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Charge promiscuity in the metal ion activation of S‐Adenosylmethionine decarboxylase
Author(s) -
Lu Zichun J,
Markham George D
Publication year - 2006
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.20.4.a43
Subject(s) - chemistry , divalent , ionic radius , metal , enzyme activator , enzyme kinetics , enzyme , lanthanide , allosteric regulation , substrate (aquarium) , inorganic chemistry , stereochemistry , ion , active site , biochemistry , organic chemistry , oceanography , geology
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 + , Tb 3+ , Gd 3+ , Eu 3+ , in addition to the known divalent cation activators Mg 2+ , Mn 2+ and Ca 2+ . All of the activating cations provide k cat 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 K M 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(NH 3 ) 6 3+ cation activates in the presence of EDTA and to a higher level than Co 2+ (NH 4 + 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 + , Zn 2+ and Rh 3+ 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