Structural and mechanistic properties of E. coli adenosylmethionine decarboxylase.

Adenosylmethionine decarboxylase catalyzes one of the first committed steps in polyamine biosynthesis. It is a member of a small class of decarboxylases that use a pyruvovyl prosthetic group rather than the more common pyridoxal cofactor. We have recently shown that AdoMet decarboxylase from E. coli is composed of stoichiometric amounts of two types of subunits; alpha (Mr = 19,000), and beta (Mr = 14,000). The NH2-terminal of the alpha subunit is blocked by the pyruvoyl group and can be sequenced only after reductive amination, which converts this to an alanine residue. The beta subunit, on the other hand, has an unblocked NH2-terminal and sequences normally. The molecular weight of the holoenzyme, estimated by gel filtration, is 136,000 suggesting that the enzyme is an alpha 4 beta 4 octamer. AdoMet decarboxylase undergoes a time dependent inactivation during turnover. The mechanism of this inactivation involves a transamination from the product, decarboxylated AdoMet, and the pyruvoyl group generating an NH2-terminal alanine. The nascent product aldehyde then eliminates methylthioadenosine, resulting in the formation of acrolein, which covalently labels the alpha subunit. How this mechanism may explain AdoMet decarboxylase turned over, and how AdoMet decarboxylase inhibitors can affect its half life will be discussed.