Re: Why do copper II ions and zinc II ions affect the growth of microbes?

You are correct in saying that many enzymes have a copper or zinc requirement; one of the best known is superoxide dismutase, an enzyme that removes toxic oxygen species from the cell. Another important example is cytochrome oxidase, the enzyme that reduces oxygen to water during respiration. So all organisms have a requirement for copper, zinc and other metals in their diet. However for microbes these are trace elements, meaning that the amount required is very small (for instance a very few micromolar in a growth medium). Once this limit is passed, the metals quickly become toxic.

There are actually many reasons why these metals become toxic, not all of which are understood. The major reason is that the metals can bind to proteins in the cell (especially those rich in the amino acids cysteine or methionine) and make them inactive. In fact this principle is often used in X-ray crystallography to help determine the structure of protein molecules. Metals can also catalyse chemical reactions in the cell, which may lead to the production of toxic molecules like free radicals. The absence of oxygen can also increase the toxicity of copper by reducing it to Cu(I), which is more reactive.

In fact there are microbes that have evolved resistance to high metal concentrations, for instance those living around the waste from old mines. They have evolved membrane proteins that can more efficiently pump metals out of the cell, or specific proteins that bind metals as they enter the cell, so "sequestering" them away from the other components.