Enantioselectivity and key residue of Herbaspirillum huttiense monooxygenase in asymmetric epoxidation of styrenes

Styrene monooxygenases (SMOs) are powerful enzymes for the synthesis of enantiopure epoxides, but these SMOs have narrow substrate spectra, and the residues in controlling enantioselectivity of SMOs remains unclear. A monooxygenase from Herbaspirillum huttiense (HhMO) was found to have excellent enantioselectivities and diastereoselectivities in the epoxidation of unconjugated terminal alkenes. Here we found that HhMO could also transfer styrene into styrene epoxide with 75% ee, and it could also catalyze the epoxidation of styrene derivatives into the corresponding epoxides with enantioselectivities up to 99% ee. Meanwhile, site 199 in the substrate access channel of HhMO was found to play an important role in the controlling enantioselectivity of the epoxidation. The E199L variant catalyzed the epoxidation of styrene with > 99% ee. The identification of critical residue that affects the enantioselectivity of SMOs would thus be valuable for creating efficient monooxygenases for the preparation of essential enantiopure epoxides. KEY POINTS: • Bioexpoxidation of both conjugated and unconjugated alkenes by HhMO with excellent enantioselectivities. • Gating residue 199 played an essential role in controlling the enantioselectivity of SMO. • HhMO E199L catalyzed the epoxidation of styrenes with up to > 99% ee.

Structural and mechanistic insights into enantioselectivity toward near-symmetric esters of a novel carboxylesterase RoCE

A novel carboxylesterase RoCE was identified with relatively high enantioselectivity toward “hard-to-be-discriminated” oxyheterocyclic esters. Molecular basis of enantioselectivity was elucidated and applied in increasing enantioselectivity of RoCE.

Significantly increased catalytic activity of Candida antarctica lipase B for the resolution of cis-(±)-dimethyl 1-acetylpiperidine-2,3-dicarboxylate

Structure-based semi-rational engineering approach was applied to alter the binding pocket and substrate channel for enhancing the activity of CALB towards moxifloxacin chiral intermediate.

Structure-guided engineering of the substrate specificity of a fungal β-glucuronidase toward triterpenoid saponins

Glycoside hydrolases (GHs) have attracted special attention in research aimed at modifying natural products by partial removal of sugar moieties to manipulate their solubility and efficacy. However, these modifications are challenging to control because the low substrate specificity of most GHs often generates undesired by-products. We previously identified a GH2-type fungal β-glucuronidase from Aspergillus oryzae (PGUS) exhibiting promiscuous substrate specificity in hydrolysis of triterpenoid saponins. Here, we present the PGUS structure, representing the first structure of a fungal β-glucuronidase, and that of an inactive PGUS mutant in complex with the native substrate glycyrrhetic acid 3-O-mono-β-glucuronide (GAMG). PGUS displayed a homotetramer structure with each monomer comprising three distinct domains: a sugar-binding, an immunoglobulin-like β-sandwich, and a TIM barrel domain. Two catalytic residues, Glu⁴¹⁴ and Glu⁵⁰⁵, acted as acid/base and nucleophile, respectively. Structural and mutational analyses indicated that the GAMG glycan moiety is recognized by polar interactions with nine residues (Asp¹⁶², His³³², Asp⁴¹⁴, Tyr⁴⁶⁹, Tyr⁴⁷³, Asp⁵⁰⁵, Arg⁵⁶³, Asn⁵⁶⁷, and Lys⁵⁶⁹) and that the aglycone moiety is recognized by aromatic stacking and by a π interaction with the four aromatic residues Tyr⁴⁶⁹, Phe⁴⁷⁰, Trp⁴⁷², and Tyr⁴⁷³ Finally, structure-guided mutagenesis to precisely manipulate PGUS substrate specificity in the biotransformation of glycyrrhizin into GAMG revealed that two amino acids, Ala³⁶⁵ and Arg⁵⁶³, are critical for substrate specificity. Moreover, we obtained several mutants with dramatically improved GAMG yield (>95%). Structural analysis suggested that modulating the interaction of β-glucuronidase simultaneously toward glycan and aglycone moieties is critical for tuning its substrate specificity toward triterpenoid saponins.

Engineering Streptomyces coelicolor Carbonyl Reductase for Efficient Atorvastatin Precursor Synthesis

Streptomyces coelicolor CR1 (ScCR1) has been shown to be a promising biocatalyst for the synthesis of an atorvastatin precursor, ethyl-(S)-4-chloro-3-hydroxybutyrate [(S)-CHBE]. However, limitations of ScCR1 observed for practical application include low activity and poor stability. In this work, protein engineering was employed to improve the catalytic efficiency and stability of ScCR1. First, the crystal structure of ScCR1 complexed with NADH and cosubstrate 2-propanol was solved, and the specific activity of ScCR1 was increased from 38.8 U/mg to 168 U/mg (ScCR1_I158V/P168S) by structure-guided engineering. Second, directed evolution was performed to improve the stability using ScCR1_I158V/P168S as a template, affording a triple mutant, ScCR1_{A60T/I158V/P168S}, whose thermostability (T₅₀¹⁵, defined as the temperature at which 50% of initial enzyme activity is lost following a heat treatment for 15 min) and substrate tolerance (C₅₀¹⁵, defined as the concentration at which 50% of initial enzyme activity is lost following incubation for 15 min) were 6.2°C and 4.7-fold higher than those of the wild-type enzyme. Interestingly, the specific activity of the triple mutant was further increased to 260 U/mg. Protein modeling and docking analysis shed light on the origin of the improved activity and stability. In the asymmetric reduction of ethyl-4-chloro-3-oxobutyrate (COBE) on a 300-ml scale, 100 g/liter COBE could be completely converted by only 2 g/liter of lyophilized ScCR1_{A60T/I158V/P168S} within 9 h, affording an excellent enantiomeric excess (ee) of >99% and a space-time yield of 255 g liter^-1 day^-1 These results suggest high efficiency of the protein engineering strategy and good potential of the resulting variant for efficient synthesis of the atorvastatin precursor.IMPORTANCE Application of the carbonyl reductase ScCR1 in asymmetrically synthesizing (S)-CHBE, a key precursor for the blockbuster drug Lipitor, from COBE has been hindered by its low catalytic activity and poor thermostability and substrate tolerance. In this work, protein engineering was employed to improve the catalytic efficiency and stability of ScCR1. The catalytic efficiency, thermostability, and substrate tolerance of ScCR1 were significantly improved by structure-guided engineering and directed evolution. The engineered ScCR1 may serve as a promising biocatalyst for the biosynthesis of (S)-CHBE, and the protein engineering strategy adopted in this work would serve as a useful approach for future engineering of other reductases toward potential application in organic synthesis.