ANAEROBIC OXIDATION OF PHENYLACETATE AND 4-HYDROXYPHENYLACETATE TO BENZOYL-COENZYME-A AND CO2 IN DENITRIFYING PSEUDOMONAS SP - EVIDENCE FORAN ALPHA-OXIDATION MECHANISM

Anaerobic degradation of (4-hydroxy)phenylacetate in denitrifying Pseu domonas sp. was investigated. Evidence is presented for alpha-oxidatio n of the coenzyme A (CoA)-activated carboxymethyl side chain, a reacti on which has not been described. The C6-C2 compounds are degraded to b enzoyl-CoA and furtheron to CO2 via the following intermediates: Pheny lacetyl-CoA, phenylglyoxylate, benzoyl-CoA plus CO2;4-hydroxyphenylace tyl-CoA, 4-hydroxyphenylglyoxylate, 4-hydroxybenzoyl-CoA plus CO2, ben zoyl-CoA. Trace amounts of mandelate possibly derived from mandelyl-Co A were detected during phenylacetate degradation in vitro. The reactio ns are catalyzed by (i) phenylacetate-CoA ligase which converts phenyl acetate to phenylacetyl-CoA and by a second enzyme for 4-hydroxyphenyl acetate; (ii) a (4-hydroxy)-phenylacetyl-CoA dehydrogenase system whic h oxidizes phenylacetyl-CoA to (4-hydroxy)phenylglyoxylate plus CoA; a nd (iii) (4-hydroxy)phenylglyoxylate: acceptor oxidoreductase (CoA acy lating) which catalyzes the oxidative decarboxylation of (4-hydroxy)ph enylglyoxylate to (4-hydroxy)benzoyl-CoA and CO2. (iv) The degradation of 4-hydroxyphenylacetate in addition requires the reductive dehydrox ylation of 4-hydroxybenzoyl-CoA to benzoyl-CoA, catalyzed by 4-hydroxy benzoyl-CoA reductase (dehydroxylating). The whole cell regulation of these enzyme activities supports the proposed pathway. An ionic mechan ism for anaerobic alpha-oxidation of the CoA-activated carboxymethyl s ide chain is proposed.