Competitive Binding of Protons, Calcium, Cadmium, and Zinc to Isolated Cell Walls of a Gram-Positive Soil Bacterium

Metal ion binding to the bacterial cell wall is the first step in the interactions of a metal with a bacterium. Cadmium and zinc ion binding to isolated cell walls of Rhodococcus erythropolis A177 has been studied for a wide range of proton and calcium concentrations. The release of calcium ions and protons during metal ion binding is an indication of the competitive nature of the binding. Calculations, based on the metal binding data and the pH-dependent charging behavior, reveal that the cell wall becomes positively charged at high coverage with bivalent ions. A cooperative effect of the presence of calcium on metal ion binding was observed. Apparently, the cell wall structure is altered in the presence of bivalent ions. Since calcium is a dominant bivalent cation in most natural systems, one may regard the calcium as a “structure determining ion”. On the basis of a qualitative interpretation of the data, the NICA model was selected for a quantitative description of the data.