In muscle and microtubule contraction {muscle contraction}|, protein slides along another protein by grabbing and pulling, using ATP.
process
Calcium ions are in muscle-cell sarcoplasmic reticulum. Calcium-ion release initiates sliding. If calcium ion is present, tropomyosin goes into actin helix groove, and calcium opens binding sites, so actin can bind. Then ATPase globule tilts 45 degrees, pulling actin along. Then actin releases. After contraction, tissue elasticity passively returns muscle to normal length.
myosin
Myosin has four light chains and two long alpha-helix chains, which make two beads at myosin end. Beads are ATPases, connect actin to myosin, and are where calcium ions act. Three thick myosins surround each thin actin.
actin
Actin is globular protein that polymerizes into globule helix. Six thin actin proteins surround each thick myosin protein.
tropomyosin
Tropomyosin molecule helically wraps around actin.
troponin
Troponin molecule has binding sites for calcium, tropomyosin, and actin.
Transverse tubules are adjacent to sarcoplasmic reticulum at structures {triad junction}.
Muscle fibers have fine-tube {tubule} networks on cell surfaces and insides.
Skeletal muscle has parallel protein filaments {myofibril} lined up along elongated muscle-cell axis. Dark A bands have overlapping actin and myosin. Light I bands have actin, tropomysin, and troponin.
Repeating units {sarcomere} can have alternating dark A bands and light I bands. Dark Z line is in light I-band middle, where actins connect. Light H zone is in dark A-band middle and has thick myosin. Light H zone has middle dark M line, where myosins meet.
5-Chemistry-Biochemistry-Protein
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Date Modified: 2022.0225