There are several kinds of regulation that enable microbes to cope with rapidly changing supplies of nutrients. This is exemplified by sugar metabolism in Saccharomyces cerevisiae. Some readily reversible controls affect the activity of enzymes, either by allosteric activation and deactivation, which often occur within seconds, or by covalent modification, within minutes. Other controls regulate the amount of enzyme present in the cells, either by irreversible proteolytic inactivation of the enzyme, or by influencing enzymic synthesis. The nomenclature of these processes is often confused.