Protein engineering of toluene ortho-monooxygenase of Burkholderia cepacia G4 for regiospecific hydroxylation of indole to form various indigoid compounds.

Previous work showed that random mutagenesis produced a mutant of toluene ortho-monooxygenase (TOM) of Burkholderia cepacia G4 containing the V106A substitution in the hydroxylase alpha-subunit (TomA3) that changed the color of the cell suspension from wild-type brown to green in rich medium. Here, DNA shuffling was used to isolate a random TOM mutant that turned blue due to mutation TomA3 A113V. To better understand the TOM reaction mechanism, we studied the specificity of indole hydroxylation using a spectrum of colored TOM mutants expressed in Escherichia coli TG1 and formed as a result of saturation mutagenesis at TomA3 positions A113 and V106. Colonies expressing these altered enzymes ranged in color from blue through green and purple to orange; and the enzyme products were identified using thin-layer chromatography, high performance liquid chromatography, and liquid chromatography-mass spectroscopy. Derived from the single TOM template, enzymes were identified that produced primarily isoindigo (wild-type TOM), indigo (A113V), indirubin (A113I), and isatin (A113H and V106A/A113G). The discovery that wild-type TOM formed isoindigo via C-2 hydroxylation of the indole pyrrole ring makes this the first oxygenase shown to form this compound. Variant TOM A113G was unable to form indigo, indirubin, or isoindigo (did not hydroxylate the indole pyrrole ring), but produced 4-hydroxyindole and unknown yellow compounds from C-4 hydroxylation of the indole benzene ring. Mutations at V106 in addition to A113G restored C-3 indole oxidation, so along with C-2 indole oxidation, isatin, indigo, and indirubin were formed. Other TomA3 V106/A113 mutants with hydrophobic, polar, or charged amino acids in place of the Val and/or Ala residues hydroxylated indole at the C-3 and C-2 positions, forming isatin, indigo, and indirubin in a variety of distributions. Hence, for the first time, a single enzyme was genetically modified to produce a wide range of colors from indole.