A theory is proposed to explain the physical bases of the ionic control of the activity of an enzyme system, located on a plant cell surface, and probably involved in cell-wall synthesis and extension. The model, which is based on various previously published experimental results, involves several assumptions: a cell-wall pectin methyl esterase de-esterifies pectins and thus creates the fixed negative charges of the cell wall; various enzymes incorporate uncharged carbohydrates in cell-wall material; cell-wall extension implies the sliding of cellulose microfibrils; the enzymes responsible for carbohydrate incorporation are activated by protons in the pH range 4–8 and have very similar pH dependencies: the cell-wall pectin methyl esterase is inhibited by protons in the same pH range.
The mathematical derivation of this model, written in the form of a hypercycle, indicates that it is equivalent to a set of two antagonistic enzyme reactions: an enzyme reaction conditioned by pectin methyl esterase which results in the increase of fixed charge density of the cell wall; a number of ‘growth enzymes’, which produce extension and building up of the cell wall and therefore a decrease of charge density. The mathematical study of this model shows it may display a very high co-operativity of its response to slight changes of pH. This co-operativity means that the cell wall charge density may dramatically increase or decrease, within a very narrow pH range. The steep response of this system appears to be the direct consequence of different pH sensitivities of pectin methyl esterase and of the other cell-wall enzymes involved in cell growth. Calcium, which tightly binds to the cell wall, may diminish or even suppress this abrupt charge transition.
This model suggests a novel theory of the ionic control of cell-wall expansion. The very basis of this theory is the existence of an electrostatic potential difference, ΔΨ, between the inside and the outside of the cell wall. When this ΔΨ value is large, the local proton concentration is high. Therefore the enzymes involved in cell wall extension and building up are active, but pectin methyl esterase is not. Therefore, the cell wall extends and the charge density decreases. The ΔΨ value then declines, as well as the local proton concentration. Under these conditions, the pectin methyl esterase becomes activated, whereas the ‘growth enzymes’ are not. This activation of pectin methyl esterase restores the initial, or an even higher, electrostatic potential difference, which in turn results in a decrease of local pH.
In the following papers the basic ideas of this model are subjected to experimental control.