Charge promiscuity in the metal ion activation of S‐Adenosylmethionine decarboxylase

Abstract

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+, Tb3+, Gd3+, Eu3+, in addition to the known divalent cation activators Mg2+, Mn2+ and Ca2+. All of the activating cations provide kcat values within an order of magnitude of one another, suggesting that role of the cation does not predominantly lie in the rate limiting step of the reaction. The KM for AdoMet varies less than 4-fold with the different metal ion activators. Cation concentrations for half maximal activation decrease with each increment of increase in the cation charge, from ~ 300 mM with Li+ to ~ 2 μM with trivalent lanthanide ions. The exchange inert Co(NH3)63+ cation activates in the presence of EDTA and to a higher level than Co2+ (NH4+ does not activate) indicating that direct metal coordination to the substrate is not required for activation Equilibrium binding studies monitoring protein fluorescence changes show that K+, Zn2+ and Rh3+interact with the protein, but these cations do not activate. The binding of metal ion activators and enzyme activation are both cooperative with Hill coefficients as large as 4, indicating that the binding of one equivalent of some cations can activate the entire (αβ )4 enzyme. 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. Supported by NIH grant GM31186