Monomeric S-adenosylmethionine decarboxylase from plants provides an alternative to putrescine stimulation.
S-Adenosylmethionine decarboxylase has been implicated in cell growth and differentiation and is synthesized as a proenzyme, which undergoes autocatalytic cleavage to generate an active site pyruvoyl group. In mammals, S-adenosylmethionine decarboxylase is active as a dimer in which each protomer contains one alpha subunit and one beta subunit. In many higher organisms, autocatalysis and decarboxylation are stimulated by putrescine, which binds in a buried site containing numerous negatively charged residues. In contrast, plant S-adenosylmethionine decarboxylases are fully active in the absence of putrescine, with rapid autocatalysis that is not stimulated by putrescine. We have determined the structure of the S-adenosylmethionine decarboxylase from potato, Solanum tuberosum, to 2.3 A resolution. Unlike the previously determined human enzyme structure, the potato enzyme is a monomer in the crystal structure. Ultracentrifugation studies show that the potato enzyme is also a monomer under physiological conditions, with a weak self-association constant of 6.5 x 10(4) M(-)(1) for the monomer-dimer association. Although the potato enzyme contains most of the buried charged residues that make up the putrescine binding site in the human enzyme, there is no evidence for a putrescine binding site in the potato enzyme. Instead, several amino acid substitutions, including Leu13/Arg18, Phe111/Arg114, Asp174/Val181, and Phe285/His294 (human/potato), provide side chains that mimic the role of putrescine in the human enzyme. In the potato enzyme, the positively charged residues form an extensive network of hydrogen bonds bridging a cluster of highly conserved negatively charged residues and the active site, including interactions with the catalytic residues Glu16 and His249. The results explain the constitutively high activity of plant S-adenosylmethionine decarboxylases in the absence of putrescine and are consistent with previously proposed models for how putrescine together with the buried, negatively charged site regulates enzyme activity.
Bennett
Eric M
EM
Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853-1301, USA.
Ekstrom
Jennifer L
JL
Pegg
Anthony E
AE
Ealick
Steven E
SE
D012437:Adenosylmethionine Decarboxylase / Q000737:chemistry*; D019943:Amino Acid Substitution; D000818:Animals; D001665:Binding Sites; D018360:Crystallography, X-Ray; D019281:Dimerization; D006801:Humans; D006860:Hydrogen Bonding; D008958:Models, Molecular; D010940:Plant Proteins / Q000737:chemistry*; D011487:Protein Conformation; D015800:Protozoan Proteins / Q000737:chemistry; D011700:Putrescine / Q000737:chemistry*; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D011198:Solanum tuberosum / Q000201:enzymology*; D055672:Static Electricity
D010940:Plant Proteins; D015800:Protozoan Proteins; D012437:Adenosylmethionine Decarboxylase; D011700:Putrescine