Mechanistic basis for the enantioselectivity of the anaerobic hydroxylation of alkylaromatic compounds by ethylbenzene dehydrogenase

• Published in: Journal of inorganic biochemistry 2014-10, Vol.139, p.9-20
• Main Authors: Szaleniec, Maciej, Dudzik, Agnieszka, Kozik, Bartłomiej, Borowski, Tomasz, Heider, Johann, Witko, Małgorzata
• Format: Article
• Language: English
• Subjects: Bacterial Proteins - chemistry; Benzene Derivatives - chemistry; Catalytic Domain; Chiral chromatography; DFT; Enantioselective reaction; Ethylbenzene dehydrogenase; Hydrogen Bonding; Hydroxylation; Models, Chemical; Models, Molecular; Molybdenum enzymes; Oxidation-Reduction; Oxidoreductases - chemistry; Oxygen - chemistry; QM:MM; Quantum Theory; Rhodocyclaceae - enzymology; Stereoisomerism; Substrate Specificity; Thermodynamics
• ISSN: 0162-0134; 1873-3344
• DOI: 10.1016/j.jinorgbio.2014.05.006

The enantioselectivity of reactions catalyzed by ethylbenzene dehydrogenase, a molybdenum enzyme that catalyzes the oxygen-independent hydroxylation of many alkylaromatic and alkylheterocyclic compounds to secondary alcohols, was studied by chiral chromatography and theoretical modeling. Chromatographic analyses of 22 substrates revealed that this enzyme exhibits remarkably high reaction enantioselectivity toward (S)-secondary alcohols (18 substrates converted with >99% ee). Theoretical QM:MM modeling was used to elucidate the structure of the catalytically active form of the enzyme and to study the reaction mechanism and factors determining its high degree of enantioselectivity. This analysis showed that the enzyme imposes strong stereoselectivity on the reaction by discriminating the hydrogen atom abstracted from the substrate. Activation of the pro(S) hydrogen atom was calculated to be 500 times faster than of the pro(R) hydrogen atom. The actual hydroxylation step (i.e., hydroxyl group rebound reaction to a carbocation intermediate) does not appear to be enantioselective enough to explain the experimental data (the calculated rate ratios were in the range of only 2–50 for pro(S): pro(R)-oriented OH rebound). Enantioselective CH activation: ethylbenzene dehydrogenase is achieving high reaction enantioselectivity (100% ee) by imposing steric constrains on CH activation process. As a result pro(S) H atom is preferentially activated leading to (S)-1-alkylaromatic and alkylheterocyclic alcohols. [Display omitted]