Class II decarboxylases

With respect to class II enzymes, the primary structure analysis and site-directed mutagenesis study of 2,6-dihydroxybenzoate decarboxylase have been reported.37 38 2,6-Dihydroxybenzoate decarboxylase shows 20-50% identities to 2,3-dihydroxybenzoate decarboxylase, 5-carboxyvanillate decarboxylase, 2-amino-3-carboxymuconate-6-semialdehyde decarboxylase, and several hypothetical proteins. The multiple alignment of 2,6-dihydroxybenzoate decarboxylase of Rhizobium strains and the orthologous proteins including hypothetical proteins indicated that two histidine residues were conserved. A site-directed mutagenesis study revealed the histidine residues to be essential for the catalytic activity.38 As for 2,3-dihydroxybenzoate decarboxylase, the presence of histidine, tryptophan, and cystein residues was suggested at the active site of the enzyme from Aspergillus niger.57,58 To elucidate the reaction mechanism of decarboxylation/carboxylation reaction without the involvement of cofactor, further site-directed mutagenesis studies and X-ray crystallographic analysis are required.

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