The influence of hydroxypropyl-β-cyclodextrin on the enantioselective hydrolysis of 2-amino phenylpropionitrile catalyzed by recombinant nitrilase
© Li and Wang; licensee Springer 2014
Received: 30 January 2014
Accepted: 19 May 2014
Published: 24 July 2014
Hydrolysis of 2-amino phenylpropionitrile by nitrilase is a fundamental biochemical reaction that produces chiral phenylalanine. For practical application of this biochemical reaction, researchers have attempted to improve enzyme enantioselectivity and the reaction rate.
The substrate concentration was increased from 100 to 200 mM without substrate inhibition because of the formation of a substrate-hydroxypropyl-β-cyclodextrin (HP-β-CD) complex. Meanwhile, the activity of recombinant nitrilase increased 2.5 times because the addition of HP-β-CD solubilized hydrophobic substrates in the aqueous system. Furthermore, the formation of the substrate-HP-β-CD inclusion improved the enantioselectivity of the enzymatic reaction toward producing l-phenylalanine (l-Phe). The enantiomeric excess (e.e.) value of l-Phe increased from 65% to 83% when the conversion rate reached 50%.
The recombinant nitrilase enantioselectively hydrolyzed 2-amino phenylpropionitrile to produce l-Phe. The addition of HP-β-CD to the reaction system enhanced the solubility and bioavailability of hydrophobic substrates as well as the enantioselectivity. The results showed that this additive has potential advantages in biochemical reactions of hydrophobic substrates, particularly for enantioselective biosynthesis.
l-Phenylalanine (l-Phe) is an essential amino acid generally used in food industries, human nutrients, and pharmaceuticals. For example, it is a precursor of some anticancer drugs and the dipeptide sweetener aspartame []. In early industrial processes, l-Phe was mainly produced by chemical synthesis. Because of the specific demand for the stereospecific form and the consideration of eco-friendly chemical synthesis, this approach was gradually replaced with bioprocesses, such as microbial fermentation and enzymatic transformation [].
Nitrilase (EC 18.104.22.168) is an enzyme that converts nitrile to its carboxylic acid or amide []. Some important chiral pharmaceutical intermediates and bulk products, such as acrylic acid [], (R)-(−)-mandelic acid [], 3-hydroxyvaleric acid [], and nicotinic acid [], are produced by nitrile hydrolysis. For example, (R)-(−)-mandelic acid, widely used for the production of semisynthetic cephalosporins and anti-obesity agents, is produced by hydrolyzing mandelonitrile [,]. In addition, other studies focused on strategic optimizations for increasing the reaction rate, reducing substrate inhibition, and improving enzyme enantioselectivity [–].
Cyclodextrin (CD) is the generic term for cyclic oligosaccharides that are used frequently as host molecules in supramolecular chemistry. Despite the outside of the CD molecule being hydrophilic, CD contains a hydrophobic cavity that entraps most hydrophobic molecules to form inclusion complexes []. Thus, it improves the solubility and bioavailability of hydrophobic compounds. Its use is of interest for reactions in which hydrophobic compounds are to be delivered. In previous reports, CD has been proven to increase the availability of insoluble substrates, reduce substrate inhibition, and enhance the efficiency of catalysis by increasing the reaction rate in other catalytic reactions []. It has also been used in an enzymatic enantioselective reaction to increase the enantiomeric excess (e.e.) value of the product [].
In our previous study, a novel nitrilase from Rhodobacter sp. LHS-305 was cloned and expressed in Escherichia coli []. This nitrilase displayed high activity toward both aliphatic and aromatic nitriles, similar to the nitrilase from Rhodococcus rhodochrous ATCC 33278 []. It also showed regioselectivity toward dinitriles to produce cyanocarboxylic acids. Because this nitrilase shows properties different from those of typical nitrilases, it potentially has industrial applications in the future.
In this study, the stereoselective hydrolysis of 2-amino phenylpropionitrile by this novel nitrilase was investigated. The influences of CD on this hydrolysis in terms of substrate inhibition and the e.e. value of the product were found to improve the catalytic reaction. The investigations comprehensively increased our knowledge of this unique nitrilase for its application.
Materials and methods
Hydroxypropyl-β-cyclodextrin (HP-β-CD) (≥98.5%) was purchased from Shanghai Lingfeng Chemical Reagent Company (Shanghai, China), biochemical-grade l-Phe from Sigma-Aldrich (St. Louis, MO, USA), and glucose and other chemicals from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
Microorganism and culture media
Recombinant E. coli expressing a novel nitrilase gene from Rhodobacter sphaeroides LHS-305 were used in this study.
Seed medium (g/L) contained peptone 10, yeast extract 5, and NaCl 10, pH 7.0 to 7.5.
Fermentation medium (g/L) contained yeast extract 10, peptone 5, NaCl 5, glucose 3, and MgSO4·7H2O 3, pH 7.0 to 7.5. Media were sterilized for 20 min at 115°C.
Precultures cultured in the seed medium for 3 to 4 h at 37°C were inoculated into 100 mL of fermentation medium in a 500-mL Erlenmeyer flask, with the addition of α-lactose (1 g/L), and cultured on a shaker at 200 rpm and 20°C for 12 h.
Preparation of recombinant nitrilase
The recombinant nitrilase was purified by affinity chromatography as described in our previous report [].
Preparation of the 2-amino phenylpropionitrile/HP-β-CD complex
2-Amino phenylpropionitrile (73 mg) was added to 100 μL methanol to obtain a 5 M substrate stock solution. The stock solution was added to 50 mM PB solution (pH 7.0) with HP-β-CD and stirred for 20 min at 40°C. This yielded the 2-amino phenylpropionitrile/HP-β-CD inclusion complex.
Assay of enzyme activity toward bioconversion of 2-amino phenylpropionitrile
The catalytic reaction was performed in 1 mL of sodium phosphate buffer (50 mM, pH 7.0) containing the substrate 2-amino phenylpropionitrile (20 mM, final concentration) and nitrilase (1 mg/mL, final concentration). Reaction mixtures were incubated at 40°C for 10 min, and reactions were quenched by the addition of 10% (v/v) 1 mol/L HCl. Enzyme activity (U) was defined as the amount of enzyme required for the hydrolysis of l μmol of the 2-amino phenylpropionitrile substrate to the corresponding acid within 1 min. All experiments were performed in triplicate.
Analysis of l-Phe
The enantiomeric purity of Phe was determined by reversed-phase HPLC (Agilent, Santa Clara, CA, USA) equipped with a Chirobiotic T column (Sigma-Aldrich Co.) at a flow rate of 0.5 mL/min with a solvent system (75:25, v/v) of phosphate buffer (25 mM, adjusted to pH 3.5 with H3PO4) and methanol. Peaks were detected using an ultraviolet detector at 210 nm.
Results and discussion
Effects of HP-β-CD addition on the catalytic reaction
Effect of HP-β-CD inclusions on substrate inhibition
Effect of HP-β-CD inclusion on the e.e. value of the product
The recombinant nitrilase enantioselectively hydrolyzed 2-amino phenylpropionitrile to produce l-Phe. Using the HP-β-CD-substrate reaction system, the reaction rate was greatly improved by enhancing the solubility and bioavailability of the hydrophobic substrate. Meanwhile, the e.e. value of the product was also improved significantly because of the formation of inclusion complexes. In this manner, HP-β-CD enhanced both the efficiency of the catalytic reaction and the optical purity of the product. The properties of CDs to form inclusion complexes with hydrophobic molecules led to their practical application in a biochemical reaction with a hydrophobic substrate. In particular, they were used to promote enantioselective biosynthesis.
We thank Guinan Li and Hualei Wang who provided the fundamental works, such as clone and expression of the novel nitrilase in E. coli.
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