Phenylacetyl-CoA : acceptor oxidoreductase, a membrane-bound molybdenum-iron-sulfur enzyme involved in anaerobic metabolism of phenylalanine in the denitrifying bacterium Thauera aromatica

Phenylacetic acids are common intermediates in the microbial metabolism of various aromatic substrates including phenylalanine. In the denitrifying ba cterium Thauera aromatica phenylacetate is oxidized, under anoxic condition s, to the common intermediate benzoyl-CoA via the intermediates phenylacety l-CoA and phenylglyoxylate (benzoylformate). The enzyme that catalyzes the four-electron oxidation of phenylacetyl-CoA has been purified from this bac terium and studied. The enzyme preparation catalyzes the reaction phenylace tyl-CoA + 2 quinone + 2 H2O --> phenylglyoxylate + 2 quinone H-2 + CoASH. P henylacetyl-CoA:acceptor oxidoreductase is a membrane-bound molybdenum-iron -sulfur protein. The purest preparations contained three subunits of 93, 27 , and 26 kDa. Ubiquinone is most likely to act as the electron acceptor, an d the oxygen atom introduced into the product is derived from water. The pr otein preparations contained 0.66 mol Mo, 30 mol Fe, and 25 mol acidlabile sulfur per mol of native enzyme, assuming a native molecular mass of 280 kD a. Phenylglyoxylyl-CoA, but not mandelyl-CoA, was observed as a free interm ediate. All enzyme preparations also catalyzed the subsequent hydrolytic re lease of coenzyme A from phenylglyoxylyl-CoA but not from phenylacetyl-CoA. The enzyme is reversibly inactivated by a low concentration of cyanide, bu t is remarkably stable with respect to oxygen. This new member of the molyb doproteins represents the first example of an enzyme which catalyzes the al pha-oxidation of a CoA-activated carboxylic acid without utilizing molecula r oxygen.