How many different amino acids are found in living proteins?

The correct number of different amino acids found in living proteins is 20. These 20 amino acids are the building blocks of proteins and are standard across all living organisms. They combine in various sequences to form different proteins, each with unique functions.

There are additional amino acids, such as selenocysteine and pyrrolysine, that are incorporated into proteins in certain organisms, which can sometimes lead to the belief that there are 22 amino acids. However, the primary set widely accepted and found in most proteins is strictly the original 20. The presence of selenocysteine and pyrrolysine indicates that while variations exist, they do not count as standard amino acids for the majority of living organisms.

Understanding the role of these 20 standard amino acids is fundamental for grasping the composition and function of proteins in biology. Each of these amino acids has a specific side chain that determines its properties and role in protein structure and function.