Protein

Proteins( / ˈ p r oʊ ˌ t iː n z /or / ˈ p r oʊ t i . ɨ n z /) are large biological molecules, or macromolecules, consisting of one or more long chains of amino acid residues. Proteins perform a vast array of functions within living organisms, including catalyzing metabolic reactions, replicating DNA, responding to stimuli, 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 sequenceof their genes, and which usually results in foldingof the protein into a specific three- dimensional structurethat determines its activity. A linear chain of amino acid residues is called a polypeptide. A protein contains at least one long polypeptide. Short polypeptides, containing less than about 20-30 residues, are rarely considered to be proteins and are commonly called peptides, or sometimes oligopeptides. The individual amino acid residues are bonded together by peptide bondsand adjacent amino acid residues. The sequenceof amino acid residues in a protein is defined by the sequenceof a gene, which is encoded in the genetic code. In general, the genetic code specifies 20 standard amino acids; however, in certain organisms the genetic code can include selenocysteineand—in certain archaea— pyrrolysine. Shortly after or even during synthesis, the residues in a protein are often chemically modified by posttranslational modification, which alters the physical and chemical properties, folding, stability, activity, and ultimately, the function of the proteins. Sometimes proteins have non-peptide groups attached, which can be called prosthetic groupsor cofactors. Proteins can also work together to achieve a particular function, and they often associate to form stable protein complexes. Once formed, proteins only exist for a certain period of time and are then degradedand recycled by the cell's machinery through the process of protein turnover. A protein's lifespan is measured in terms of its half-lifeand covers a wide range. They can exist for minutes or years with an average lifespan of 1-2 days in mammalian cells. Abnormal and or misfolded proteins are degraded more rapidly either due to being targeted for destruction or due to being unstable. Like other biological macromoleculessuch as polysaccharidesand nucleic acids, proteins are essential parts of organisms and participate in virtually every process within cells. Many proteins are enzymesthat catalyzebiochemical reactions and are vital to metabolism. Proteins also have structural or mechanical functions, such as actinand myosinin muscle and the proteins in the cytoskeleton, which form a system of scaffoldingthat maintains cell shape. Other proteins are important in cell signaling, immune responses, cell adhesion, and the cell cycle. Proteins are also necessary in animals' diets, since animals cannot synthesizeall the amino acids they need and must obtain essential amino acidsfrom food. Through the process of digestion, animals break down ingested protein into free amino acids that are then used in metabolism. Proteins may be purifiedfrom other cellular components using a variety of techniques such as ultracentrifugation, precipitation, electrophoresis, and chromatography; the advent of genetic engineeringhas made possible a number of methods to facilitate purification. Methods commonly used to study protein structure and function include immunohistochemistry, site-directed mutagenesis, X-ray crystallography, nuclear magnetic resonanceand mass spectrometry.