Halohydrin dehalogenases (HHDHs) are of biotechnological interest due to their promiscuous epoxide ring-opening activity with a set of negatively charged nucleophiles, enabling the formation of C−C, C−N, or C−O bonds. The recent discovery of HHDH-specific sequence motifs aided the identification of a large number of halohydrin dehalogenases from public sequence databases, enlarging the biocatalytic toolbox substantially. During the characterization of 17 representatives of these phylogenetically diverse enzymes, one HHDH, namely HheG from Ilumatobacter coccineus, was identified to convert cyclic epoxide substrates. The enzyme exhibits significant activity in the azidolysis of cyclohexene oxide and limonene oxide with turnover numbers of 7.8 and 44 s −1 , respectively. As observed for other HHDHs, the cyanide-mediated epoxide ring-opening proceeded with lower rates. Wild-type HheG displays modest enantioselectivity, as the resulting azido-and cyanoalcohols of cyclohexene oxide ring-opening were obtained in 40% enantiomeric excess. These biocatalytic findings were further complemented by the crystal structure of the enzyme refined to 2.3 Å. Analysis of HheG's structure revealed a large open cleft harboring the active site. This is in sharp contrast to other known HHDH structures and aids in explaining the special substrate scope of HheG.

HheG from Ilumatobacter coccineus is a halohydrin dehalogenase with synthetically useful activity in the ring opening of cyclic epoxides with various small anionic nucleophiles. This enzyme provides access to chiral β-substituted alcohols that serve as building blocks in the pharmaceutical industry. Wild-type HheG suffers from low thermostability, which poses a significant drawback for potential applications. In an attempt to thermostabilize HheG by protein engineering, several single mutants at position 123 were identified which displayed up to 14 °C increased apparent melting temperatures and up to three-fold higher activity. Aromatic amino acids at position 123 resulted even in a slightly higher enantioselectivity. Crystal structures of variants T123W and T123G revealed a flexible loop opposite to amino acid 123. In variant T123G, this loop adopted two different positions resulting in an open or partially closed active site. Classical molecular dynamics simulations confirmed a high mobility of this loop. Moreover, in variant T123G this loop adopted a position much closer to residue 123 resulting in denser packing and increased buried surface area. Our results indicate an important role for position 123 in HheG and give first structural and mechanistic insight into the thermostabilizing effect of mutations T123W and T123G.

Catalytic activity of protein crystals correlates with their particle size due to diffusion limitations. However, because the particles need to be restrained during, e.g., a filtration process, keeping a minimum particle size is required. Thus, knowledge of mechanical properties of enzyme crystals is needed for downstream process design in the biopharmaceutical industry to avoid particle breakage. In this study, hardness and Young's modulus of cross-linked lysozyme crystals and cross-linked halohydrin dehalogenase from Ilumatobacter coccineus (HheG) crystals are evaluated using atomic force microscopy. The results show that hardness of lysozyme and HheG crystals is in the range of 2−22 MPa. For Young's modulus of lysozyme crystals, values between 40 and 1820 MPa are measured, while HheG crystals ranging between 40 and 1200 MPa, respectively. The results for lysozyme crystals are comparable to those published in the current literature on native protein crystals. Hence, the cross-linking seems not to significantly affect both values. The investigation of the mechanical properties of HheG is pioneered in this study. Moreover, the mechanical properties are described as a cumulative distribution and fitted using the Weibull theory. The results show that hexagonal protein crystals have a multimodal distribution of mechanical properties, and hence, hardness and Young's modulus should not be reduced to their average values. In addition, this observation should be taken into consideration by further mechanical studies about protein crystals.

Biotransformation of testosterone into 15β-hydroxytestosterone by the cyanobacterium Synechocystis expressing the heterologous monooxygenase CYP110D1. The reaction is sustained by reducing equivalents and oxygen provided by oxygenic photosynthesis.

Halohydrin dehalogenases are promiscuous biocatalysts, which enable asymmetric ring opening reactions of epoxides with various anionic nucleophiles. However, despite the increasing interest in such asymmetric transformations, the substrate scope of G-type halohydrin dehalogenases toward cyclic epoxides has remained largely unexplored, even though this subfamily is the only one known to display activity with these sterically demanding substrates. Herein, we report on the exploration of the substrate scope of the two G-type halohydrin dehalogenases HheG and HheG2 and a newly identified, more thermostable member of the family, HheG3, with a variety of sterically demanding cyclic epoxides and anionic nucleophiles. This work shows that, in addition to azide and cyanide, these enzymes facilitate ring-opening reactions with cyanate, thiocyanate, formate, and nitrite, significantly expanding the known repertoire of accessible transformations.

No abstract

Halohydrin dehalogenases are promiscuous biocatalysts, which enable asymmetric ring opening reactions of epoxides with various anionic nucleophiles. However, despite the increasing interest in such asymmetric transformations, the substrate scope of G-type halohydrin dehalogenases toward cyclic epoxides has remained largely unexplored, even though this family is the only one known to display activity with these sterically demanding substrates. Herein, we report on the exploration of the substrate scope of the two G-type halohydrin dehalogenases HheG and HheG2 and a newly identified, more thermostable member of the family, HheG3, with a variety of sterically demanding cyclic epoxides and anionic nucleophiles. This work shows that, in addition to azide and cyanide, these enzymes facilitate ring-opening reactions with cyanate, thiocyanate, formate, and nitrite, significantly expanding the known repertoire of accessible transformations.

No abstract