Electrostatic effects and calcium ion concentration as modulators of acid phosphatase bound to plant cell walls

At 'low' ionic strength, acid phosphatase bound to plant cell walls exhibits an apparent negative co-operativity, whereas it displays classic Michaelis-Menten kinetics in free solution. Conversely, at 'high' ionic strength, the bound enzyme and the soluble enzyme behave identically. This apparent negative co-operativity is explained by the existence of an electrostatic partition of the charged substrate by the fixed negative charges of the cell wall. Raising the ionic strength suppresses these electrostatic repulsion effects. Calcium may be removed from the cell walls by acid treatment and the acid phosphatase is apparently strongly inhibited. This inhibition occurs together with an increased apparent negative co-operativity of the enzyme. Incubating cell wall fragments previously depleted of calcium with CaCl2 restores the initial behaviour of the enzyme. Calcium, which tightly binds to cell wall pectic compounds, has by itself no effect on the enzyme in free solution. It affects the net charge of the cell wall and therefore the amplitude of electrostatic repulsion effects. Non-linear least-square fitting methods make it possible to estimate the density of fixed negative charges as well as the electrostatic partition coefficient, for both the 'native' and 'calcium-deprived' cell wall fragments. It may be shown directly that calcium loading and unloading in the cell wall controls the electrostatic effects, by monitoring proton extrusion from cell wall fragments upon raising the ionic strength. Proton outflux in the bulk phase is considerably enhanced upon removal of calcium from the cell walls. The main conclusion is that loading and unloading of calcium during cell elongation and division may regulate the activity of cell wall enzymes.