 Enzymes act as catalysts – they allow a reaction to proceed more rapidly – and they also allow the regulation of the rate of a metabolic reaction, for example in response to changes in the cell's environment or to signals from other cells.The metabolic system of a particular organism determines which substances it will find nutritious and which poisonous.For example, some prokaryotes use hydrogen sulfide as a nutrient, yet this gas is poisonous to animals.