Post-translational modifications are the chemical modifications a polypeptide chain receives after it is translated that convert it to the mature protein.
A protein is made up of a chain of amino acids, also known as a polypeptide. During translation, 20 different amino acids can be incorporated to make up a polypeptide chain. Once a polypeptide is create, post-translational modifications can help convert it into a mature protein and extend its range of functions. Many post-translational modifications happen in the endoplasmic reticulum and the Golgi apparatus.
Post-translational modifications include the addition of biochemical functional groups, a change in the chemical nature of an amino acid, a change in protein structure (such as the formation of disulfide bonds), or proteolysis that cuts part of the polypeptide chain. Some very common post-translational modifications that involve the addition of a functional group to a protein include glycosylation, lipidation, ubiquitination, and phosphorylation.
Proteins that are destined to be embedded in the plasma membrane often undergo glycoslylation or lipidation. Glycosylation describes the addition of a carbohydrate group to a protein, which can act as a cell surface marker, while lipidation describes the addition of a lipid to a protein, which can help anchor that protein to the cell membrane. In ubiquitination, the protein ubiquitin is appended to another protein of interest, marking it for degradation by the proteasome. Protein degradation occurs when there are damaged proteins, or as a normal part of regulating protein activity in the cell.
Phosphorylation and dephosphorylation are common modifications that are used in biochemical signal transduction. The only amino acids that can be phosphorylated are serine, tyrosine and threonine making this a very specific modification to amino acid chains.
It may be necessary for enzymes to cut the polypeptide chain to remove single amino acids or even entire regions of the polypeptide before it can become a functional protein. For example, the first amino acid in a polypeptide (methionine) is often removed, as this usually corresponds to the start codon that initiated translation.
In addition to the standard 20 amino acids, there are also many other non-standard amino acids that can be formed by post-translational modifications.
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Key Points
• During protein synthesis, 20 different amino acids can be incorporated into a polypeptide chain to become a protein.
• Post-translational modifications can extend a proteins range of functions.
• A common modification to proteins is the addition of phosphate groups in phosphorylation using ATP, Ubiquitination which marks a protein for destruction or the addition of carbohydrate chains to form glycoproteins.
• Post-translational modifications can involve a functional group being added to a protein, a chemical change in amino acids, a structural change in the protein (such as the formation of disulfide bonds), or proteolysis.
• The only amino acids that can be phosphorylated are serine, tyrosine and threonine making this a very specific modification to amino acid chains.
Key Terms
Post-translational modification: The chemical modification of a protein after its translation.
Amino acid: Any organic compound containing both an amino and a carboxylic acid functional group.
Disulfide bonds: A covalent bond formed between the thiol groups in two cysteine amino acids.
Proteolysis: The breakdown of a protein into polypeptides, usually carried out by an enzyme.
Glycosylation: The addition of a carbohydrate group to a protein.
Lipidation: The addition of a lipid to a protein.
Ubiquitination: The addition of a ubiquitin protein to another protein.
Phosphorylation: The addition of a phosphoryl group to a protein.
Proteasome: Protein complex in the cell that degrades proteins