Isopeptide bond
An isopeptide bond is an amide bond that is not present on the main chain of a protein. The bond forms between a side-chain carboxyl group and amino group.
Isopeptide bonds can occur between the side chain amine of lysine and the side chain carboxyl groups of either glutamate or aspartate[1]. Bond formation can be either enzyme catalyzed, as in the case for the bond formed between lysine and glutamine catalyzed by transglutaminases, or it can form spontaneously as observed in HK97 bacteriophage capsid formation [2] and Gram-positive bacterial pili [3]. Spontaneous isopeptide bond formation requires the presence of another residue, glutamic acid, which catalyzes bond formation in a proximity induced manner.
An example of a small peptide containing an isopeptide bond is glutathione, which has a bond between the side chain of a glutamate residue and the amino group of a cysteine residue. An example of a protein involved in isopeptide bonding is ubiquitin, which gets attached to other proteins with a bond between the C-terminal glycine residue of ubiquitin and a lysine side chain of the substrate protein.
Applications
Recently, researchers have exploited spontaneous isopeptide bond formation to develop a new peptide tag called the isopeptag. The isopeptag can spontaneously and irreversibly react with its binding partner to through a covalent isopeptide bond. Application for this exciting new molecular tool include in vivo protein targeting, fluorescent microscopy imaging, and the development of new protein architectures for synthetic biology [4].
See also
References
- ^ Stryer, Lubert. Biochemistry 4th Edition, 942-943 (1995)
- ^ Wikoff, W. R. et al. Topologically linked protein rings in the bacteriophage HK97 capsid. Science 289, 2129-2133 (2000).
- ^ Kang, H. J., Coulibaly, F., Clow, F., Proft, T. & Baker, E. N. Stabilizing isopeptide bonds revealed in gram-positive bacterial pilus structure. Science 318, 1625-1628 (2007).
- ^ Zakeri,B. and Howarth,M. (2010). Spontaneous intermolecular amide bond formation between side chains for irreversible peptide targeting. J. Am. Chem. Soc. 132, 4526-4527.
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