Dihydroxybenzoate decarboxylase

HO OH HO OH

COOH

COOH

Scheme 5

2,3-Dihydroxybenzoate decarboxylase has been reported to be involved in the metabolism of indole, tryptophan, and anthranilic acid.4041 The primary structure of 2,3-dihydroxybenzoate decaroboxylase of Aspergillus niger has been revealed by gene analysis, which exhibits significant homology with those of 2,6-dihydroxybenzoate decarboxylases (unpublished data). This suggests that 2,3-dihydroxybenzoate decarboxylase also catalyzes the regioselective carboxylation of catechol to 2,3-dihydroxybenzoate, although the carboxylation activity has not been examined.

2,3-Dihydroxybenzoate decarboxylase purified from Aspergillus niger has a molecular mass of 120 kDa and consists of four identical 28 kDa sub-units. 2,3-Dihydroxybenzoate has a Km value of 340 jxM and does not act on salicylate, anthranilate, 2,3-dihydroxybenzaldehyde, 2,4-dihydroxybenzoate,

3-hydroxyanthranilate, 3-hydroxybenzoate, 2,3-dihydroxybenzoate, benzoate, or

4-hydroxybenzoate.42 The enzyme of Trichosporon cutaneum is a homodimer of identical 36.5 kDa subunits, which also catalyzes the decarboxylation of 2,3,5-trihydroxybenzoate and 2,3,6-trihydroxybenzoate.41 2,3-Dihdroxybenzoate decarboxylase can be purified, without the addition of sulfhydryl-protecting reagents to the purification buffer, indicating that the decarboxylases are insensitive to O2.

Recently, the distribution of 2,3-dihydroxybenzoate decarboxylase has been found in a variety of fungal strains (unpublished data), and the carboxylation activity for catechol is confirmed by the reaction using resting cells (or cell-free extract) in the presence of 3 M KHCO3. The detailed comparative studies of enzyme structures and catalytic properties between 2,3-dihydroxybenzoate decarboxylase and 3,4-dihyroxybenzoate decarboxylase might explain how the decarboxylases catalyze the regioselective carboxylation of catechol.

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