Question

When a vertebrate dies, its muscles stiffen as they are deprived of ATP, a state called rigor mortis. Using your knowledge of the catalytic cycle of myosin in muscle contraction, explain the molecular basis of the rigor state.

Short answer

Rigor mortis occurs because ATP is unavailable, preventing myosin heads from detaching from actin.

Step-by-step solution

Understanding Muscle Contraction

In living muscles, contraction involves the actomyosin complex. Myosin heads bind to actin filaments using energy from ATP hydrolysis to pull and generate force, leading to muscle contraction.

Role of ATP in the Myosin Catalytic Cycle

The hydrolysis of ATP is crucial for muscle function. ATP binds to myosin heads, causing them to release actin, allowing muscles to relax. Hydrolysis then converts ATP to ADP and Pi, which repositions the myosin head for another contraction cycle.

Initiation of Rigor Mortis

Upon death, the supply of ATP ceases. Without ATP, myosin heads cannot detach from actin, causing muscles to remain in a contracted state, resulting in rigidity.

Molecular Basis of Rigor State

The lack of ATP post-mortem means that the cross-bridge cycling in muscles cannot continue. Hence, myosin heads remain attached to actin filaments, maintaining stiffness in the muscles.

Key concepts

Muscle Contraction

Muscle contraction is a fundamental process that enables movement in vertebrates. At the core of this process is the interaction between two proteins: actin and myosin. These proteins form structures known as sarcomeres, which are the basic units of muscle fibers. During muscle contraction, myosin heads attach to actin filaments creating cross-bridges. Using energy derived from the hydrolysis of ATP, myosin heads pivot, pulling the actin filaments closer together and thereby shortening the muscle. This action is not a single event but occurs as part of a cycle where numerous cross-bridges form and break, leading to the overall contraction of the muscle. Each cycle repeats as long as ATP is available, enabling sustained contraction and movement.
This cycle is essential for activities such as walking, running, and even the beating of our heart, illustrating the critical role of muscle contraction in everyday life. When this process halts, as in rigor mortis, muscle fibers lock in place, showcasing the importance of continuous ATP supply for muscle function.

Actomyosin Complex

The actomyosin complex is a crucial structure formed during muscle contraction. It consists of actin filaments and myosin molecules. Actin is a helical polymer of globular actin subunits, while myosin is a motor protein capable of converting chemical energy into mechanical work through ATP hydrolysis. Within the actomyosin complex, the myosin head attaches to the actin filament, creating a linkage that is pivotal for muscle contraction. This linkage is termed a cross-bridge.

• Formation of the actomyosin complex is central to the muscle's ability to contract and generate force.
• The dynamic interaction between actin and myosin heads is responsible for the sliding filament theory of muscle contraction.
• The actomyosin complex ensures coordinated movement and effective force transmission within muscles.

Therefore, the actomyosin complex is not just a static formation, but a highly dynamic entity that is key to the contraction and relaxation cycle of muscles.

ATP Hydrolysis

ATP hydrolysis is the chemical reaction that provides the energy for muscle contraction. ATP, or adenosine triphosphate, is a molecule that stores energy which is released when it is hydrolyzed into ADP (adenosine diphosphate) and inorganic phosphate (Pi). This energy release is what powers the movement of myosin heads in the actomyosin complex.
During the myosin catalytic cycle, ATP binds to the myosin head, leading to a conformational change that causes the release of the actin filament from the myosin head. The hydrolysis of ATP then takes place, which re-cocks the myosin head to a high-energy state, ready for the next cycle of muscle contraction. Without the constant supply and hydrolysis of ATP, the cross-bridges formed between myosin and actin cannot be broken, resulting in muscle stiffness observed in rigor mortis. This is why ATP hydrolysis is so vital; it is not only needed for contraction but also for relaxation and resetting the muscle fibers for the next contraction cycle.

Myosin Catalytic Cycle

The myosin catalytic cycle is an essential sequence of steps enabling muscle contraction and relaxation. It begins with ATP binding to the myosin head, causing it to detach from the actin filament. This detachment is crucial for muscle relaxation. Once myosin is free, ATP is hydrolyzed to ADP and Pi, which primes the myosin head into a cocked state.

• The cocked myosin head attaches to a new position on the actin filament, forming a cross-bridge.
• The release of ADP and Pi leads to the power stroke, where the myosin head pivots and pulls the actin filament toward the center of the sarcomere.
• The cycle completes when a new ATP molecule binds to myosin, allowing for the detachment of the myosin head from actin.

The myosin catalytic cycle is thus a repeating cycle of attachment, movement, and release, driven by ATP. Its continuous nature is disrupted upon death due to ATP depletion, leading to the permanency of actin-myosin binding, which we observe as rigor mortis. Understanding this cycle is key to grasping how muscles function and the pivotal role ATP plays in maintaining muscle dynamics.