preproenzyme

A preproenzyme is a precursor form of an enzyme that contains both a signal sequence at the N terminus (the "pre" segment) and a propeptide (the "pro" segment) in addition to the mature catalytic domain. The signal peptide directs the nascent polypeptide into the secretory pathway or into an organelle, typically entering the endoplasmic reticulum for further processing and trafficking through the cell.

The propeptide serves to inhibit catalytic activity during synthesis, folding, and intracellular transport, helping prevent premature enzymatic action. Activation of a preproenzyme usually involves proteolytic removal of the pre and pro regions, along with any additional maturation steps such as folding or cofactor incorporation, to yield the active enzyme. In many systems, activation occurs in a specific cellular compartment or external environment where the enzyme is required.

Examples of preproenzymes include the zymogens of secreted proteases, such as pepsinogen and trypsinogen, which possess both signal and pro segments. The pre segment targets secretion, while the pro segment maintains inactivity until appropriate activation signals are encountered in the digestive tract. The concept is closely related to zymogens and proenzymes; the term preproenzyme emphasizes the presence of both the pre and pro regions within a single polypeptide before activation.

In contemporary usage, the term preproenzyme is not always employed; many texts refer to these molecules as proenzymes or zymogens, with the broader idea that a protein can require processing steps to become active. Nonetheless, preproenzyme remains a descriptive label for multi-segment precursors that are destined for secretion or organelle localization and require proteolytic maturation.