Calpain is an enzyme that degrades proteins, mainly for the degradation of various muscle proteins. Calpain degrades the protein in myofibrils, so the muscle fibers are degraded, the shearing force of the skeletal muscle is reduced, and the tenderness of the meat is increased, which is a calcium ion-dependent protease that plays an important role in meat quality. Calpain activity is affected by the concentration of calcium ions in myocytes. Calcium ions activate this protein and are classified into u-calpain (micromolar) and m-calpain (mmol) based on the calcium ion concentration calpain required to activate this protein.
Figure 1. Protein structure of calpain.
Calpain (which is an intracellular calcium-dependent cysteine endopeptidase. In vivo, it exhibits proteolytic activity by Ca2+ activation and autolysis, and inhibits activated calpain activity by calpain. The widely existing calpains are calpain 1 (mu-calpain), calpain 2 (m-calpain), and calpain 3 (p94). The former two are composed of two subunits with similar molecular weights, but the two Ca2+ concentrations required for the highest activity are different. Their small subunits are the same, while the large subunits are the products of two related genes, two There is a certain immune cross-reaction between the large subunits. The functional calpain is composed of a large subunit (catalytic region, 80kd) and a small subunit (regulatory region, 30kd), and the small subunit is the product of a single gene. Large subunits are products of different genes. The calpain cDNA sequences from different sources have found that their large subunits are composed of four domains, of which domain II is a key site for hydrolyzing activity, which accounts for about a large subunit. About 35% of the number of amino acid residues; domain IV, located at the carboxy terminus, accounting for 20% of the number of amino acid residues, is the calcium ion binding site; domain I and domain III account for 10% and 35% of the amino acid residues, respectively. , may play a role in the regulation of activity. calpain3 (p94) is also a protease closely related to muscle tenderness.
Figure 2. Protein structure of calpain 2.
1. Functions in skeletal muscle growth
Skeletal troponin is present in muscle cells. The first step in the degradation of myofibrillar protein must be the disassembly of myofibrils into myofilaments. Numerous studies have shown that the calpain system participates in this disassembly process. The possible mechanisms by which calpain triggers the degradation of myofibrillar proteins are as follows: calpain makes the Z line (fixing the thin filaments to myofibrils) and the muscle protein, with actin (fixing the thick and thin muscles to the muscle) Fibrils are degraded and myofibril releases myofilaments. C-protein degradation of myofibrillar and tropomyosin and thin myofilament of fine muscle filaments, and myofibrillar and actin are dissociated from thick myofilament and thin filament, respectively, releasing thick filaments and thin muscles. The silk can be reassembled with the parent or other myofibrils, and can also be degraded into amino acids by cytosolic or lysosomal cathepsins. The residual part of myofibrils is functional and the contraction strength is weakened.
2. Functions in muscle tenderization
The calpain system participates in the growth and metabolism of the body in cells, and plays an important role in the regeneration of myofibrils and post-mortem tenderization. Decreased expression of calpain and increased expression of calpastatin lead to a decrease in muscle protein hydrolysis rate and post-mortem meat tenderness, suggesting that calpain hydrolysis is a major factor in the tenderization of meat.
Many studies have shown that calpain plays an important regulatory role in the degradation of myofibrillar proteins, especially in early muscle transformation, and inhibiting its activity may inhibit protein degradation and increase muscle growth. However, in addition to inhibiting the activity of calpain, the existing inhibitors also inhibit the activity of other proteases more or less, and lack specificity. Calpastatin is a highly efficient and highly specific calpain activity inhibitory protein, so it can inhibit the activity of calpain through gene regulation technology to increase its expression in muscle tissue, thereby achieving the purpose of improving muscle growth. We can recombinantly express DNA technology to overexpress calpastatin or defective calpain, and then develop a cell line in which overexpression of calpastatin or calpain may be initiated by a muscle-specific promoter to control myofibrillar protein. Degradation to accelerate protein accumulation. The tenderization of meat is related to the activity of calpain. Therefore, it can improve the tenderness of meat and improve the meat quality by activating calpain or reducing the activity of calpastadn. That is to say, it can increase the activity of calpastadn during the growth stage of livestock to inhibit the activity of calpain and promote protein. Deposition, increase the growth rate of skeletal muscle, increase the activity of calpain before slaughter to increase the tenderness of skeletal muscle. However, in general, the specific regulation mechanism of the calpain system is still not clear, and it is necessary to conduct in-depth research using molecular biology and other technical means.
1. Suzuki K.; et al. Calpain: novel family members, activation, and physiologic function. Biol. Chem. Hoppe-Seyler. 1995, 376 (9): 523–529