Anaerobic metabolism of aromatic compounds via the benzoyl-CoA pathway

Aromatic compounds are important growth substrates for microorganisms. They form a large group of diverse compounds including lignin monomers, amino acids, quinones, and flavonoids. Aerobic aromatic metabolism is characterized by the extensive use of molecular oxygen which is essential for the hydroxylation and cleavage of aromatic ring structures. The anaerobic metabolism of low molecular mass soluble aromatic compounds requires, of necessity, a quite different strategy. In most known cases, aromaticity is broken by reduction and the ring is subsequently opened hydrolytically. A small number of different central aromatic intermediates can be reduced, the most common of which is benzoyl-CoA, a compound that is formed as a central intermediate in the degradation of a large number of aromatic growth substrates. This review concentrates on the anaerobic aromatic metabolism via the benzoyl-CoA pathway. The peripheral pathways that transform growth substrates to benzoyl-CoA include various types of novel reactions, for example carboxylation of phenolic compounds, reductive elimination of ring substituents like hydroxyl or amino groups, oxidation of methyl substituents, O-demethylation reactions and shortening of aliphatic side chains. The central benzoyl-CoA pathway differs in several aspects in the denitrifying, phototrophic and fermenting bacteria studied. In denitrifying and phototrophic bacteria it starts with the two-electron reduction of benzoyl-CoA to a cyclic dienoyl-CoA driven by the hydrolysis of two molecules of ATP to ADP+Pi. This ring reduction is catalyzed by benzoyl-CoA reductase and requires a low-potential ferredoxin as an electron donor. In Rhodopseudomonas palustris the cyclic diene is further reduced to cyclohex-1-ene-1-carboxyl-CoA. In the denitrifying species Thauera aromatica, the cyclic diene is hydrated to give 6-hydroxycyclohex-1-ene-1-carboxyl-CoA. Subsequent β-oxidation results in the formation of a cyclic β-oxo compound, followed by hydrolytic carbon ring opening yielding 3-hydroxypimelyl-CoA in the case of T. aromatica and pimelyl-CoA in the case of R. palustris. These intermediates are further β-oxidized via glutaryl-CoA; final products are 3 acetyl-CoA and 1 CO2. In fermenting bacteria benzoyl-CoA may possibly be reduced to the level of cyclohex-1-ene-1-carboxyl-CoA in an ATP-independent reaction. The genes coding for the enzymes of the central benzoyl-CoA pathway have been cloned and sequenced from R. palustris, T. aromatica, and Azoarcus evansii. Sequence analyses of the genes support the concept that phototrophic and denitrifying bacteria use two slightly different pathways to metabolize benzoyl-CoA. The gene sequences have in some cases been very helpful for the identification of possible catalytic mechanisms that were not obvious from initial characterizations of purified enzymes.