Protein

Proteins are relatively large organic compounds made of amino acids arranged in a linear chain and joined together between the carboxyl atom of one amino acid and the amine nitrogen of another. This bond is called a peptide bond. more...

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The sequence of amino acids in a protein is defined by a gene and encoded in the genetic code. Although this genetic code specifies 20 \"standard\" amino acids, the residues in a protein are often chemically altered in post-translational modification: either before the protein can function in the cell, or as part of control mechanisms. Proteins can also work together to achieve a particular function, and they often associate to form stable complexes.

Like other biological macromolecules such as polysaccharides and nucleic acids, proteins are essential parts of all living organisms and participate in every process within cells. Many proteins are enzymes that catalyze biochemical reactions, and are vital to metabolism. Other proteins have structural or mechanical functions, such as the proteins in the cytoskeleton, which forms a system of scaffolding that maintains cell shape. Proteins are also important in cell signaling, immune responses, cell adhesion, and the cell cycle. Protein is also a necessary component in our diet, since animals cannot synthesise all the amino acids and must obtain essential amino acids from food. Through the process of digestion, animals break down ingested protein into free amino acids that can be used for protein synthesis.

The name protein comes from the Greek πρώτα (\"prota\"), meaning \"of primary importance\" and were first described and named by Jöns Jakob Berzelius in 1838. However, their central role in living organisms was not fully appreciated until 1926, when James B. Sumner showed that the enzyme urease was a protein. The first protein structures to be solved included insulin and myoglobin; the first was by Sir Frederick Sanger who won a 1958 Nobel Prize for it, and the second by Max Perutz and Sir John Cowdery Kendrew in 1958. Both proteins' three-dimensional structures were amongst the first determined by x-ray diffraction analysis; the myoglobin structure won the Nobel Prize in Chemistry for its discoverers.

Biochemistry

Proteins are linear polymers built from 20 different L-alpha-amino acids. All amino acids share common structural features including an alpha carbon to which an amino group, a carboxyl group, and a variable side chain are bonded. Only proline shows little difference in a fashion by containing an unusual ring to the N-end amine group, which forces the CO-NH amide sequence into a fixed conformation. The side chains of the standard amino acids, detailed in the list of standard amino acids, have varying chemical properties that produce proteins' three-dimensional structure and are therefore critical to protein function. The amino acids in a polypeptide chain are linked by peptide bonds formed in a dehydration reaction. Once linked in the protein chain, an individual amino acid is called a residue and the linked series of carbon, nitrogen, and oxygen atoms are known as the main chain or protein backbone. The peptide bond has two resonance forms that contribute some double bond character and inhibit rotation around its axis, so that the alpha carbons are roughly coplanar. The other two dihedral angles in the peptide bond determine the local shape assumed by the protein backbone.

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