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Proteins (originally meaning first thing when discovered in 1838 by Berzelius) are one of the basic classes of substances studied in biochemistry.
Proteins are biopolymers consisting of strings of amino acids joined head-to-tail via peptide bonds. The sequence of amino acids is known as the primary structure of a protein. The folding of these strings into 3-dimensional structures is referred to as the secondary strucgture. The two most common secondary structural motifs in proteins are alpha helices and beta sheets, which are stabilized by hydrogen bonds. Often, proteins consist of two or more subunits, which are each a continuous string of amino acids. This level of structure is considered tertiary. The subunits can be held together by hydrogen bonds, hydrophobic interactions, and/or disulfide bonds, etc.
Proteins are generally large molecules, sometimes having molecular masses of up to 100,000. Such long chains of amino acids are almost universally referred to as proteins, but shorter strings of amino acids are referred to as "polypeptides" and "oligopeptides". The dividing line 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.
Nearly all the biological catalysts known as enzymes are proteins. (Some RNA was shown in the late 20th century to have catalytic properties as well.) Immunoglobulins, membrane-bound receptors, and trans-membrane transporters are also proteins. Finally, protein also makes up much of the structure of animals: collagen and keratin are components of skin, hair, and cartilage; and muscles are composed largely of proteins.
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, which modifies a protein indirectly, by modifying the gene that codes for it.)
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 rely heavily on carbohydrates and there is societal pressure to be thin. Protein deficiency can lead to sympotoms such as fatigue, insulin resistance, hair loss, loss of hair pigment (hair that should be black becomes reddish), loss of muscle mass, low body temperature, and hormonal irregularities. Severe protein deficiency is fatal.
Excess protein can cause problems as well, such as foundering (foot problems) in horses.
See also: