Catalytic activity of maize leaf phosphoenolpyruvate carboxylase in relation to oligomerization

The relationship between the state of oligomerization and activity of purified maize leaf phosphoenolpyruvate carboxylase using size exclusion high performance liquid chromatography was examined. Maximum activities of 35 to 38 micromoles per minute per milligram protein were found when 100% of the enzyme was in its tetrameric form. The effects of the sulfhydryl group modifiers CuCl(2) and p-chloromercuribenzoate on enzyme inhibition and the state of aggregation of the protein complex were examined. Aggregation of the enzyme is temperature and pH sensitive with low temperature and high pH favoring depolymerization. Stability of the tetrameric form is largely dependent upon histidyl residues, and to some extent this explains the biphasic response of enzyme activity to changes in MgCl(2) concentrations. Modification of the tetramer's histidyl residues by the inhibitor diethylpyrocarbonate (0.125 millimolar) results in its dissociation to the dimeric form and loss of activity. Subsequent treatment with 0.4 molar hydroxylamine results in reassociation to the tetramer and restoration of enzymic activity.