Committed step

In enzymology, the committed step (also known as the "first" committed step) is an effectively irreversible enzymatic reaction that occurs at a branch point during the biosynthesis of some molecules.
As the name implies, after this step, the molecules are "committed" to the pathway and will ultimately end up in the pathway's final product. The first committed step should not be confused with the rate-determining step, which is the slowest step in a reaction or pathway. However, it is sometimes the case that the first committed step is in fact the rate-determining step as well.
Regulation.
Metabolic pathways require tight regulation so that the proper compounds get produced in the proper amounts. Often, the first committed step is regulated by processes such as feedback inhibition and activation. Such regulation ensures that pathway intermediates do not accumulate, a situation that can be wasteful or even harmful to the cell.
Other uses.
The term has also been applied to other processes that involve a series of steps. For example, the binding of egg and sperm can be thought of as the first committed step in metazoan fertilization.