1KFU

Calpain is an intracellular, cytostolic, calcium dependent cysteine protease containing a conserved papain-like catalytic fold. Thus, in the presence of clacium, calpain catalyses the hydrolysis of peptide bonds in proteins. Two major, heterodimeric isoforms exist, micro- and m-calpain, which differ in their concentration requirements of calcium ions for activation. Calpains are the only known mammalian enzymes that combine protease activity with dependence on calcium ion binding to EF-hands in one molecule.

Calcium regulates calpain-1 activity by affecting the orientation of the two catalytic lobes (domains I and II). calcium binding occurs at multiple sites, including EF-hands and loops near the active site itself. At low calcium levels, domains I and II are locked by the regulatory domains of the proteins in an open, inhibited conformation, where the catalytic residues are too far apart to form the triad. (Cys of domain I, His and Asp of domain II).

Calpain exists in animals, but not in plants, yeast or bacteria. Through proteolysis of key cellular components, calpain plays a significant physiological role and is involved in a variety of cellular functions, including nerve growth, muscle homeostasis, signal transduction and apoptosis. Calpains have been known as biomodulators since their physiological activity involves cleavage of substrates at inter-domain boundaries, hence modulating the function of these substrates rather than digesting them.

Excessive activation and calcium dependent proteolysis by calpain has been implicated in neurodegenerative and demyelinating diseases including cerebral ischemia, ischemic myocardial infarction, spinal cord injury, muscular dystrophy and cataract. Calpain inhibition should help to control symptoms, hence calpains are ideal targets for pharmacological intervention.

The catalytic triad is not assembled prior to calcium binding. The calcium ion induces conformational changes to bring together the domain I Cis residue and the domain II His and Asp residues. In the inactive form, the residues are too far apart to form the triad since the Cis-His ion pair cannot be formed over such a distance. Calpain is also capable of autoproteolysis.