Second-Sphere Interactions between the C93-Y157 Cross-Link and the Substrate-Bound Fe Site Influence the O2 Coupling Efficiency in Mouse Cysteine Dioxygenase

Cysteine dioxygenase (CDO) is a non-heme iron enzyme that catalyzes the O-2-dependent oxidation of L-cysteine (L-Cys) to produce cysteinesulfinic acid (CSA). Adjacent to the Fe site of CDO is a covalently cross-linked cysteine tyrosine pair (C93-Y157). While several theories have been proposed for the function of the C93-Y157 pair, the role of this post-translational modification remains unclear. In this work, the steady-state kinetics and O-2/CSA coupling efficiency were measured for wild-type CDO and selected active site variants (Y157F, C93A, and H155A) to probe the influence of second-sphere enzyme-substrate interactions on catalysis. In these experiments, it was observed that both k(cat) and the O-2/CSA coupling efficiency were highly sensitive to the presence of the C93-Y157 cross-link and its proximity to the substrate carboxylate group. Complementary electron paramagnetic resonance (EPR) experiments were performed to obtain a more detailed understanding of the second-sphere interactions identified in O-2/CSA coupling experiments. Samples of the catalytically inactive substrate-bound Fe-III-CDO species were treated with cyanide, resulting in a low-spin (S = 1/2) ternary complex. Remarkably, both the presence of the C93-Y1S7 pair and interactions with the Cys carboxylate group could be readily identified by perturbations to the rhombic EPR signal. Spectroscopically validated active site quantum mechanics/molecular mechanics and density functional theory computational models are provided to suggest a potential role for Y157 in the positioning of the substrate Cys in the active site and to verify the orientation of the g-tensor relative to the CDO Fe site molecular axis.