Characteristic shared by all enzymes involved in protein digestion is that are?

The correct answer is: they are all secreted as inactive zymogens.

Explanation:

All enzymes involved in protein digestion are secreted as inactive zymogens. This is an important safety measure, because these enzymes are capable of breaking down proteins, and if they were secreted in their active form, they could damage the tissues of the digestive tract. Instead, they are secreted as inactive zymogens, which are then activated in the gut by various factors, such as low pH or the presence of other enzymes.

This mechanism ensures that the protein-digesting enzymes are only active when they are needed, and that they do not damage the tissues of the digestive tract.