Protein are biopolymers consisting of strings of amino acids. They are generally large molecules, sometimes having molecular masses of up to 100 000. Although RNA was shown in the late 20th century to have catalytic properties as well, nearly all of the biological catalysts known as enzymes are composed of protein.
Protein also makes up much of the structure of animals: collagen (a component of skin, hair, and cartilage) and muscle are composed largely of protein. The two most common structural motifs in proteins are alpha helices and beta pleated sheets.
While longer chains of amino acids are almost universally referred to as proteins, as one approaches shorter strings of amino acids, the term "polypeptide" is used. The distinction is somewhat undefined, although a polypeptide may be less likely to have tertiary structure, and may be more likely to act as a hormone (like insulin) rather than as an enzyme or structural element.
During the 1980s scientists started developing a technology known as protein engineering which can alter the structure of a protein and hence its properties. Genetic engineering is another technique developed.
Protein deficiency is often discussed in relation to starvation and malnourishment in Third World Countries. It may be an overlooked health factor even in developed countries such as the United States, where diets may lean heavily on carbohydrates and there is societal pressure to be thin. Protein deficiency can lead to sympotoms such as fatique, symptoms of insulin resistance, hair loss, loss of muscle mass, low body temperature, and hormonal irregularities. Severe protein deficiency is fatal.