S-Adenosylmethionine decarboxylase (AdoMetDC) is a pyruvoyl-dependent enzyme that catalyzes the formation of the aminopropyl group donor in the biosynthesis of the polyamines spermidine and spermine. The enzyme is synthesized as a protein precursor and is activated by an autocatalytic serinolysis reaction that creates the pyruvoyl group. The autoprocessing reaction proceeds via an N --> O acyl rearrangement, generating first an oxyoxazolidine anion intermediate followed by an ester intermediate. A similar strategy is utilized in self-catalyzed protein splicing reactions and in autoproteolytic activation of protein precursors. Mutation of Ser68 to alanine in human AdoMetDC prevents processing by removing the serine side chain necessary for nucleophilic attack at the adjacent carbonyl carbon atom. We have determined the X-ray structure of the S68A mutant and have constructed models of the proenzyme and the oxyoxazolidine intermediate. Formation of the oxyoxazolidine intermediate is promoted by a hydrogen bond from Cys82 and stabilized by a hydrogen bond from Ser229. These observations are consistent with mutagenesis studies, which show that the C82S and C82A mutants process slowly and that the S229A mutant does not process at all. Donation of a proton by His243 to the nitrogen atom of the oxyoxazolidine ring converts the oxyoxazolidine anion to the ester intermediate. The absence of a base to activate the hydroxyl group of Ser68 suggests that strain may play a role in the cleavage reaction. Comparison of AdoMetDC with other self-processing proteins shows no common structural features. Comparison to histidine decarboxylase and aspartate decarboxylase shows that these pyruvoyl-dependent enzymes evolved different catalytic strategies for forming the same cofactor.