engleski

Biocatalytic scope of halohydrin dehalogenase from Mycobacterium sp. GP1 (HheB2) and mutant HheB2- T120A

Halohydrin dehalogenases (HHDHs) belong to a distinct group of enzymes catalyzing the removal of a halide ion and a proton from a vicinal halohydrin with formation of an epoxide. Moreover, these enzymes can catalyze the epoxide ring- opening reactions with a range of anionic nucleophiles affording β-substituted alcohols as products. Based on sequence similarities, they are divided into 7 phylogenetic groups: A, B, C, D, E, F and G. Activity, enantioselectivity and enantiopreference are dependent on the type of enzyme and the substrate structure. Among all, the enzyme HheC from Agrobacterium radiobacter (group C) is the most studied because of its high enantioselectivity and wide nucleophile scope. However, the major shortcoming of HheC is its narrow substrate tolerance, due to the relatively small active site. To expand the catalytic relevance of HHDHs we focus our attention to an enzyme from different organism. There are two similar enzymes in group B, one found in Corynebacterium sp. N1074 (HheB) and the other in Mycobacterium sp. GP1 (HheB2).They share high sequence identity (95%) with only 4 amino acid substitutions HheB/HheB2: F36/I36, T120/A120, C124/Y124, H125/Q125. HheB exhibits higher enantioselectivity than does HheB2. Because of that HheB2 has been previously characterised as non-enantioselective enzyme, and neglected as biocatalyst. In this work HheB2 was investigated in the ring-opening reaction on a set of 21 structurally different aliphatic and aromatic epoxides using sodium azide as nucleophile. In order to gain more insight into the difference in enantioselectivity of HheB2 and HheB, mutant HheB2-T120A was also investigated in the ring opening reactions. The screening confirmed low to moderate enantioselectivity of HheB2 towards monosubstituted epoxides, however high activity and enantioselectivity in conversion of 2, 2- disubstituted (E-values up to >200). Similar to HheC, the large enhancement of enantioselectivity is obtained when a second substituent (methyl or ethyl) is present at the chiral centre. T120A mutation has been shown to be a key mutation for increasing enantioselectivity in aliphatic epoxides, which is not the case for aromatic epoxides. The results further extend the repertoire of enantioselective HHDHs and their application in the kinetic resolution of epoxides.

biocatalyst ; HHDH ; enantioselectivity

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