Final answer:
Proteins serve numerous functions including roles as enzymes and antibodies, but they are not commonly used outside of biological systems where other materials are more suitable for non-biological applications.
Step-by-step explanation:
Proteins have a vast array of functions in living organisms but they are not commonly used for certain applications. While proteins do include enzymes that catalyze biochemical reactions and antibodies that defend against foreign particles, they are generally not used in other non-biological contexts such as industrial catalysts or as structural materials in construction.
The versatility of proteins stems from the 20 different chemically distinct amino acids that can combine in various sequences to forge the vast diversity of proteins, each tailored for specific functions within living systems. An example is hemoglobin, a protein that transports oxygen in the blood. However, proteins are not commonly used for applications outside biological systems where other materials are favored due to stability, cost, and functional requirements.