Surface charge measurements on Micrococcus lysodeikticus and the catalytic implications for lysozyme.

Electrophoresis measurements on Micrococcus lysodeikticus have shown that the net surface charge density on the cell wall is constant at around -1.5 microC/cm2 for the pH range 4-8. This result has enabled a quantitative analysis to be made of how the electrostatic field associated with the negatively charged cell wall influences the ionic strength and pH dependency of the lytic activity of lysozyme towards M. lysodeikticus. A dominant effect is the creation of a local pH gradient at the cell wall, and at high ionic strengths the lytic activity is found to be controlled by an electrostatic force of attraction between the lysozyme molecule and the cell wall. As the ionic strength of the supporting electrolyte is decreased, however, an electrostatic force of repulsion becomes dominant and is associated with a negative charge carried by the lysozyme molecule, which could possibly be the ionized Asp-52 residue at the active site. This is considered to arise from the fact that at low ionic strengths the fine details of the heterogeneous charge distribution on the cell wall and lysozyme molecule are only partially screened by counter ions.