Muscle Contraction Mechanisms: Use of Synchrotron X‐ray Diffraction

AbstractMuscle contraction occurs when the constitutive proteins,actinandmyosin, interact via crossbridge formation, powered by the hydrolysis of adenosine triphosphate (ATP). It is a physical process that transduces chemical energy into mechanical work, producing directional motion. The central question is how force generation and movement in muscle contraction are associated with major conformational changes in contractile and regulatory proteins. The best available technique for such an approach is X‐ray diffraction which can provide structural information with a submolecular resolution. X‐ray diffraction with intense synchrotron radiation has demonstrated a structural basis for the molecular mechanism underlying muscle contraction with high spatial‐ and time‐resolution. The structural alterations in vertebrate skeletal muscles undergoing contraction by synchrotron X‐ray diffraction are outlined by putting a lot of weight on the thin filament as the locus of actomyosin interaction.Key Concepts:Muscle contraction occurs when constitutive proteins actin and myosin in muscle interact with each other, powered by the hydrolysis of adenosine triphosphate (ATP), leading to a force generation or a shortening of the sarcomere.Sliding filament theory is a proposal on the basis of the discovery that when a sarcomere shortens, two types of filaments slide each other with little change in their lengths, resulting in an overall shortening of muscle.Elastic elements sustaining the tension that muscle exerts during contraction are thought to be resided somewhere in a sarcomere. In the current hypothesis, the only elastic elements are assumed to reside somewhere around or within the myosin heads.Extensibility of the thin actin filament: The force is transmitted to the ends of the contractile unit through the thin actin filaments, which have been thought to contribute very little compliance to the mechanochemical machinery. Evidence that the thin actin filaments are purely elastic under active force generation has been obtained.Twisting of the thin actin filament suggests that the force acting at a myosin crossbridge contains torque components around the axis of the thin actin filament.X‐ray diffraction is the interference of the X‐rays scattered from the electron densities of the matter, and the Fourier transformation of the interference pattern yields the structure of the matter.Synchrotron radiations are electromagnetic waves that are emitted in the tangential direction of the orbit when electrons or positrons circulating near the speed of light in an accelerating ring (synchrotron). Synchrotron X‐rays with high brilliance are an indispensable tool for muscle structural research.