The cystatins are a family of cysteine

Cysteine

Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...

 protease inhibitor

Protease inhibitor (biology)

In biology and biochemistry, protease inhibitors are molecules that inhibit the function of proteases. Many naturally occurring protease inhibitors are proteins....

s with homology

Homology (biology)

Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 to chicken cystatin. Cystatins typically comprise about 115 amino acid

Amino acid

Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s, are largely acidic, contain four conserved cysteine residue

Residue (chemistry)

In chemistry, residue is the material remaining after a distillation or an evaporation, or to a portion of a larger molecule, such as a methyl group. It may also refer to the undesired byproducts of a reaction....

s known to form two disulfide bond

Disulfide bond

In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...

s, may be glycosylated and/or phosphorylated, with similarity to fetuin

Fetuin

Fetuins are blood proteins that are made in the liver and secreted into the blood stream. They belong to a large group of binding proteins mediating the transport and availability of a wide variety of cargo substances in the blood stream. The best known representative of these carrier proteins is...

s, kininogens, stefins, histidine

Histidine

Histidine Histidine, an essential amino acid, has a positively charged imidazole functional group. It is one of the 22 proteinogenic amino acids. Its codons are CAU and CAC. Histidine was first isolated by German physician Albrecht Kossel in 1896. Histidine is an essential amino acid in humans...

-rich glycoprotein

Glycoprotein

Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...

s and cystatin-related protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s. They mainly inhibit peptidases belonging to peptidase families C1 (papain

Papain

Papain, also known as papaya proteinase I, is a cysteine protease enzyme present in papaya and mountain papaya .-Papain family:...

 family) and C13 (legumain family).

Types

The cystatin family includes:

• The Type 1 cystatins, which are intracellular cystatins that 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 bond
  Disulfide bond
  In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...
  
  s nor carbohydrate side-chains.
• The Type 2 cystatins, which are mainly extracellular secreted polypeptides 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) and L- (low-molecular-mass) kininogen, which are found in a number of species, and T-kininogen, which is found only in rat.
• 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.

Human cystatins

• CST1
  CST1
  Cystatin-SN is a protein that in humans is encoded by the CST1 gene.-External Links:* The MEROPS online database for peptidases and their inhibitors: -Further reading:...
  
  , CST2
  CST2
  Cystatin-SA is a protein that in humans is encoded by the CST2 gene.-External Links:* The MEROPS online database for peptidases and their inhibitors: -Further reading:...
  
  , CST3, CST4
  CST4
  Cystatin-S is a protein that in humans is encoded by the CST4 gene.-External Links:* The MEROPS online database for peptidases and their inhibitors: -Further reading:...
  
  , CST5
  CST5
  Cystatin-D is a protein that in humans is encoded by the CST5 gene.-External Links:* The MEROPS online database for peptidases and their inhibitors: -Further reading:...
  
  , CST6
  CST6 (gene)
  Cystatin-M is a protein that in humans is encoded by the CST6 gene.-External Links:* The MEROPS online database for peptidases and their inhibitors: -Further reading:...
  
  , CST7
  CST7 (gene)
  Cystatin-F is a protein that in humans is encoded by the CST7 gene.-External Links:* The MEROPS online database for peptidases and their inhibitors: -Further reading:...
  
  , CST8
  CST8 (gene)
  Cystatin-8 is a protein that in humans is encoded by the CST8 gene.-External Links:* The MEROPS online database for peptidases and their inhibitors: -Further reading:...
  
  , CST9, CST11
  CST11
  Cystatin-11 is a protein that in humans is encoded by the CST11 gene.-Further reading:...
  
  , CSTA
  Cystatin A
  Cystatin-A is a protein that in humans is encoded by the CSTA gene.-Interactions:Cystatin A has been shown to interact with Cathepsin B and CTSL1.-External Links:* The MEROPS online database for peptidases and their inhibitors:...
  
  , CSTB