Impact of organic solvents on the catalytic performance of halohydrin dehalogenase.

Biocatalytic transformations in organic synthesis often require the use of organic solvents to improve substrate solubility and promote the product formation. Halohydrin dehalogenases (HHDHs) are enzymes that catalyze the formation and conversion of epoxides, important synthetic class of compounds that are often sparingly soluble in water and prone to hydrolysis. In this study, the activity, stability, and enantioselectivity of HHDH from Agrobacterium radiobacter AD1 (HheC) in form of cell-free extract were evaluated in various aqueous-organic media. A correlation was discovered between the enzyme activity in the ring-closure reaction and logP of the solvent. Knowledge of such a relationship makes biocatalysis with organic solvents more predictable, which may reduce the need to experiment with a variety of solvents in the future. The results revealed a high enzyme compatibility with hydrophobic solvents (e.g., n-heptane) in terms of activity and stability. Regarding the HHDH applicability in an organic medium, inhibitions by a number of solvents (e.g., THF, toluene, chloroform) proved to be a more challenging problem than the protein stability, especially in the ring-opening reaction, thus suggesting which solvents should be avoided. In addition, solvent tolerance of the thermostable variant ISM-4 was also evaluated, revealing increased stability and to a lesser extent enantioselectivity compared to the wild-type. This is the first time such a systematic analysis has been reported, giving insight into the behavior of HHDHs in nonconventional media and opening new opportunities for the future biocatalytic applications. KEY POINTS: • HheC performs better in the presence of hydrophobic than hydrophilic solvents. • Enzyme activity in the PNSHH ring-closure reaction is a function of the logP. • Thermostability of ISM-4 variant is accompanied by superior solvent tolerance.

Biocatalytic approach to chiral fluoroaromatic scaffolds.

Ten different fluorinated aromatic epoxides have been tested as potential substrates for halohydrin dehalogenase (HHDH) HheC. The majority of investigated epoxides are useful building blocks in synthetic chemistry applications, with a number of them being polysubstituted. Moderate to high enantioselectivities (ER = 15 → 200) were observed in azidolysis, allowing the synthesis of enantioenriched (R)-azido alcohols containing fluorine in the molecule. In the case where a reaction runs over 50% conversion, enantiopure (S)-epoxides are also available. While o-F-styrene oxide was easily converted into a product, a sterically challenging o-CF3-derivative was not accepted by HheC. In silico probing of the binding site indicates that, in order to accommodate an o-CF3-derivative in the HheC active site, it is necessary to eliminate steric hindrance. Hence, we extended our research by probing several available HheC variants containing relevant modifications in the active site. The active mutant P84V/F86P/T134A/N176A (named HheC-M4) was identified, showing not only high activity towards o-CF3-styrene oxide, but also inverted enantioselectivity (ES = 27). Since (S)-enantioselective HHDHs are rare and therefore valuable for their synthetic application, this enzyme was screened on the initial panel of substrates. The observed (S)-enantioselectivity (ES = 1-111) is ascribed to the formation of the additional space by introduced mutations in HheC-M4, which is also confirmed by classical MD simulations. Successive molecular docking demonstrated that this newly formed tunnel located close to the protein surface is a critical feature of HheC-M4, representing a novel binding site.

Inhibitory Effect of DMSO on Halohydrin Dehalogenase: Experimental and Computational Insights into the Influence of an Organic Co-solvent on the Structural and Catalytic Properties of a Biocatalyst.

Invited for the cover of this issue are Zlatko Brkljaca, Maja Majeric Elenkov and co-workers at the Ruder Boskovic Institute and University of Zagreb. The image depicts the enzyme halohydrin dehalogenase HheC, which is made up of four identical subunits, with marked catalytic residues and volumetric maps of water and DMSO in the active site. Read the full text of the article at 10.1002/chem.202201923.

Inhibitory Effect of DMSO on Halohydrin Dehalogenase: Experimental and Computational Insights into the Influence of an Organic Co-solvent on the Structural and Catalytic Properties of a Biocatalyst.

Although the application of organic solvents in biocatalysis is well explored, in-depth understanding of the interactions of solvent with proteins, in particular oligomeric ones, is still scant. Understanding these interactions is essential in tailoring enzymes for industrially relevant catalysis in nonaqueous media. In our study, the homotetrameric enzyme halohydrin dehalogenase (HHDH) from Agrobacterium radiobacter AD1 (HheC) was investigated, as a model system, in DMSO/water solvent mixtures. DMSO, the most commonly used co-solvent for biocatalytic transformations, was found to act as a mixed-type inhibitor with a prevalent competitive contribution. Even 5 % (v/v) DMSO inhibits the activity of HheC by half. Molecular dynamics (MD) simulations showed that DMSO keeps close to Ser-Tyr catalytic residues forming alternate H-bonds with them. Stability measurements paired with differential scanning calorimetry, dynamic light scattering methods and MD studies revealed that HheC maintains its structural integrity with as much as 30 % (v/v) DMSO.

A single point mutation enhances hydroxynitrile synthesis by halohydrin dehalogenase.

The cyanide-mediated ring opening of epoxides catalyzed by halohydrin dehalogenases yields ß-hydroxynitriles that are of high interest for synthetic chemistry. The best studied halohydrin dehalogenase to date is the enzyme from Agrobacterium radiobacter, but this enzyme (HheC) exhibits only low cyanolysis activities. Sequence comparison between a pair of related halohydrin dehalogenases from Corynebacterium and Mycobacterium suggested that substitution of a threonine that interacts with the active site might be responsible for the higher cyanolytic activity of the former enzyme. Here we report that a variant of HheC in which this substitution (T134A) is adopted displays an up to 11-fold higher activity in cyanide-mediated epoxide ring-opening. The mutation causes removal of the hydrogen bond between residue 134 and the side chain O of the active site serine 132, which donates a hydrogen bond to the substrate oxygen. The mutation also increases dehalogenase rates with various substrates. Structural analysis revealed that the anion-binding site of the mutant enzyme remained unaltered, showing that the enhanced activity is due to altered interactions with the substrate oxygen rather than changes in the nucleophile binding site.

Catalytic activity of halohydrin dehalogenases towards spiroepoxides.

A novel activity of halohydrin dehalogenases towards spiroepoxides has been found. The enzyme from Arthrobacter sp. (HheA) catalysed highly regioselective azidolysis of spiroepoxides containing 5, 6 and 7-membered cycloalkane rings, while the enzyme from Agrobacterium radiobacter (HheC), besides high regioselectivity, also displayed moderate to high enantioselectivity (E up to >200) that can be applied for the kinetic resolution of chiral spiroepoxides. The orientations of spiroepoxides in the active site of halohydrin dehalogenases were studied by quantum-chemical calculations and docking simulations. Analyses of the complexes obtained revealed the origins of diastereoselectivity and enantioselectivity of the investigated biotransformations.

Key residues for controlling enantioselectivity of Halohydrin dehalogenase from Arthrobacter sp. strain AD2, revealed by structure-guided directed evolution.

Halohydrin dehalogenase from Agrobacterium radiobacter AD1 (HheC) is a valuable tool in the preparation of R enantiomers of epoxides and ß-substituted alcohols. In contrast, the halohydrin dehalogenase from Arthrobacter sp. AD2 (HheA) shows a low S enantioselectivity toward most aromatic substrates. Here, three amino acids (V136, L141, and N178) located in the two neighboring active-site loops of HheA were proposed to be the key residues for controlling enantioselectivity. They were subjected to saturation mutagenesis aimed at evolving an S-selective enzyme. This led to the selection of two outstanding mutants (the V136Y/L141G and N178A mutants). The double mutant displayed an inverted enantioselectivity (from S enantioselectivity [E(S)] = 1.7 to R enantioselectivity [E(R)] = 13) toward 2-chloro-1-phenylethanol without compromising enzyme activity. Strikingly, the N178A mutant showed a large enantioselectivity improvement (E(S) > 200) and a 5- to 6-fold-enhanced specific activity toward (S)-2-chloro-1-phenylethanol. Further analysis revealed that those mutations produced some interference for the binding of nonfavored enantiomers which could account for the observed enantioselectivities. Our work demonstrated that those three active-site residues are indeed crucial in modulating the enantioselectivity of HheA and that a semirational design strategy has great potential for rapid creation of novel industrial biocatalysts.

Sequential kinetic resolution catalyzed by halohydrin dehalogenase.

A sequential kinetic resolution catalyzed by halohydrin dehalogenase was employed for the synthesis of two valuable enantiopure building blocks. Resolution of methyl 4-chloro-3-hydroxybutanoate methylester ((R,S)-2) with use of a Trp249Phe mutant of halohydrin dehalogenase yielded methyl 4-cyano-3-hydroxybutanoate methylester ((S)-4) with 96.8% ee (40% yield) and (S)-2 with 95.2% ee (41% yield). This reaction is carried out in aqueous solution under mild conditions and provides access to a useful statin side-chain building block.