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About CST / Cystatin:
Cystatins are a group of cysteine protease inhibitors that share a sequence homology and a common tertiary structure. Their common tertiary structure is an alpha helix which lies on top of an anti-parallel beta sheet. Cystatins are similar to fetuins, kininogens, histidine-rich glycoproteins and cystatin-related proteins.
Type 1 Cystatins
These intracellular cystatins referred to as “Stefins”. They can be found in the cytosol of many cell types and can also appear in bodily fluids. Sometimes, when in bodily fluids, they can appear in significant concentrations. These cystatins are single-chain polypeptides of roughly 100 residues. They have neither disulfide bonds nor carbohydrate side-chains.
Type 2 Cystatins
These types of cystatins are extracellular. They are secreted polypeptides that are largely acidic and contain four conserved cysteine residues. Type 2 cystatins are found in most bodily fluids. They can form disulfide bonds and can be glycosylated and / or phosphorylated.
Type 3 Cystatins
Type 3 cystatins are multidomain proteins. Their mammalian representatives are kinonogens and there are three different types found within mammals: H, L and T. T-kininogens are only found in rats.
Unclassified Cystatins
This group of cystatins are proteins that are found in a number of organisms, ranging from plant phytocystatins to insect cystatins, puff adder venom cystatin and fetuin (which is found in mammals).