In plants such as Zea mays that carry out C4 metabolism, phosphoenolpyruvate carboxylase catalyses the primary fixation of atmospheric CO2. The properties of this enzyme from Z. mays leaves kept in light and in darkness are different. In brightly illuminated leaves, which are actively fixing CO2, the enzyme is less sensitive to feedback inhibition by malate and is phosphorylated on one or more serine residues. In darkened leaves, which are not photosynthesising, the enzyme is more sensitive to inhibition by malate and is much less phosphorylated. This indicates that the activity of the enzyme is controlled by a reversible phosphorylation.
