Proteins are often synthesized in an inactive precursor form typically, an N-terminal or C-terminal segment blocks the active site of the protein, inhibiting its function. In addition to those listed above, the most important modification of primary structure is peptide cleavage (by chemical hydrolysis or by proteases). Many other chemical reactions (e.g., cyanylation) have been applied to proteins by chemists, although they are not found in biological systems. Most of the polypeptide modifications listed above occur post-translationally, i.e., after the protein has been synthesized on the ribosome, typically occurring in the endoplasmic reticulum, a subcellular organelle of the eukaryotic cell. Ubiquitin is the most common of these, and usually signals that the ubiquitin-tagged protein should be degraded. Various full-length, folded proteins can be attached at their C-termini to the sidechain ammonium groups of lysines of other proteins. This modification is a target for the powerful toxins of disparate bacteria, e.g., Vibrio cholerae, Corynebacterium diphtheriae and Bordetella pertussis. The large ADP-ribosyl group can be transferred to several types of side chains within proteins, with heterogeneous effects. This is used to strengthen the binding to "hard" metal ions such as calcium. Unlike the GPI and myritoyl anchors, these groups are not necessarily added at the termini.Ī relatively rare modification that adds an extra carboxylate group (and, hence, a double negative charge) to a glutamate side chain, producing a Gla residue. In particular, the L-amino acids normally found in proteins can spontaneously isomerize at the C α atom of cysteine residues to anchor proteins to cellular membranes. Although it does not change the sequence, it does affect the chemical properties of the sequence. The chiral centers of a polypeptide chain can undergo racemization. However, proteins can become cross-linked, most commonly by disulfide bonds, and the primary structure also requires specifying the cross-linking atoms, e.g., specifying the cysteines involved in the protein's disulfide bonds. In general, polypeptides are unbranched polymers, so their primary structure can often be specified by the sequence of amino acids along their backbone.
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