The Three-His Triad in Dke1: Comparisons to the Classical Facial Triad

The oxygen activating mononuclear non-heme ferrous enzymes catalyze a diverse range of chemistry yet typically maintain a common structural motif: two histidines and a carboxylate coordinating the iron center in a facial triad. A new Fe-II coordinating triad has been observed in two enzymes, diketone-cleaving dioxygenase, Dke1, and cysteine dioxygenase (CDO), and is composed of three histidine residues. The effect of this three-His motif in Dke1 on the geometric and electronic structure of the Fe-II center is explored via a combination of absorption, CD, MCD, and VTVH MCD spectroscopies and DFT calculations. This geometric and electronic structure of the three-His triad is compared to that of the classical (2-His-1-carboxylate) facial triad in the a-ketoglutarate (alpha KG)-dependent dioxygenases clavaminate synthase 2 (CS2) and hydroxyphenylpyruvate dioxygenase (HPPD). Comparison of the ligand fields at the Fe-II shows little difference between the three-His and 2-His-1-carboxylate facial triad sites. Acetylacetone, the substrate for Dke1, will also bind to HPPD and is identified as a strong donor, similar to alpha KG. The major difference between the three-His and 2-His-1-carboxylate facial triad sites is in MLCT transitions observed for both types of triads and reflects their difference in charge. These studies provide insight into the effects of perturbation of the facial triad ligation of the non-heme ferrous enzymes on their geometric and electronic structure and their possible contributions to reactivity.