Answer:

Introduction:
Proteins are essential macromolecules in living organisms that perform a wide range of functions. Some proteins act as enzymes, which are biological catalysts that facilitate chemical reactions in the body. Enzymes are typically proteins, although some RNA molecules can also exhibit enzymatic activity.

Explanation:
Among the given options, trypsin (option A) is the protein that acts as an enzyme. Trypsin is a protease enzyme that plays a crucial role in the digestion of proteins. It is produced in the pancreas and released into the small intestine, where it helps break down dietary proteins into smaller peptides. Trypsin specifically cleaves peptide bonds after the basic amino acids lysine and arginine.

Distinguishing Features:
To distinguish trypsin from the other options, let's look at the properties of the other proteins mentioned:

1. Insulin (option B):
- Insulin is a hormone, not an enzyme.
- It is produced and secreted by the beta cells of the pancreas.
- Insulin regulates glucose metabolism and helps lower blood sugar levels.

2. Collagen (option C):
- Collagen is a structural protein found in connective tissues such as skin, bones, tendons, and cartilage.
- It provides strength and support to various body tissues.
- Collagen is not an enzyme.

3. Antibody (option D):
- Antibodies, also known as immunoglobulins, are proteins produced by the immune system in response to foreign substances (antigens).
- Antibodies bind to specific antigens and help in immune responses, such as neutralizing pathogens or marking them for destruction.
- Antibodies are not enzymes.

Conclusion:
Based on the provided options, trypsin is the only protein that acts as an enzyme. Trypsin is a protease enzyme involved in protein digestion. Insulin, collagen, and antibodies have different functions in the body but do not possess enzymatic activity.