TheArabidopsis thaliana S-Adenosylmethionine decarboxylase (AdoMetDC) cDNA (GenBank TM U63633) was cloned. Site-specific mutagenesis was performed to introduce mutations at the conserved cysteine Cys 50 , Cys 83 , and Cys 230 , and lys 81 residues. In accordance with the human AdoMetDC, the C50A and C230A mutagenesis had minimal effect on catalytic activity, which was further supported by DTNB-mediated inactivation and reactivation. However, unlike the human AdoMetDC, the Cys 50 and Cys 230 mutants were much more thermally unstable than the wild type and other mutant AdoMetDC, suggesting the structural significance of cysteines. Furthermore, according to a circular dichroism spectrum analysis, the Cys 50 and Cys 230 mutants show a higher a-helix content and lower coiled-coil content when compared to that of wild type and the other mutant AdoMetDC. Also, the three-dimensional structure of Arabidopsis thaliana AdoMetDC could further support all of the data presented here. Summarily, we suggest that the Cys 50 and Cys 230 residues are structurally important.