Post Translational Modifications
Most of the proteins that are translated from mRNA undergo chemical modifications before becoming functional in different body cells. The modifications collectively, are known as post-translational modifications. The protein post translational modifications play a crucial role in generating the heterogeneity in proteins and also help in utilizing identical proteins for different cellular functions in different cell types. How a particular protein sequence will act in most of the eukaryotic organisms is regulated by these post translational modifications.
Post translational modifications occurring at the peptide terminus of the amino acid chain play an important role in translocating them across biological membranes. These include secretory proteins in prokaryotes and eukaryotes and also proteins that are intended to be incorporated in various cellular and organelle membranes such as lysosomes, chloroplast, mitochondria and plasma membranes.
Expression of proteins is important in diseased conditions. Post translational modifications play an important part in modifying the end product of expression and contribute towards biological processes and diseased conditions. The amino terminal sequences are removed by proteolytic cleavage when the proteins cross the membranes. These amino terminal sequences target the proteins for transporting them to their actual point of action in the cell.
Protein post translational modifications may happen in several ways. Some of them are listed below:
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Glycosylation: Many proteins, particularly in eukaryotic cells, are modified by the addition of carbohydrates, a process called glycosylation. Glycosylation in proteins results in addition of a glycosyl group to either asparagine, hydroxylysine, serine, or threonine. Software for studying glycosylation by glycan structure prediction.
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Acetylation: the addition of an acetyl group, usually at the N-terminus of the protein.
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Alkylation: The addition of an alkyl group (e.g. methyl, ethyl).
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Methylation: The addition of a methyl group, usually at lysine or arginine residues. (This is a type of alkylation.)
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Biotinylation: Acylation of conserved lysine residues with a biotin appendage.
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Glutamylation: Covalent linkage of glutamic acid residues to tubulin and some other proteins.
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Glycylation: Covalent linkage of one to more than 40 glycine residues to the tubulin C-terminal tail of the amino acid sequence.
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Isoprenylation: The addition of an isoprenoid group (e.g. farnesol and geranylgeraniol).
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Lipoylation: The attachment of a lipoate functionality.
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Phosphopantetheinylation, The addition of a 4'-phosphopantetheinyl moiety from coenzyme A, as in fatty acid, polyketide, non-ribosomal peptide and leucine biosynthesis.
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Phosphorylation, the addition of a phosphate group, usually to serine, tyrosine, threonine or histidine.
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Sulfation: The addition of a sulfate group to a tyrosine.
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Selenation
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C-terminal amidation
