What is "cystatin"

n. (context biochemistry English) Any of a group of polypeptides that are cysteine protease inhibitors

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.