Cysteine Proteinase Inhibitors
 
 Cysteine proteinase inhibitors attracted wide attention and, after serine proteinase inhibitors, have the most frequently described members. Most but not all of these belong to the cystatin superfamily, which is frequently divided into three families: the stefin, the cystatin, and the kininogen families. The stefins are low molecular weight (about 100 residues( molecules without attached carbohydrate and without any intramolecular disulfide bridges. They are extremely stable intracellular proteins. The word stefin comes from the name of the J. Stefin Institute in Ljubljana (Slovenia), where the first stefin was discovered. Cystatins proper are only slightly bigger than stefins (120–130 residues). They contain two disulfide bridges near their COOH terminals and typically are secretory proteins. The family and superfamily are named after chicken egg white cystatin, which was discovered long before the other cystatins became popular. The third family of cystatins is the kininogen family. Proteins are assigned to it on the basis of the high molecular weights of its members, predominantly HMW (high molecular weight) (120 kD) and LMW (low molecular weight) (68 kD) kininogens. Kininogens are glycosylated and contain numerous disulfide bridges. Each contains three cystatin like repeats.
All cystatin family members inhibit only cysteine proteinases and only those related to papain but, within this broad group, they exhibit little discrimination between individual enzymes. In strong contrast to serine proteinase inhibitors, cystatins react with enzymes whose catalytic sulfhydryl group is significantly blocked. The mode of association was elucidated in detail from the three-dimensional structure of a complex of stefin A with papain.
 In avian and mammalian cells, there are present cysteine proteinases called calcium-activated neutral proteinases (CANP), or calpains. These complicated multidomain enzymes are of great importance in the metabolism of muscles. They are controlled by a closely related family of molecularly complex endogenous protein inhibitors—calpastatins.