Troponin turning

Inositol triphosphate is water soluble and therefore diffuses into the cytoplasm, where it mobilizes calcium from its stores in microsomes or the endoplasmic reticulum. The Ca ions then activate Ca-dependent kinases (like troponin C in muscle) directly or bind to the ubiquitous Ca-binding protein calmodulin, which activates calmodulin-dependent kinases. These kinases, in turn, phosphorylate cell-specific enzymes. 

Each cell within vertebrate striated muscle contains within its sarcoplasm many parallel myofibrils which in turn are made up of repeating sarcomere units. Within the sarcomere are the alternating dark A band and light I band, in the middle of which are the H zone and Z line, respectively. A myofibril contains two types of filaments the thick filaments consisting of myosin which are present only in the A band, and the thin filaments consisting of actin, tropomyosin and troponin. When muscle contracts, the thick and thin filaments slide over one another, shortening the length of the sarcomere. 

ATPase pump, causing an increase in intracellular Na. This, in turn, increases intracellular Ca by slowing down Na/Ca exchange. The increase in intracellular Ca leads to its binding to the troponin-tropomyosin complex, causing an allosteric change and facilitating the interaction between actin and myosin. 

Constraints imposed by caldesmon that disrupt potentiation may not block myosin docking on actin. This process of modulating ATPase could in turn lead to or permit the development of the latch-state of tension maintenance displayed by tonic smooth muscles and observed at low Ca + concentration. This phenomenon probably involves stable actin-myosin binding and is associated with low actomyosin ATPase activity (Hai and Murphy, 1988 McDaniel et al, 1990). As envisioned, such tension maintenance would be incompatible with a troponin-tropomyosin form of regulation since tropomyosin would in that case block myosin docking at low Ca + concentrations. Hence, the caldesmon-tropomyosin system may be adapted for muscles that enter a latch-state. 

Redesign of Ca /M Specificity. Proteins in the calmodulin superfamily (including troponin C, parvalbumin (PV) and oncomodulin (OM)) share similar overall structure and yet have different selectivity for Ca and Mg see Calcium-binding Proteins, Cation-activated Enzyme. For example, PV and OM have four helix-turn-helix domains, two of which contain mixed Ca VMg sites and two Ca -specific sites. The mixed Ca VMg sites have been converted to Ca -specific sites by replacing amino acids with the corresponding residues in the Ca Dgpecific site. ... 

Myocardial stunning , a reversible contractile failure often found in ischaemic cardiomyopathy is associated with selective degradation of troponin 1, resulting in a truncated molecule that shows considerably reduced calcium sensitivity. It was postulated that intracellular calcium overload due to severe ischaemia could activate the calcium-dependent protease calpain 1, which in turn would cleave troponin I (Gao etal. 1996). Klainguti etal. (2000) have shown that myocardial endo-thelin-1 is increased after 20 min of ischaemia and have suggested that endothelin-1 might play a role as an inducer of apoptosis. 

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