Binding of Sulfur Ligands to Cysteine Dioxygenase: A Crystallographic, Spectroscopic and Computational Study

Abstract

Cysteine dioxygenase (CDO) is a non-heme mono-iron enzyme that catalyses the oxidation of cysteine to cysteine sulfinic acid (CSA) by the addition of the two oxygen atoms from dioxygen to the thiol group of cysteine. In many species, this is the first step of the catabolism of cysteine to its constituent parts. Elevated levels of cysteine in the body have been found to be associated with many disease states. Many well studied non-heme mono-iron enzymes contain the iron bound to two histidines and a carboxylate containing residue. However, as the iron in CDO is bound to three histidine residues, information about the mechanism of other non-heme mono-iron enzymes cannot be easily extrapolated to CDO.

The aim of this work was to structurally and electronically characterise CDO in the resting state and with the addition of the substrate (cysteine), two catalytically inactive cysteine analogues (homocysteine and 3-mercaptopropionic acid (3MPA)) and the product (CSA). These states were characterised using X-ray crystallography, Mössbauer spectroscopy and density functional theory calculations. The resting state of CDO contained an octahedral iron(II) with three labile water ligands. Cysteine bound bidentate to the iron(II) via the thiol and the amino group, while homocysteine bound to the ferrous iron by the sulfur and possibly by the amino group as well. Homocysteine was not a substrate of CDO as its increased size blocked the oxygen binding site to iron, preventing the reaction from occurring. 3MPA by itself did not interact strongly with the iron of CDO, however, a persulfide derivative of 3MPA bound directly to the iron of CDO. Information about CSA binding is consistent with CSA bound in a tridentate manner by the two oxygen atoms of the sulfinic acid group and the amine nitrogen. In all states, the iron remained as high spin iron(II). It was concluded that cysteine required the correct length and both the amine and carboxylate to bind to CDO and be catalytically active and that the mechanism occurred via oxygen activation with the iron playing a redox active role.