Fig. 1. Rieske oxygenases adopt a trimeric architecture that sequesters the metal-based active site and reactive pathway intermediates in the interior of the protein.

a The quaternary architecture of an α₃β₃ Rieske oxygenase that is observed primarily in Rieske oxygenases that catalyze cis-dihydroxylation reactions^15–23. This panel is shown in two orientations that differ by a 90° rotation (PDB: 1NDO²²). Here, the three catalytic α-subunits are shown in gray and the β-subunits are shown in pink, cyan, and purple. b The quaternary architecture of a homotrimeric α₃ Rieske oxygenase is observed in 10 of the 18 available non-redundant Rieske oxygenase crystal structures^7–14 (PDB: 6WN3⁷). The active site of the α₃ Rieske oxygenases is also buried deep within the protein core. c The architecture of α₃β₃ and α₃ Rieske oxygenases require the substrate to travel to reach the active site before catalysis can occur. The extra architectural region that the substrate must traverse is known as the “keyhole” and unlike the traditional Lock-Key model of enzyme catalysis, requires the substrate to interact with protein regions outside of the active site³⁶. The identity and importance of these auxiliary “keyhole” regions and their relationship to the selectivity of a Rieske oxygenase catalyzed reaction is yet to be determined. d SxtT catalyzes the conversion β-saxitoxinol (β-STOH) to saxitoxin (STX) and e GxtA catalyzes the conversion of β-STOH and STX to 11-β-hydroxy-β-saxitoxinol and 11-β-hydroxysaxitoxin, respectively⁴². The selectivity exhibited by SxtT and GxtA can be partially attributed to two active site residues that position the substrate correctly for hydroxylation at the respective positions (marked with a red asterisk)⁷.