Are the carboxyl groups of pectin polymers the only metal-binding sites in plant cell walls?

Abstract

Background and Aims

Are the carboxyl groups of pectin polymers the only metal-binding sites in plant cell walls? To answer this question we investigated Cu²⁺ and Ni²⁺ -exchange capacities of isolated root cell walls of seven plant species. As the ionization degree of carboxyl groups and consequently their ability to bind metal ions is dependent upon the solution pH it is possible to reveal the predominant sites of metal binding at a certain pH.

Methods

Amounts of different ionogenic groups in the isolated cell walls was determined by potentiometric titration, Cu²⁺ and Ni²⁺ binding capacities – by measuring the amount of adsorbed metal ions after incubation in 1 mM CuCl₂ or NiCl₂ at different pHs.

Results

Carboxyl groups of polygalacturonic acid (PGA) are the only Cu²⁺ and Ni²⁺ binding sites at pH < 5. At рН ≥ 5 the ratio between the amount of the metal bound and the amount of PGA carboxyl groups is >1 in some plant species, indicating the involvement of carboxyl groups of hydroxycinnamic acids (HCA) in Cu²⁺ and Ni²⁺ binding. HCA carboxyl groups in cell walls of grasses bind 40–85 % of total cell wall Cu²⁺ and Ni²⁺ at рН ≥ 5, but in dicotyledons their contribution does not exceed 45 % and varies widely between species.

Conclusion

Carboxyl groups of both polygalacturonic and hydroxycinnamic acids provide Cu²⁺ and Ni²⁺ adsorption sites in root cell walls. Their relative content depends on plant species, and ability to bind Cu²⁺ and Ni²⁺ – on the solution pH.