Alternations to polypeptides/proteins that occur after the completion of their synthesis.
This modification can include removal of amino acids such as cleavage signal sequence. Cleavage of the protein into two or more larger parts can also occur which, depending on the system, may or may not subsequently serve as separate proteins and indeed may or may not separate following cleavage (contrast the proteins associated with HIV with, for example, insulin).
In addition, there is a substantial amount of covalent modification that can occur including phosphorylation, glycosolyation, and addition of lipids. Such modifications can also be reversible or permanent, e.g., particularly phosphorylation as a means of controlling enzyme activity tends to be reversible.
Though these modifications generally are carried out by enzymes that are distinct from the protein being modified, in fact whether or not modification should take place is indicated by the to-be-modified proteins themselves. Indeed, all of a protein's final structure are determined, ultimately, by a protein's or polypeptide's primary structure.
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