of 120 mM (corresponding to 50 g/L). Additionally, the performance of the laccase/mediator system was compared with a classical ADH/acetone regeneration system and with other regeneration systems reported in literature.

Enhanced activity and substrate tolerance of 7α-hydroxysteroid dehydrogenase by directed evolution for 7-ketolithocholic acid production

7-Ketolithocholic acid (7-KLCA) is an important intermediate for the synthesis of ursodeoxycholic acid (UDCA). UDCA is the main effective component of bear bile powder that is used in traditional Chinese medicine for the treatment of human cholesterol gallstones. 7α-Hydroxysteroid dehydrogenase (7α-HSDH) is the key enzyme used in the industrial production of 7-KLCA. Unfortunately, the natural 7α-HSDHs reported have difficulty meeting the requirements of industrial application, due to their poor activities and strong substrate inhibition. In this study, a directed evolution strategy combined with high-throughput screening was applied to improve the catalytic efficiency and tolerance of high substrate concentrations of NADP+-dependent 7α-HSDH from Clostridium absonum. Compared with the wild type, the best mutant (7α-3) showed 5.5-fold higher specific activity and exhibited 10-fold higher and 14-fold higher catalytic efficiency toward chenodeoxycholic acid (CDCA) and NADP+, respectively. Moreover, 7α-3 also displayed significantly enhanced tolerance in the presence of high concentrations of substrate compared to the wild type. Owing to its improved catalytic efficiency and enhanced substrate tolerance, 7α-3 could efficiently biosynthesize 7-KLCA with a substrate loading of 100 mM, resulting in 99% yield of 7-KLCA at 2 h, in contrast to only 85% yield of 7-KLCA achieved for the wild type at 16 h.

Biosynthesis of Phenylglyoxylic Acid by LhDMDH, a Novel d-Mandelate Dehydrogenase with High Catalytic Activity

d-Mandelate dehydrogenase (DMDH) has the potential to convert d-mandelic acid to phenylglyoxylic acid (PGA), which is a key building block in the field of chemical synthesis and is widely used to synthesize pharmaceutical intermediates or food additives. A novel NAD⁺-dependent d-mandelate dehydrogenase was cloned from Lactobacillus harbinensi (LhDMDH) by genome mining and expressed in Escherichia coli BL21. After being purified to homogeneity, the oxidation activity of LhDMDH toward d-mandelic acid was approximately 1200 U·mg^-1, which was close to four times the activity of the probe. Meanwhile, the k_cat/ K_m value of LhDMDH was 28.80 S^-1·mM^-1, which was distinctly higher than the probe. By coculturing two E. coli strains expressing LhDMDH and LcLDH, we developed a system for the efficient synthesis of PGA, achieving a 60% theoretical yield and 99% purity without adding coenzyme or cosubstrate. Our data supports the implementation of a promising strategy for the chiral resolution of racemic mandelic acid and the biosynthesis of PGA.

Latest development in the synthesis of ursodeoxycholic acid (UDCA): a critical review

Ursodeoxycholic acid (UDCA) is a pharmaceutical ingredient widely used in clinics. As bile acid it solubilizes cholesterol gallstones and improves the liver function in case of cholestatic diseases. UDCA can be obtained from cholic acid (CA), which is the most abundant and least expensive bile acid available. The now available chemical routes for the obtainment of UDCA yield about 30% of final product. For these syntheses several protection and deprotection steps requiring toxic and dangerous reagents have to be performed, leading to the production of a series of waste products. In many cases the cholic acid itself first needs to be prepared from its taurinated and glycilated derivatives in the bile, thus adding to the complexity and multitude of steps involved of the synthetic process. For these reasons, several studies have been performed towards the development of microbial transformations or chemoenzymatic procedures for the synthesis of UDCA starting from CA or chenodeoxycholic acid (CDCA). This promising approach led several research groups to focus their attention on the development of biotransformations with non-pathogenic, easy-to-manage microorganisms, and their enzymes. In particular, the enzymatic reactions involved are selective hydrolysis, epimerization of the hydroxy functions (by oxidation and subsequent reduction) and the specific hydroxylation and dehydroxylation of suitable positions in the steroid rings. In this minireview, we critically analyze the state of the art of the production of UDCA by several chemical, chemoenzymatic and enzymatic routes reported, highlighting the bottlenecks of each production step. Particular attention is placed on the precursors availability as well as the substrate loading in the process. Potential new routes and recent developments are discussed, in particular on the employment of flow-reactors. The latter technology allows to develop processes with shorter reaction times and lower costs for the chemical and enzymatic reactions involved.

Cloning, expression, and biochemical characterization of a novel NADP+-dependent 7α-hydroxysteroid dehydrogenase from Clostridium difficile and its application for the oxidation of bile acids

A gene encoding a novel 7α-specific NADP⁺-dependent hydroxysteroid dehydrogenase from Clostridium difficile was cloned and heterologously expressed in Escherichia coli. The enzyme was purified using an N-terminal hexa-his-tag and biochemically characterized. The optimum temperature is at 60°C, but the enzyme is inactivated at this temperature with a half-life time of 5min. Contrary to other known 7α-HSDHs, for example from Clostridium sardiniense or E. coli, the enzyme from C. difficile does not display a substrate inhibition. In order to demonstrate the applicability of this enzyme, a small-scale biotransformation of the bile acid chenodeoxycholic acid (CDCA) into 7-ketolithocholic acid (7-KLCA) was carried out with simultaneous regeneration of NADP⁺ using an NADPH oxidase that resulted in a complete conversion (<99%). Furthermore, by a structure-based site-directed mutagenesis, cofactor specificity of the 7α-HSDH from Clostridium difficile was altered to accept NAD(H). This mutant was biochemically characterized and compared to the wild-type.

Cloning and characterization of two distinct water-forming NADH oxidases from Lactobacillus pentosus for the regeneration of NAD

Two uncharacterized nicotinamide adenine dinucleotide (NADH) oxidases (named as LpNox1, LpNox2) from Lactobacillus pentosus ATCC 8041 were cloned and overexpressed in Escherichia coli BL21 (DE3). The sequence analysis revealed that the two enzymes are water-forming Noxs with 64 % and 52 % identity to LbNox from Lactobacillus brevis DSM 20054. The optimal pH and temperature of the purified LpNox1 and LpNox2 were 7.0 and 8.0 and 35 and 40 °C, respectively, with K M of 99.0 μM (LpNox1) and 27.6 μM (LpNox2), and yielding catalytic efficiency k cat/K M of 1.0 and 0.2 μM(-1) s(-1), respectively. Heat inactivation studies revealed that the two enzymes are relatively instable. The application of LpNox1 for the regeneration of NAD(+) was demonstrated by coupling with a glycerol dehydrogenase-catalyzed oxidation of glycerol to 1,3-dihydroxyacetone. The characteristics of the LpNox1 could prove to be of interest in industrial application such as NAD(+) regeneration in dehydrogenase-catalyzed oxidations.

One-step synthesis of 12-ketoursodeoxycholic acid from dehydrocholic acid using a multienzymatic system

12-ketoursodeoxycholic acid (12-keto-UDCA) is a key intermediate for the synthesis of ursodeoxycholic acid (UDCA), an important therapeutic agent for non-surgical treatment of human cholesterol gallstones and various liver diseases. The goal of this study is to develop a new enzymatic route for the synthesis 12-keto-UDCA based on a combination of NADPH-dependent 7β-hydroxysteroid dehydrogenase (7β-HSDH, EC 1.1.1.201) and NADH-dependent 3α-hydroxysteroid dehydrogenase (3α-HSDH, EC 1.1.1.50). In the presence of NADPH and NADH, the combination of these enzymes has the capacity to reduce the 3-carbonyl- and 7-carbonyl-groups of dehydrocholic acid (DHCA), forming 12-keto-UDCA in a single step. For cofactor regeneration, an engineered formate dehydrogenase, which is able to regenerate NADPH and NADH simultaneously, was used. All three enzymes were overexpressed in an engineered expression host Escherichia coli BL21(DE3)Δ7α-HSDH devoid of 7α-hydroxysteroid dehydrogenase, an enzyme indigenous to E. coli, in order to avoid formation of the undesired by-product 12-chenodeoxycholic acid in the reaction mixture. The stability of enzymes and reaction conditions such as pH value and substrate concentration were evaluated. No significant loss of activity was observed after 5 days under reaction condition. Under the optimal condition (10 mM of DHCA and pH 6), 99 % formation of 12-keto-UDCA with 91 % yield was observed.

In search of sustainable chemical processes: cloning, recombinant expression, and functional characterization of the 7α- and 7β-hydroxysteroid dehydrogenases from Clostridium absonum

Nicotinamide adenine dinucleotide phosphate-dependent 7α-hydroxysteroid dehydrogenase (7α-HSDH) and 7β-hydroxysteroid dehydrogenases (7β-HSDH) from Clostridium absonum catalyze the epimerization of primary bile acids through 7-keto bile acid intermediates and may be suitable as biocatalysts for the synthesis of bile acids derivatives of pharmacological interest. C. absonum 7α-HSDH has been purified to homogeneity and the N-terminal sequence has been determined by Edman sequencing. After PCR amplifications of a gene fragment with degenerate primers, cloning of the complete gene (786 nt) has been achieved by sequencing of C. absonum genomic DNA. The sequence coding for the 7β-HSDH (783 nt) has been obtained by sequencing of the genomic DNA region flanking the 5' termini of 7α-HSDH gene, the two genes being contiguous and presumably part of the same operon. After insertion in suitable expression vectors, both HSDHs have been successfully produced in recombinant form in Escherichia coli, purified by affinity chromatography and submitted to kinetic analysis for determination of Michaelis constants (K (m)) and specificity constants (k (cat)/K (m)) in the presence of various bile acids derivatives. Both enzymes showed a very strong substrate inhibition with all the tested substrates. The lowest K (S) values were observed with chenodeoxycholic acid and 12-ketochenodeoxycholic acid as substrates in the case of 7α-HSDH, whereas ursocholic acid was the most effective inhibitor of 7β-HSDH activity.

The catalytic promiscuity of a microbial 7α-hydroxysteroid dehydrogenase. Reduction of non-steroidal carbonyl compounds

A thermostable 7α-hydroxysteroid dehydrogenase from Bacteroides fragilis ATCC 25285 was found to catalyze the reduction of various benzaldehyde analogues to their corresponding benzyl alcohols. The enzyme activity was dependent upon the substituent on the benzene ring of the substrates. Benzaldehydes with electron-withdrawing substituent usually showed higher activity than those with electron-donating groups. Furthermore, this enzyme was tolerant to some organic solvents. These results together with previous studies suggested that 7α-hydroxysteroid dehydrogenase from B. fragilis might play multiple functional roles in biosynthesis and metabolism of bile acids, and in the detoxification of xenobiotics containing carbonyl groups in the large intestine. In addition, its broad substrate spectrum offers great potential for finding applications not only in the synthesis of steroidal compounds of pharmaceutical importance, but also for the production of other high-value fine chemicals.

Identification, cloning, heterologous expression, and characterization of a NADPH-dependent 7β-hydroxysteroid dehydrogenase from Collinsella aerofaciens

A gene encoding an NADPH-dependent 7β-hydroxysteroid dehydrogenase (7β-HSDH) from Collinsella aerofaciens DSM 3979 (ATCC 25986, formerly Eubacterium aerofaciens) was identified and cloned in this study. Sequence comparison of the translated amino acid sequence suggests that the enzyme belongs to the short-chain dehydrogenase superfamily. This enzyme was expressed in Escherichia coli with a yield of 330 mg (5,828 U) per liter of culture. The enzyme catalyzes both the oxidation of ursodeoxycholic acid (UDA) forming 7-keto-lithocholic acid (KLA) and the reduction of KLA forming UDA acid in the presence of NADP(+) or NADPH, respectively. In the presence of NADPH, 7β-HSDH can also reduce dehydrocholic acid. SDS-PAGE and gel filtration of the expressed and purified enzyme revealed a dimeric nature of 7β-HSDH with a size of 30 kDa for each subunit. If used for the oxidation of UDA, its pH optimum is between 9 and 10 whereas for the reduction of KLA and dehydrocholic acid it shows an optimum between pH 4 to 6. Usage of the enzyme for the biotransformation of KLA in a 0.5-g scale showed that this 7β-HSDH is a useful biocatalyst for producing UDA from suitable precursors in a preparative scale.

Design of a cytochrome P450BM3 reaction system linked by two-step cofactor regeneration catalyzed by a soluble transhydrogenase and glycerol dehydrogenase

A cytochrome P450BM3-catalyzed reaction system linked by a two-step cofactor regeneration was investigated in a cell-free system. The two-step cofactor regeneration of redox cofactors, NADH and NADPH, was constructed by NAD(+)-dependent bacterial glycerol dehydrogenase (GLD) and bacterial soluble transhydrogenase (STH) both from Escherichia coli. In the present system, the reduced cofactor (NADH) was regenerated by GLD from the oxidized cofactor (NAD(+)) using glycerol as a sacrificial cosubstrate. The reducing equivalents were subsequently transferred to NADP(+) by STH as a cycling catalyst. The resultant regenerated NADPH was used for the substrate oxidation catalyzed by cytochrome P450BM3. The initial rate of the P450BM3-catalyzed reaction linked by the two-step cofactor regeneration showed a slight increase (approximately twice) when increasing the GLD units 10-fold under initial reaction conditions. In contrast, a 10-fold increase in STH units resulted in about a 9-fold increase in the initial reaction rate, implying that transhydrogenation catalyzed by STH was the rate-determining step. In the system lacking the two-step cofactor regeneration, 34% conversion of 50 microM of a model substrate (p-nitrophenoxydecanoic acid) was attained using 50 microM NADPH. In contrast, with the two-step cofactor regeneration, the same amount of substrate was completely converted using 5 microM of oxidized cofactors (NAD(+) and NADP(+)) within 1 h. Furthermore, a 10-fold dilution of the oxidized cofactors still led to approximately 20% conversion in 1 h. These results indicate the potential of the combination of GLD and STH for use in redox cofactor recycling with catalytic quantities of NAD(+) and NADP(+).

7alpha- and 12alpha-Hydroxysteroid dehydrogenases from Acinetobacter calcoaceticus lwoffii: a new integrated chemo-enzymatic route to ursodeoxycholic acid

We report the very efficient biotransformation of cholic acid to 7-keto- and 7,12-diketocholic acids with Acinetobacter calcoaceticus lwoffii. The enzymes responsible of the biotransformation (i.e. 7alpha- and 12alpha-hydroxysteroid dehydrogenases) are partially purified and employed in a new chemo-enzymatic synthesis of ursodeoxycholic acid starting from cholic acid. The first step is the 12alpha-HSDH-mediated total oxidation of sodium cholate followed by the Wolf-Kishner reduction of the carbonyl group to chenodeoxycholic acid. This acid is then quantitatively oxidized with 7alpha-HSDH to 7-ketochenodeoxycholic acid, that was chemically reduced to ursodeoxycholic acid (70% overall yield).