|Proteinase inhibitor I25, cystatin|
The cystatins are a family of cysteine protease inhibitors which share a sequence homology and a common tertiary structure of an alpha helix lying on top of an anti-parallel beta sheet. The family is subdivided as described below.
Cystatins show similarity to fetuins, kininogens, histidine-rich glycoproteins and cystatin-related proteins. Cystatins mainly inhibit peptidase enzymes (another term for proteases) belonging to peptidase families C1 (papain family) and C13 (legumain family). They are known to mis-fold to form amyloid deposits and are implicated in several diseases.
The cystatin family includes:
- The Type 1 cystatins, which are intracellular and are present in the cytosol of many cell types, but can also appear in body fluids at significant concentrations. They are single-chain polypeptides of about 100 residues, which have neither disulfide bonds nor carbohydrate side-chains. Type 1 cystatins are also known as Stefins (after the Stefan Institute where they were first discovered )
- The Type 2 cystatins, which are mainly extracellular secreted polypeptides are largely acidic, contain four conserved cysteine residues known to form two disulfide bonds, may be glycosylated and/or phosphorylated. They are synthesised with a 19- to 28-residue signal peptide. They are broadly distributed and found in most body fluids.
- The Type 3 cystatins, which are multidomain proteins. The mammalian representatives of this group are the kininogens. There are three different kininogens in mammals: H- (high-molecular-mass, IPR002395) and L- (low-molecular-mass) kininogen, which are found in a number of species, and T-kininogen, which is found only in rats.
- Unclassified cystatins. These are cystatin-like proteins found in a range of organisms: plant phytocystatins, fetuin in mammals, insect cystatins, and a puff adder venom cystatin, which inhibits metalloproteases of the MEROPS peptidase family M12 (astacin/adamalysin). Also, a number of the cystatin-like proteins have been shown to be devoid of inhibitory activity.
- CST1, CST2, CST3 (cystatin C, a marker of kidney function), CST4, CST5, CST6, CST7, CST8, CST9, CST11, CSTA (cystatin A), CSTB (cystatin B)
- Affimer, a type of engineered protein that is based on the cystatin scaffold
- ; Salát J, Paesen GC, Rezácová P, Kotsyfakis M, Kovárová Z, Sanda M, Majtán J, Grunclová L, Horká H, Andersen JF, Brynda J, Horn M, Nunn MA, Kopácek P, Kopecký J, Mares M (June 2010). "Crystal structure and functional characterization of an immunomodulatory salivary cystatin from the soft tick Ornithodoros moubata". Biochem. J. 429 (1): 103–12. doi:10.1042/BJ20100280. PMC 3523712. PMID 20545626.; rendered with PyMOL
- Rawlings ND, Barrett AJ (1990). "Evolution of proteins of the cystatin superfamily". J. Mol. Evol. 30 (1): 60–71. doi:10.1007/BF02102453. PMID 2107324.
- Abrahamson M, Alvarez-fernandez M, Nathanson CM (2003). "Cystatins". Biochem. Soc. Symp. (70): 179–199. PMID 14587292.
- Bode W, Turk V (1991). "The cystatins: protein inhibitors of cysteine proteinases". FEBS Lett. 285 (2): 213–219. doi:10.1016/0014-5793(91)80804-C. PMID 1855589.
- Machleidt, W.; Borchart, U.; Fritz, H.; Brzin, J.; Ritonja, A.; Turk, V. (1983). "Protein inhibitors of cysteine proteinases. II. Primary structure of stefin, a cytosolic protein inhibitor of cysteine proteinases from human polymorphonuclear granulocytes". Hoppe-Seyler's Zeitschrift fur physiologische Chemie 364 (11): 1481–1486. doi:10.1515/bchm2.1983.364.2.1481. PMID 6689312.
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