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Posttranslational modification may involve the formation of disulfide bridges and attachment of any of a number of biochemical functional groups, such as acetate, phosphate, various lipids and carbohydrates. Enzymes may also remove one or more amino acids from the amino end of the polypeptide chain, or cut the polypeptide in the middle of the chain. For instance, the peptide hormone insulin is cut twice after disulfide bond formation to remove a propeptide from the middle of the chain, leaving a protein consisting of two polypeptide chains connected by disulfide bonds. In other cases, two or more polypeptide chains that are synthesized separately may associate to become subunits of a protein with quaternary structure.
A protein is a chain composed of a long sequence of 20 possible amino acids, also called a polypeptide. Some posttranslational modification extends the range of possible functions a protein can have by introducing other chemical groups into the makeup of a protein (e.g., carbohydrate chains). Such chemical changes may alter the hydrophobicity of a protein and thus determine if the modified protein is cytosolic or membrane-bound. Other modifications like phosphorylation are part of common mechanisms for controlling the behavior of a protein, for instance, activating or inactivating an enzyme.
Types of posttranslational modifications include:
- phosphorylation, the addition of a phosphate group, usually to serine, tyrosine, threonine or histidine
- acetylation, the addition of an acetyl group, usually at the N-terminus of the protein
- alkylation, the addition of an alkyl group (e.g. methyl, ethyl)
- isoprenylation, the addition of an isoprenoid group (e.g. farnesol and geranylgeraniol)
- glycosylation, the addition of a glycosyl group to either asparagine, hydroxylysine, serine, or threonine, resulting in a glycoprotein
- ubiquitination, the covalent linkage of the protein ubiquitin to a target protein, typically interpreted by the cellular machinery as a "destroy this protein" tag.
- SUMOylation, the covalent linkage of the SUMO protein (Small Ubiquitin-related MOdifier) to a target protein (1).
- ISGylation, the covalent linkage of the ISG15 protein (Interferon-Stimulated Gene 15) to a target protein (2). ISG15 bears significant sequence similarity to ubiquitin.
- the GPI anchor
2. Malakhova, Oxana A.; Yan, Ming; Malakhov, Michael P.; Yuan, Youzhong; Ritchie, Kenneth J.; Kim, Keun Il; Peterson, Luke F.; Shuai, Ke; and Dong-Er Zhang. (2003). Protein ISGylation modulates the JAK-STAT signaling pathway. Genes & Development 17 (4), 455-460.
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