The upcoming discussion will update you about the difference between constitutive enzymes and inductive enzymes.
The enzymes which are always present in the organism in constant amounts regardless of its metabolic state are called as constitutive enzymes. For example, the enzymes involved in central pathway of catabolism such as glycolysis are constitutive enzymes. Sometimes, the enzymes appear in cells only when they are needed in the presence of their substrates or other agents which cause their de novo synthesis through DNA dependent synthesis of specific messenger RNA for the specific enzymic protein.
Such enzymes are called as inducible or inductive enzymes or induced enzymes and this process of their synthesis is called as enzyme induction. The substrate or any other agent capable of inducing the synthesis of an enzyme is called as inducer or inducing agent. Enzyme induction is widespread in micro-organisms but fewer examples are known in higher plants and animals. In bacterium Escherichia coli (E. coli) an example of the inducible enzyme is β-galactosidase which catalyses the hydrolysis of lactose to yield D-Glucose and D-Galactose.
Normally, E. coli cells do not utilise lactose if glucose is available in plenty due to insignificant amount of β-galactosidase present in them. But, if E. coli cells are placed in a culture medium which contains only lactose as the source of energy and carbon, then within one or two, minutes they start synthesizing β-galactosidase enzyme in large quantities which readily utilizes the lactose. On transferring these lactose induced cells to another culture medium which contains only glucose and not lactose, further synthesis of β-galactosidase by E. coli cells is immediately stopped.
Among higher plants an excellent example of inducible enzyme is the enzyme nitrate reductase which catalyses the reduction of nitrate to nitrite. This enzyme is synthesized in the cells only when its substrate i.e., nitrate is available. This synthesis of the enzyme nitrate reductase is, however, repressed by ammonium ions (NH+4) and inhibitors of protein synthesis.
During germination of cereal grains gibberellic acid is known to cause de novo synthesis of the enzyme a-amylase in the aleurone cells which is then secreted into the endosperm to contribute to the hydrolysis of complex insoluble carbohydrates. The enzyme induction appears to be an economical process. By not synthesizing the enzymes when they are not needed, the cells do conserve energy.