• In the carboxypeptidases, the activation segment is 95-99 residues long; in thermolysin, it is several hundred

• In the case of the carboxypeptidases, evidence suggests the the activation segment also has chaperonin-like functionality as well, facilitating the folding the the protein and conveying heat stability

Here are two views showing how the activation segment of human CP blocks access to the catalytic site:

Activation Segment of Human Carboxypeptidase

Note from the right-hand picture that unlike the pro-segments of most of the other proteases we have examined, this one has secondary structure of its own.

If separated from the enzyme, however, it adopts a rather different structure:

Activation Segment of Carboxypeptidase A

The two helical segments that block the catalytic site are still present, but the third helix has become unstructured.

No crystal structure is yet available for the zymogen of thermolysin, but it has been sequenced. Here is a comparison of the zymogen and the active enzyme:

Sequence alignment of Thermolysin and Its Zymogen