Proteinase inhibitors are molecules that help regulate the activity of proteinases, which are enzymes that break down proteins. By binding to these enzymes, proteinase inhibitors prevent them from functioning, thereby controlling various biological processes such as digestion, immune response, and cell signaling. They play a crucial role in maintaining the balance of protein turnover in the body. These inhibitors are found in many organisms, including plants, animals, and microorganisms. In plants, for example, proteinase inhibitors can protect against pests and pathogens by inhibiting the digestive enzymes of herbivores. Additionally, they have potential therapeutic applications in treating diseases like cancer and viral infections by targeting specific proteinases involved in disease progression.