Regulation of phosphoenolpyruvate carboxylase of Zea mays by metabolites

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Abstract
Crude preparations of phosphoenolpyruvate carboxylase obtained from aetiolated seedlings of Zea mays are unstable but can be stabilized with glycerol. At the pH optimum of 8.3, the Km value for phosphoenolpyruvate is 80μm. When assayed at 30°C, the enzyme shows normal Michaelis–Menten kinetics, but when assayed at 45°C sigmoid kinetics are exhibited. At pH7.0 the enzyme is inhibited by a number of dicarboxylic acids and by glutamate and aspartate. d and l forms of the hydroxy acids and amino acids are inhibitory and the kinetics approximate to simple non-competitive inhibition. The same compounds produce less inhibition at pH7.6 than at pH7.0 and the kinetics of inhibition are more complex. The enzyme is activated by Pi, by SO42- and by a number of sugar phosphates. Maximum activation occurs at acid pH values, where enzyme activity is lowest. The enzyme is activated by AMP and inhibited by ADP and ATP so that the response to energy charge is of the R type and is thus at variance with Atkinson's (1968) concept of energy charge. The physiological significance of the response to metabolites is discussed.