Biochemical mechanism of muscular contraction.
Factors providing muscle contraction:
- Affinity complex "myosin-ATP" to actin is very low;
- Affinity complex "myosin-ADP" to actin is very high;
actin accelerates the cleavage of ADP and F of myosin, which is accompanied by conformational rearrangement (rotation of the myosin head).
Stages of muscle contraction:
- Fixation of ATP on the myosin head;
- Hydrolysis of ATP. Hydrolysis products (ADP and P) are fixed, and the released energy is stored in the head. Muscle ready to reduce;
- The formation of strong complex "actin-myosin" collapsing only when sorption new ATP molecules;
- Conformational changes of the myosin molecule, resulting in the rotation of the myosin head. Releasing the reaction products (ADP and F) from the active center of the myosin head.
Proteins - regulators of muscle contraction:
1) tropomyosin - fibril protein, has the form of a-helix. In a thin thread on one molecule of tropomyosin molecules accounted for 7 G-actin. Located in the groove between the two helices G-actin. Connected "end to end" chain continuous. Tropomyosin molecule closes the active actin binding sites on the surface of actin globules;
2) troponin - a globular protein consisting of three subunits: troponin "T" troponin "C" and troponin «I». Located on the tropomyosin at regular intervals, the length of which is equal to the length of the tropomyosin molecule. Troponin T (TnT) - is responsible for binding to troponin tropomyosin, troponin through "T" conformational changes are transmitted to the troponin tropomyosin. Troponin C (TNS) - Ca2 + - binding subunit contains 4 sites for calcium binding structurally similar to the protein calmodulin. Troponin I (TnI) - inhibitory subunit - fake inhibitor, creating a spatial obstacle interaction of actin and myosin when the troponin "C" is not associated with Ca2 +.
Regulation of muscle contraction and relaxation in the living cell:
- Muscle contraction begins with a nerve impulse. Develops under the influence of acetylcholine stimulation of the cell membrane and dramatically increases its permeability to Ca2 +;
- Ca2 + enters the cytoplasm of the muscle cell (sarcoplasm) of the depot - tanks cytoplasmic reticulum. Ca2 + concentration in the sarcoplasm instantly increases;
- Calcium binds to troponin "C". Conformational changes occur troponin molecules, as a result of steric hindrance is removed in the form of troponin «I», as tropomyosin molecule is pulled to the side and opens on the surface of actin, myosin-binding sites. Further muscle contraction follows the scheme.
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