Structural Basis of the Substrate Specificity of Rieske Nonheme Iron Oxygenase

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Carbazole 1,9a-dioxygenase (CARDO) catalyzes angular dioxygenation (AD) for carbazole (at 1, 9a carbons), lateral dioxygenation (LD) for polycyclic aromatic hydrocarbons and biphenyl, and monooxygenation for methylene carbon of fluorene and sulfide sulfur of dibenzothiophene. To clarify the molecular mechanism determining the substrate specificity of this novel enzyme, 28 site-specific mutants having single (17) or double (11) amino acid substitution (s) of I262, F275, Q282, and/or F329, which located close vicinity to active site iron in substrate-binding pocket, were created. Then, their oxygenation activities for various aromatic compounds were determined. In addition, some of the mutant enzymes showing markedly different substrate specificity than wild-type CARDO were crystallized and their three-dimensional structures were determined as single and/or substrate-bound forms.As the results, it was revealed that I262 and F275 were important amino acid residues, which largely affected the substrate binding manner of carbazole, fluorene, and/or fluoranthene. Structural analyses clearly indicated that these amino acid substitutions resulted in the change in the docking manners of the some of the substrates. Different docking positions lead different carbon atoms of substrates to situate closer to active site iron than the case with wild type enzyme. This is a reason why CARDO mutants catalyze the different type of oxygenation reactions.