Ethylbenzene dehydrogenase, a novel hydrocarbon-oxidizing molybdenum/iron-sulfur/heme enzyme

The initial enzyme of ethylbenzene metabolism in denitrifying Azoarcus stra in EbN1, ethylbenzene dehydrogenase, was purified and characterized. The so luble periplasmic enzyme is the first known enzyme oxidizing a nonactivated hydrocarbon without molecular oxygen as cosubstrate, It is a novel molybde num/iron-sulfur/ heme protein of 155 kDa, which consists of three subunits (96, 43, and 23 kDa) in an alpha beta gamma structure. The N-terminal amino acid sequence of the alpha subunit is similar to that of other molybdenum proteins such as selenate reductase from the related species Thauera selena tis, Ethylbenzene dehydrogenase is unique in that it oxidizes the hydrocarb on ethylbenzene, a compound without functional groups, to (S)-1-phenylethan ol. Formation of the product was evident by coupling tot an enantiomer-spec ific (S)-1-phenylethanol dehydrogenase from the same organism. The apparent K-m of the enzyme for ethylbenzene is very low at <2 <mu>M, Oxygen does no t affect ethylbenzene dehydrogenase activity in extracts but inactivates th e purified enzyme, if the heme b cofactor is in the reduced state. A varian t of ethylbenzene dehydrogenase exhibiting significant activity also with t he homolog n-propylbenzene was detected in a related Azoarcus strain (PbN1) .