Like other biological macromolecules such as polysaccharides and nucleic acids, proteins are essential parts of organisms and participate in virtually every process within cells. Many proteins are enzymes that catalyse biochemical reactions and are vital to metabolism. Proteins also have structural or mechanical functions, such as actin and myosin in muscle and the proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape. Other proteins are important in cell signaling, immune responses, cell adhesion, and the cell cycle. In animals, proteins are needed in the diet to provide the essential amino acids that cannot be synthesized. Digestion breaks the proteins down for metabolic use. Proteins can be informally divided into three main classes, which correlate with typical tertiary structures: globular proteins, fibrous proteins, and membrane proteins. Almost all globular proteins are soluble and many are enzymes. Fibrous proteins are often structural, such as collagen, the major component of connective tissue, or keratin, the protein component of hair and nails. Membrane proteins often serve as receptors or provide channels for polar or charged molecules to pass through the cell membrane. [31] :165–85 Kauzmann W (May 1956). "Structural factors in protein denaturation". Journal of Cellular Physiology. 47 (Suppl 1): 113–31. doi: 10.1002/jcp.1030470410. PMID 13332017. Margolin W (January 2000). "Green fluorescent protein as a reporter for macromolecular localization in bacterial cells". Methods. 20 (1): 62–72. doi: 10.1006/meth.1999.0906. PMID 10610805.

Sleator RD (2012). "Prediction of Protein Functions". Functional Genomics. Methods in Molecular Biology. Vol.815. pp.15–24. doi: 10.1007/978-1-61779-424-7_2. ISBN 978-1-61779-423-0. PMID 22130980. A representation of the 3D structure of the protein myoglobin showing turquoise α-helices. This protein was the first to have its structure solved by X-ray crystallography. Toward the right-center among the coils, a prosthetic group called a heme group (shown in gray) with a bound oxygen molecule (red). Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, and transporting molecules from one location to another. Proteins differ from one another primarily in their sequence of amino acids, which is dictated by the nucleotide sequence of their genes, and which usually results in protein folding into a specific 3D structure that determines its activity. Rüdiger H, Siebert HC, Solís D, Jiménez-Barbero J, Romero A, von der Lieth CW, Diaz-Mariño T, Gabius HJ (April 2000). "Medicinal chemistry based on the sugar code: fundamentals of lectinology and experimental strategies with lectins as targets". Current Medicinal Chemistry. 7 (4): 389–416. doi: 10.2174/0929867003375164. PMID 10702616. See also: Glycan-protein interactions Ribbon diagram of a mouse antibody against cholera that binds a carbohydrate antigenThe best-known role of proteins in the cell is as enzymes, which catalyse chemical reactions. Enzymes are usually highly specific and accelerate only one or a few chemical reactions. Enzymes carry out most of the reactions involved in metabolism, as well as manipulating DNA in processes such as DNA replication, DNA repair, and transcription. Some enzymes act on other proteins to add or remove chemical groups in a process known as posttranslational modification. About 4,000 reactions are known to be catalysed by enzymes. [48] The rate acceleration conferred by enzymatic catalysis is often enormous—as much as 10 17-fold increase in rate over the uncatalysed reaction in the case of orotate decarboxylase (78 million years without the enzyme, 18 milliseconds with the enzyme). [49] In animals, amino acids are obtained through the consumption of foods containing protein. Ingested proteins are then broken down into amino acids through digestion, which typically involves denaturation of the protein through exposure to acid and hydrolysis by enzymes called proteases. Some ingested amino acids are used for protein biosynthesis, while others are converted to glucose through gluconeogenesis, or fed into the citric acid cycle. This use of protein as a fuel is particularly important under starvation conditions as it allows the body's own proteins to be used to support life, particularly those found in muscle. [90] With the development of X-ray crystallography, it became possible to sequence protein structures. [18] The first protein structures to be solved were hemoglobin by Max Perutz and myoglobin by John Kendrew, in 1958. [19] [20] The use of computers and increasing computing power also supported the sequencing of complex proteins. In 1999, Roger Kornberg succeeded in sequencing the highly complex structure of RNA polymerase using high intensity X-rays from synchrotrons. [18]

Mulder NJ (2007-09-28). "Protein Family Databases". eLS. Chichester, UK: John Wiley & Sons, Ltd. pp.a0003058.pub2. doi: 10.1002/9780470015902.a0003058.pub2. ISBN 978-0-470-01617-6.Conrotto P, Souchelnytskyi S (September 2008). "Proteomic approaches in biological and medical sciences: principles and applications". Experimental Oncology. 30 (3): 171–80. PMID 18806738.