Identification of functionally important residues of Arabidopsis thaliana S-adenosylmethionine decarboxylase

The Arabidopsis thaliana S-adenosylmethionine decarboxylase (AdoMetDC) cDNA (GenBank (TM) U63633) was cloned, and the AdoMetDC protein was expressed, purified, and characterized, The K-m value for S-adenosylmethionine (AdoMet ) is 23.1 mu M and the K-1 value for methylglyoxal bis-(guanylhydrazone) (M GBG) is 0.15 mu M. Site-specific mutagenesis was performed on the AdoMetDC to introduce mutations at conserved cysteine (Cys(50), Cys(83), and Cys(230 )) and lysine(81) residues, chosen by examination of the conserved sequence and proved to be involved in enzymatic activity by chemical modification. The AdoMetDC mutants K81A and C83A retained up to 60 and 10% of wild type a ctivity, respectively, demonstrating that lysyl and sulfhydryl groups are r equired for full catalytic activity, However, changing Cys(50) and Cys(230) to alanine had minimal effects on the catalytic activity. Changing Lys(81) to alanine produced an altered substrate specificity. When lysine was used as a substrate instead of AdoMet, the substrate specificity for lysine inc reased 6-fold. The K-m value for AdoMet is 11-fold higher than that of the wild type, but the V-max value is more than 60%. Taken together, the result s suggest that the lysine(81) residue is critical for substrate binding.