Ethylbenzene dehydrogenases

Ethylbenzene 0 1-phenylethanol

Azoarcus enzyme, which is membrane-bound, requires quinone as an electron acceptor; the product is pure (S)-1-phenylethanol. Not all analogues are substrates, but propylbenzene and p -ethylfluorobenzene are hydroxylated [K241].

Byssochlamys fulva vanillyl alcohol oxidase is a homodimer, monomeric molecular weight 58 000, which oxidizes p -hydroxyphenylethane and p -hydroxyphenylpropane to the corresponding (S)-1-(p -hydroxyphenyl)alcohols [K164].

E.coli enzyme forms mainly the (R )-isomer [G740]

Mortierella isabellina enzyme forms both (R)- and (S)-phenylethanols, and the identity of the para substituent determines which predominates. In no case was one isomer found to be the exclusive product. With bromo and nitro substitution the enantiomeric enrichment was only slight, whereas with methoxy and chloro (S) predominates (between 20 per cent and 40 per cent enantiomeric excess), but with cyano, methyl, ethyl and fluoro (R) predominates [D969].

A Penicillium simplicissimum vanillyl alcohol oxidase acts on 4-allyl- and 4-alkylphenols (1-3 C chain). Further oxidation forms the corresponding acetophenone and propiophenone. p -Propylphenol also forms a small amount of p -coumaryl alcohol, and ethylphenol and propylphenol form vinylphenol and propenylphenol respectively. A quinone methide intermediate is probable for the formation of (R )-benzylic alcohols, which are formed in about 94 per cent enantiomeric purity. The hydroxyl group comes from water [J16, J664, J680].

The (S)-isomer is formed by Pseudomonas from ethylbenzene [J219]. P. fluorescens eugenol dehydrogenase forms (S)-1-(p -hydroxyphenyl)ethanol and (S)-1-(p -hydroxyphenyl)propanol from the corresponding 4-alkylphenols [K177].

Pseudomonas putida 4-ethylphenol methylenehydroxylase is a flavocytochrome c composed of two pairs of subunits, molecular weight about 120000. One subunit, molecular weight 50 000 is a flavoprotein and the other, molecular weight 10 000 is a cytochrome c. The mechanism involves dehydrogenation followed by hydration to form 1-(p -hydroxyphenyl)ethanol. It acts on a range of 4-alkylphenols with up to nine carbon atoms in the side chain and on 5-indanol (a cyclic analogue of 4-ethylphenol). p -Cresol and 2,4-xylenol are substrates; and 1-(p -hydroxyphenyl)ethanol is further oxidized to p -hydroxyacetophenone. It is considered that the reaction proceeds via a quinone methide [F381]. In another study the enzyme was called 4-ethylphenol methylenehydroxylase [E396].

This type of reaction has also been observed in man [D482], rat [D365], rabbit [A287], chinook salmon [E710], Desulfobacula toluolica [J847], Nocardia tartaricans [B144] and in Nitrosomonas [H219]; in N. europaea ammonia monooxygenase catalyzes the reaction [H631]; in many studies, particularly the early ones the chirality of the product was not established.

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