Calcium, troponin and tropomyosin in muscle contraction
In skeletal muscle, calcium binds troponin on the thin filament (a fine protein strand), moving tropomyosin (a covering thread) so the force-producing myosin heads can attach and contraction can begin.
Remember
In skeletal muscle, calcium binding to troponin moves tropomyosin off the myosin-binding sites on actin. Smooth muscle has no troponin and uses calmodulin (a calcium-sensing protein) instead.
Memory aid
tropomyosin is a gate across the actin track, calcium-bound troponin is the hand that opens it, and with no calcium on troponin the gate stays shut
Picture tropomyosin as a gate across the actin track, which calcium-bound troponin opens.
A muscle cell, or fibre, contracts when myosin heads (the club-shaped ends of the thick filament) attach to actin, the protein that is pulled during contraction. Actin forms the backbone of the thin filament, which also carries the regulators: troponin, a protein complex that binds calcium, and tropomyosin, a long rope-like protein lying along the actin.
At rest, tropomyosin covers the myosin-binding sites on actin, so the heads have nothing to grab. When the fibre is activated, the sarcoplasmic reticulum (the fibre's internal calcium store) releases calcium. Calcium binds troponin, which changes shape and pulls tropomyosin aside. With the sites uncovered, the heads attach and form cross-bridges (temporary links between myosin heads and actin), starting the sliding filament process that shortens the sarcomere (the repeating contractile unit of the fibre).
To recall the three troponin subunits (parts), use the final letter of each abbreviation: TnC binds Calcium, TnI is the Inhibitory subunit and TnT binds Tropomyosin. In skeletal muscle, calcium works as a switch that unblocks actin and does not act on myosin directly.
Smooth muscle, found in the walls of blood vessels, the gut and the bladder, has actin and myosin but no troponin. Calcium binds calmodulin instead, and the calcium-calmodulin complex activates myosin light chain kinase, an enzyme that adds a phosphate group to the myosin heads so that they can bind actin. Striated muscle (the banded type, which includes skeletal and cardiac muscle) switches on at the thin filament, whereas smooth muscle switches on at the myosin.
Key facts
- At rest
- tropomyosin covers the myosin-binding sites on actin, so the myosin heads are blocked from attaching.
- Calcium switch in skeletal muscle
- calcium binds troponin, which moves tropomyosin away and uncovers the myosin-binding sites.
- Troponin subunits
- TnC binds Calcium, TnI is the Inhibitory subunit and TnT binds Tropomyosin.
- Smooth muscle
- it has no troponin, so calcium binds calmodulin, which activates myosin light chain kinase.
- Shared steps
- ATP splitting by myosin and filament sliding happen in both muscle types.
- Troponin test
- cardiac troponin is released when heart muscle is damaged, so a blood troponin test is used to detect damage such as a heart attack.
Exam traps
- Calcium binding calmodulin to activate myosin light chain kinase is the smooth muscle pathway, so it is not the step that sets skeletal muscle off, because skeletal muscle uses troponin instead.
- Calcium does not bind tropomyosin directly: its target is troponin, and tropomyosin moves as a consequence.
- Calcium does not act on the myosin head in skeletal muscle, because it acts on troponin C on the thin filament, and the heads stay blocked until tropomyosin is moved aside.
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