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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 one or more strings of amino acid residues joined head-to-tail via peptide bonds. Each string folds into a 3-dimensional structures. The two most regular structural motifs in proteins are alpha helices and beta sheets, which are stabilized by hydrogen bonds between the C=O and N-H groups of different peptide bonds. The string is folded further into larger 3-dimensional structures that are held together by hydrogen bonds, hydrophobic interactions, and/or disulfide bonds, etc. Many proteins consist of two or more subunits, which are each a continuous string of amino acids.

Proteins are generally large molecules, sometimes having molecular masses of up to 3,000,000 (the muscle protein titin has a single amino acid chain 27,000 subunits long). 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.

Proteins can be picky about the environment in which they are found. They may only exist in their active, or native state, in a small range of pH values and under solution conditions with a minimum quantity of electrolytes, as many proteins will not remain in solution in distilled water. A protein that loses its native state is said to be denatured. Denatured proteins generally have no secondary structure other than random coil. A protein in its native state is often described as folded.

One of the more striking discoveries of the 20th century was that the native and denatured states in many proteins were interconvertible, that by careful control of solution conditions (by for example, dialyzing away a denaturing chemical), a denatured protein could be converted to native form. The issue of how proteins arrive at their native state is an important area of biochemical study, called the study of protein folding.

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 at an earlier stage of its production in vivo, by modifying the gene that codes for it.) Another, exciting area of research in recent years is the design of proteins from scratch, de novo protein design.

Protein deficiency is often discussed in relation tonutrition especially as it relates 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, may lack essential amino acids, 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 (proteins repair muscle tissue, 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.

Proteins can often figure in allergies and allergic reactions to certain foods. This is because the structure of each form of protein is slightly different, and some may trigger a response from the immune system while others are perfectly safe. Many people are allergic to the particular proteins found in peanuts, or those in shellfish or other seafoods, for example, but it is extremely unusual for the same person to react to all three.

See also:

• Proteome
• Proteinoid
• Proteomics
• Protein targeting
• Protein biosynthesis