PROPERTIES OF THE PLANT-CELL WALL AND INTERACTIONS BETWEEN THE MATRIX AND THE BOUND ENZYMES

The plant cell wall is chiefly made of polysaccharides (mostly cellulose, hemicelluloses and pectins) associated with proteins. Among them there is a lot of enzymes, mostly hydrolases and some oxidases. Some enzymes may act on exogenous substrates allowing them to enter the cell or degrading them in relation with defense reactions against pathogens. Other enzymes may hydrolyze the wall polysaccharides and this autolytic activity probably is related to the cell growth. Owing to their binding to the wall matrix the activity of the wall enzymes is often modified. Firstly the active site accessibility may be changed because the substrates have to diffuse in a viscous milieu. Secondly the binding may change the enzyme conformation and its catalytic properties. At last the presence of polygalacturonic acids which are negatively charged may modify the enzyme efficiency. Thus a cationic enzyme may be firmly bound to the negative charges of pectic compounds so that it becomes unable to move in the structure. In the same manner small substances (substrates or protons) may be attracted in the wall and their internal concentrations will be very different of their ones in the bulk. Studies which were made on cell walls isolated from soja bean or sycamore cells cultivated in sterile suspensions allowed us to show that the parietal electrostatic potential surely play a central role in the regulation of the activity of the wall enzymes. As this potential is brought about by the wall pectin methyl esterase (PME) a simple autoregulation mechanism may be considered: the PME increases the wall electrostatic potential and this leads to its own inhibition favouring at the same time the activity of the glycosidases which take a part in the growth. Thus the wall potential slows down again and the PME is reactivated. In the normal conditions a steady state is attained which is strictly dependent of the ionic concentration of the milieu. We may conclude that the interactions in the wall, and especially the electrostatic ones, induce important regulatory properties in enzymic systems which do not show any out of their natural milieu.