Protein

Proteins ( or ) are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of adjacent amino acid residues . The sequence of amino acids in a protein is defined by the sequence of 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 selenocysteine —and 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 groups or cofactors . Proteins can also work together to achieve a particular function, and they often associate to form stable protein complexes . 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 catalyze 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 . Proteins are also necessary in animals diets, since animals cannot synthesize all the amino acids they need and must obtain essential amino acids from food. Through the process of digestion , animals break down ingested protein into free amino acids that are then used in metabolism. Proteins may be purified from other cellular components using a variety of techniques such as ultracentrifugation , precipitation , electrophoresis , and chromatography ; the advent of genetic engineering has made possible a number of methods to facilitate purification. Methods commonly used to study protein structure and function include immunohistochemistry , site-directed mutagenesis , nuclear magnetic resonance and mass spectrometry .

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