The role of calpains in muscle tissue proteolysis

Kalpainski enzimski sustav čine dvije sveprisutne proteaze, μ-kalpaina i m-kalpaina, te njihov inhibitor, kalpastatin. Kalpaini su heterodimeri koji se sastoje od dvije podjedinice: velike katalitičke veličine 80 kDa organizirane u četiri domene, te male regulatorne veličine 30 kDa organizirane u dvije domene. Na temelju dosadašnjih saznanja o katalitičkim svojstvima kalpaina, aktivnost kalpaina se regulira koncentracijom Ca2+, autproteolizom, kalpastatinom, staničnom lokalizacijom i specifičnim područjem aktivnog mjesta. Kalpaini sudjeluju u različitim staničnim procesima uključujući staničnu mobilnost, prijenos signala, stanični ciklus, apoptozu, patološke bolesti i postmortalnu proteolizu. Mehanizam posmortalne proteolize mesa je vrlo složen proces u kojem sudjeluje velik broj različitih enzima. Glavnu ulogu u miofibrilarnoj proteolizi imaju kalpaini. Miofibrilarna žilavost posljedica je mrtvačke ukočenosti, dok je miofibrilarno omekšavanje uzrokovano cijepanjem miofibrilarnih proteina kalpainima. Na temelju važnih dosadašnjih dokaza, razjašnjeni su mnogi faktori koji utječu mekoću mesa.The calpain system consist of calpains, a family of Ca2+ dependent cysteine proteases whose members are expressed ubiquitously or in a tissue-specific way. Calpains are heterodimeric proteins, composed of catalytic subunit of about 80 kDa organised in 4 domains and the small 30 kDa subunit organised in 2 domains. Based on the current knowledge of the catalytic properties of the calpains, it seems that calpain activity is regulated by Ca2+ concentration, calpastatin, intracellular location and subsite specificity of the calpains. Calpains participate in a variety of cellular processes including cell motility, signal transduction pathways, the cell cycle, apoptosis, long-term potentiation, pathological states in cells and postmortem proteolysis. The exact mechanisms involved in the postmortem meat tenderization process are complex and includes a large number of different enzymes. The main myofibrillar proteolysis can be attributed to calpains. The myofibrillar toughness is considered to be affected by the development of rigor-mortis and tenderization caused by calpains break-down of the contractile proteins. Based on the relevant evidence thus far obtained, factors affecting meat tenderness, although not fully understood, are in perspective to be clarified