Partial purification and characteristics of potato phosphoenolpyruvate carboxylase

Involvement of thiol groups in the activity of phosphoenolpyruvate carboxylase from maize leaves

Purified maize leaf phosphoenolpyruvate carboxylase (EC 4.1.1.31) was completely inactivated by several thiol-modifying reagents, including, CuCl2, CdCl2 and N-ethylmaleimide. The inactivation by CuCl2 could be reversed by dithiothreitol, suggesting the involvement of vicinal dithiols in the inactivation process.Complete inactivation of phosphoenolpyruvate carboxylase was correlated with the incorporation of two mol ((3)H)N-ethylmaleimide per 100-kilodalton subunit. The total protection of the enzyme against N-ethylmaleimide inactivation afforded by the substrate, phosphoenolpyruvate, was correlated with the protection of one mol ((3)H)N-ethylmaleimide reactive residue per mol subunit.The complete inactivation of phosphoenolpyruvate carboxylase by N-ethylmaleimide and the protection afforded by phosphoenolpyruvate against modification suggest the presence of an essential cysteine residue in the catalytic site of the C4 leaf enzyme.

Phosphoenolpyruvate carboxylase from soybean nodule cytosol. Evidence for isoenzymes and kinetics of the most active component

Phosphoenolpyruvate carboxylase (orthophosphate:oxaloacetate carboxylase (phosphorylating), EC 4.1.1.31) from plant cells of soybean nodules was studied to assess its role in providing carbon skeletons for aspartate and asparagine synthesis. The enzyme was purified 119-fold by (NH4)2SO4 fractionation and DEAE-cellulose, BioGel A-1.5m, and hydroxyapatite chromatography. Five activity bands were resolved with discontinuous polyacrylamide gel electrophoresis. A small quantity of enzyme from the most active band was separated from the others by preparative electrophoresis. The apparent Michaelis constants of this enzyme for phosphoenolpyruvate and HCO3- were 9.4.10(-2) and 4.1.10(-1) mM, respectively. A series of metabolite tested at 1 mM had no significant effect on enzyme activity. These experiments indicate that the major factors directly controlling phosphoenolpyruvate carboxylase activity in vivo are phosphoenolpypyruvate and HCO3- concentrations.

The regulation of potato phosphoenolpyruvate carboxylase in relation to a metabolic pH-stat

Phosphoenolpyruvate (PEP) carboxylase (E.C. 4.1.1.31.) was extracted from potato tubers (Solanum tuberosum L.) and investigated for regulatory response to metabolites. The enzyme was found to be activated by sugar phosphates and glycollate and non-competitively inhibited by succinate and fumarate. In both cases the effects were highly dependent on pH, being maximal between pH 7 and 7.6. Rapid extraction techniques demonstrated that the enzyme suffers a sharp decline in activity and sensitivity to metabolites during the first 2 h from extraction. The observed properties of PEP carboxylase were related to the possible role of the enzyme in a metabolic pH-stat.

Regulation of phosphoenolpyruvate carboxylase of Zea mays by metabolites

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 K(m) value for phosphoenolpyruvate is 80mum. When assayed at 30 degrees C, the enzyme shows normal Michaelis-Menten kinetics, but when assayed at 45 degrees 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 P(i), by SO(4) (2-) 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.