The influence of pH on kinetic parameters of coleoptile phosphoenolpyruvate carboxylase. Relationship to auxin-stimulated dark fixation

Abstract

In vitro phosphoenolpyruvate (PEP) carboxylase activity from Avena coleoptile tissue was investigated over a range of pH values which include cytosol pH values. Increasing the pH from 7.0 to 7.5 increased optimal PEP carboxylase activity (V_max) by over 100%. In the presence of rate-limiting 0.07 mM PEP, noncompetitive inhibition by 0.1 mM malate decreased from 80% at pH 7.1 to 50% at pH 7.5. The K_m for PEP was not influenced by malate, but as the pH was increased from 7.1 to 7.5, the K_m decreased from 0.16 to 0.08 mM. Over the same pH rise, the K_I for malate inhibition increased from 0.04 mM to 0.09 mM. Fusicoccin had no detectable influence on enzymic activity. These results are discussed in relation to the stimulation of H⁺ excretion and dark CO₂ fixation by indoleacetic acid and fusicoccin. The data indicate that any increase in cytosol pH, resulting from H⁺ excretion, would stimulate PEP carboxylase activity by promoting catalytic efficiency and binding affinity for PEP and by reducing the binding affinity for the inhibitor malate.