Hysteretic Properties of Maize Leaf Phosphoenolpyruvate Carboxylase in Crude Desalted Extracts. Effects of Metabolites and Light

Abstract

Maize leaf phosphoenolpyruvate carboxylase (PEPC, E.C. 4.1.1.31) in rapidly prepared and desalted extracts exhibits a lag in the reaction progress curves when the reaction is initiated by adding the enzyme to an otherwise complete reaction mixture. The extent of the lag period showed an inverse linear relationship with the enzyme concentration, suggesting subunit association as the molecular mechanism underlying the hysteresis. Increasing levels of phosphoenolpyruvate (PEP) and/or Mg 2+ ions in the assay or in preincubation decreased the kinetic lag. Inclusion in the assay of glucose-6-P also decreased the lag, while inclusion of malate had an opposite effect. Under identical assay conditions, the observed lag period was always higher in extracts from dark-treated plants than in extracts from light-treated plants, what may be either due to the existence of a different proportion of tetramers and dimers in the PEPC population of both kinds of extracts, or a consequence of the higher affinity for PEP of the light form of PEPC. Based on these results, we suggest that the hysteretic behavior of PEPC arises from aggregation of the enzyme induced by PEP and/or Mg 2+ ions.