Metal ion activation of S-adenosylmethionine decarboxylase reflects cation charge density

S-Adenosylmethionine decarboxylase from Escherichia coli is a pyruvoyl cofactor-containing enzyme that requires a metal cation for activity. We have found that the enzyme is activated by cations of varying charge and ionic radius, such as Li(+), A1(3+), Tb(3+), and Eu(3+), as well as the divalent cations Mg(2+), Mn(2+), and Ca(2+). All of the activating cations provide k(cat) values within 30-fold of one another, showing that the charge of the cation does not greatly influence the rate-limiting step for decarboxylase turnover. Cation concentrations for half-maximal activation decrease by > 100-fold with each increment of increase in the cation charge, ranging from similar to 300 mM with Li(+) to similar to 2 mu M with trivalent lanthanide ions. The cation affinity is related to the charge/radius ratio of the ion for those ions with exchangeable first coordination sphere ligands. The exchange-inert cation Co(NH(3))(6)(3+) activates in the presence of excess EDTA (and NH(4)(+) does not activate), indicating that direct metal coordination to the protein or substrate is not required for activation. The binding of metal ions (monitored by changes in the protein tryptophan fluorescence) and enzyme activation are both cooperative with Hill coefficients as large as 4, the active site stoichiometry of this (alpha beta)(4) enzyme. The Hill coefficients for Mg(2+) binding and activation increase from 1 to similar to 4 as the KCl concentration increases, which is also observed with NaCl or KNO(3); neither Na(+) nor K(+) activates the enzyme. The single tryptophan in the protein is located 16 residues from the carboxyl terminus of the pyruvoyl-containing alpha chain, in a 70-residue segment that is not present in metal ion independent AdoMet decarboxylases from other organisms. The results are consistent with allosteric metal ion activation of the enzyme, congruent with the role of the putrescine activator of the mammalian AdoMet decarboxylase.