Biology – Gene control | e-Consult

The key difference between reversible and inducible enzymes lies in their regulation of activity.

Reversible enzymes are enzymes whose activity can be regulated by the presence or absence of a substrate. The enzyme's activity can be switched on or off simply by adding or removing the substrate. This is a relatively simple form of regulation. A classic example is the enzyme lactase in human milk. Its activity is directly dependent on the presence of lactose; when lactose is present, lactase is active, and when it's absent, lactase is inactive.

Inducible enzymes, however, require a signal molecule (an inducer) to be activated. In the absence of the inducer, the enzyme is in an inactive state. When the inducer is present, it binds to a regulatory protein, which then alters the enzyme's structure and makes it active. This is a more complex and often more tightly regulated mechanism. A good example is galactokinase in *E. coli*. It is normally inactive, but when lactose is present, the *E. coli* cell produces allolactose (an inducer). Allolactose binds to a repressor protein, causing it to detach from the operator region of the *lac* operon, thereby allowing transcription of the *lac* operon genes, including the gene for galactokinase.

The biological significance of these mechanisms is that they allow cells to respond to changing environmental conditions. Reversible enzymes provide a basic level of control, while inducible enzymes allow for a more specific and coordinated response to specific signals. Inducible enzymes are particularly important for metabolic pathways that are not constantly required, as they prevent unnecessary energy expenditure.