Cysteine dioxygenase (CDO) is a non-heme, mononuclear-iron enzyme which catalyses the addition of molecular oxygen to the thiol group of cysteine residues forming cysteine sulfinic acid. In plants, this dioxidation occurs on N-terminal cysteine residues which promotes N-terminal arginylation and ubiquitin mediated degradation of target proteins which mediate the response to oxygen depletion. This pathway facilitates both submergence-tolerance and seed maturation, contributing to enhanced crop yield.
As mammalian CDO is involved in cysteine catabolism, (which prevents the cytotoxic and neurotoxic effects of unregulated free cysteine) determination of its mechanism is clinically relevant. Dual sequence alignment of plant and mammalian CDO suggests that although the same iron-coordinating residues are present in plant CDO, many proposed catalytic amino acids are not. Kinetic- and structural-based characterisation of plant CDO offers a unique perspective on the minimal requirements for CDO activity.
Homology modelling suggests that the central domain of plant CDO is well ordered, adopting a structure similar to mammalian CDO whilst the N- and C- terminal domains are inherently disordered. Bioinformatic analyses show that the N-terminal domain contains a bipartite nuclear localisation signal and in vitro experiments indicate that this basic domain also interacts with DNA. Preliminary 1H NMR of the product of plant CDO with free cysteine show spectral features characteristic of cystine demonstrating that plant CDO is not able to dioxidise free cysteine. In contrast to the lack of activity against free cysteine, colorimetric substrate depletion assays instead show enzymatic activity with short peptide substrates containing an N-terminal cysteine.