Systems Biology
The crossbridge cycle describes how actin and myosin interact within the sarcomeres of muscle cells and cause muscle contraction. Within the sarcomere of a muscle cell, thin filaments are mostly made of actin and also contain bound troponin and tropomyosin. At rest, tropomyosin wraps tightly around actin and covers myosin binding sites. When calcium ions are released into the cytosol by a muscular action potential, they bind to troponin, and the resulting shift in tropomyosin exposes myosin-binding sites, starting the crossbridge cycle. This process involves the binding of the myosin head to an exposed site on actin, forming a crossbridge, with the myosin head, at this point holding a bound ADP and phosphate.
The release of ADP and phosphate triggers a power stroke, during which myosin pulls on actin, drawing them closer together, causing the sarcomere to shorten and the muscle to contract. ATP binding to myosin results in the breaking of the crossbridge. The hydrolysis of ATP into ADP and phosphate primes the myosin head so that it can bind to the next section of actin and continue the cycle. As long as calcium remains bound to troponin, myosin will keep grabbing the next binding site on the thin filament, forming new crossbridges and continuing the cycle, ultimately leading to muscle contraction.
Lesson Outline
<ul> <li>Thin Filaments (Actin) <ul> <li>Structure: long strands of actin, troponin, and tropomyosin</li> <li>Role of Tropomyosin: Wraps around actin and covers myosin binding sites</li> <li>Role of Troponin: Binds on top of tropomyosin to keep it in place</li> <li>Calcium release and its effect: binding to troponin triggers conformational change to expose myosin-binding sites</li> </ul> </li> <li>Thick Filaments (Myosin) <ul> <li>Structure: made of myosin protein</li> <li>Formation of crossbridge: Myosin heads latch onto exposed actin binding sites</li> <li>Bound ADP and phosphate: initiation of power stroke</li> </ul> </li> <li>Power Stroke <ul> <li>Process: Myosin pulls actin closer, shortening sarcomere and causing muscle contraction</li> <li>Role of ATP: Binding to myosin breaks crossbridge, ending reaction</li> <li>Rigor Mortis: Lack of ATP results in continuous muscle contraction</li> </ul> </li> <li>ATP Hydrolysis <ul> <li>Process: ATP changes into ADP and phosphate, myosin head returns to high-energy state</li> </ul> </li> <li>Continuation of the Cycle <ul> <li>Process: New crossbridges form as long as calcium is bound to troponin</li> <li>Result: Sarcomere continues to shorten and muscle keeps contracting</li> </ul> </li> <li>Recap <ul> <li>Stages: Calcium binding, crossbridge formation, power stroke, ATP binding, ATP dephosphorylation, cycle continuation</li> </ul> </li> </ul>
Don't stop here!
Get access to 71 more Systems Biology lessons & 8 more full MCAT courses with one subscription!
FAQs
Actin and myosin are the two main protein components that interact during the cross-bridge cycle in muscle contraction. Myosin, which forms the thick filaments of sarcomeres, has a head region that binds to the thin filaments of actin. The myosin head uses energy from ATP hydrolysis to repetitively attach to actin filaments, pull (or 'power stroke'), and then detach, resulting in muscle contraction.
Sarcomeres are the smallest functional units of muscle fibers, consisting of overlapping actin and myosin filaments. During the cross-bridge cycle, the interaction between these filaments causes sarcomeres to shorten in a coordinated fashion, which ultimately results in muscle contraction. The sliding of actin and myosin filaments in the sarcomere generates the force required for the muscle to contract.
Calcium ions are essential in regulating the cross-bridge cycle. When a muscle is stimulated to contract, the release of calcium ions from the sarcoplasmic reticulum binds to troponin, a protein found in thin actin filaments. This binding causes a conformational change in troponin, leading to the movement of tropomyosin that exposes myosin-binding sites on actin filaments. Cross-bridge cycling can now occur, allowing muscle contraction to proceed.
ADP (adenosine diphosphate) and phosphate are the products of ATP hydrolysis. They play an essential role in the cross-bridge cycle as the energy from ATP hydrolysis powers the myosin head's movement and allows it to attach to actin, perform the power stroke, and then detach. The release of ADP and phosphate from the myosin head prepares it for a new ATP molecule's binding, allowing the cycle to continue.
During the cross-bridge cycle, the power stroke is a crucial step in which the myosin head attached to the actin filament rotates, producing a relative sliding movement between the actin and myosin filaments. This action generates the force required for muscle contraction. After the power stroke, the myosin head detaches from actin and prepares to start another cycle, ultimately leading to the shortening of sarcomeres and muscle contraction.
