Phenolytic oxygenases

Phenolytic Oxygenases. In the oxidation of tryptophan by a Pseudomonas an intermediate, catechol, is converted to czs,as-muconic acid by an enzyme named pyrocatechase (VI). This enzyme requires ferrous... 

These results were taken as evidence that pyrocatechase and probably all other phenolytic oxygenases utilize two atoms of molecular oxygen for additive oxidation, and that the oxygen atoms in the water molecule were not utilized as a source of oxygen added to the substrate. 

Metal-binding agents, including KCN, a,a -dipyridyl, o-phenanthro-line, and EDTA, did not show appreciable inhibition either by direct addition or by preincubation of the inhibitor with the enzyme (10 M, 2°C, 16 hours). The iron, therefore, must be bound extremely tightly to the protein in the case of pyrocatechase, but it can easily be dissociated and reconstructed in the case of some other phenolytic oxygenases (see Chapter 3). 

The oxidation of tryptophan by various strains of Pseudomonas has been shown to proceed in all cases via kynurine. One sequence of reactions, the aromatic pathway, continues by eliminating the alanine side chain through the action of kynureninase, and subsequently utilizes oxygen for the formation of catechol and the pyrocatechase reaction already discussed. Another pathway retains the side chain of kynurenine and forms kynurenic acid through the action of kynurenine transaminase. A sequence of reactions has been indicated by recent work of Hayaishi and his associates (Kuno et al., 1961) this sequence appears to include three oxygenase reactions one hydroxylation and two phenolytic oxygenations (Fig. 18). 

The metabolism of tryptophan has been shown to utilize oxygenation reactions in many steps, both hydroxylation and phenolytic oxygenations. Several additional reactions involving compounds of diverse pathways of tryptophan degradation may be considered as potential candidates for inclusion among the oxygenases, although there is no direct evidence yet available about their natures. 

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