Anaerobic degradation of α-resorcylate by Thauera aromatica strain AR-1 proceeds via oxidation and decarboxylation to hydroxyhydroquinone

Anaerobic degradation of alpha-resorcylate (3,5-dihydroxybenzoate) was studied with the denitrifying strain AR-1, which was assigned to the described species Thauera aromatica. alpha-Resorcylate degradation does not proceed via the benzoyl-CoA, the resorcinol, or the phloroglucinol pathway. Instead, alpha-resorcylate is converted to hydroxyhydroquinone (1,2,4-trihydroxybenzene) by dehydrogenative oxidation and decarboxylation. Nitrate, K3[Fe(CN)6], dichlorophenol indophenol, and the NAD+ analogue 3-acetylpyridine adeninedinucleotide were suitable electron acceptors for the oxidation reaction; NAD+ did not function as an electron acceptor. The oxidation reaction was strongly accelerated by the additional presence of the redox carrier phenazine methosulfate, which could also be used as sole electron acceptor. Oxidation of alpha-resorcylate with molecular oxygen in cell suspensions or in cell-free extracts of alpha-resorcylate- and nitrate-grown cells was also detected although this bacterium did not grow with alpha-resorcylate under an air atmosphere. alpha-Resorcylate degradation to hydroxyhydroquinone proceeded in two steps. The alpha-resorcylate-oxidizing enzyme activity was membrane-associated and exhibited maximal activity at pH 8.0. The primary oxidation product was not hydroxyhydroquinone. Rather, formation of hydroxyhydroquinone by decarboxylation of the unknown intermediate in addition required the cytoplasmic fraction and needed lower pH values since hydroxyhydroquinone was not stable at alkaline pH.