Titin folding powers muscle contraction

By combining springs with motors and chemical switches biological systems can achieve great scales of mechanical power amplification, for example when jumping or launching a projectile. However, the molecular mechanisms of how this is achieved remain unknown. The giant muscle protein titin, composed of hundreds of tandem Ig domains, is a complex elastic protein whose role in muscle function is still poorly understood. A crucial recent discovery that we have made is that titin domains do a surprisingly large amount of mechanical work when they fold against an opposing mechanical force. We have shown that the amount of mechanical work done by a folding titin Ig domain can be 2-3 times larger (～120 zJ) than that of the chemically powered motor myosin II; ～38 zJ. Titin molecules store mechanical energy by unfolding and extending under force. Elastic energy is stored this way by stretching caused by gravitational pulling during locomotion, inertia, chemical modifications, and ATP powered sources to name a few. Titin unfolding occurs at varying rates over a very wide range of forces above >8 pN. By contrast, most of the stored mechanical energy is delivered back only over a small range of forces where the folding probability increases from 0 to 1 (< 6 pN) and the folding protein does large amounts of mechanical work. Thus, protein folding/unfolding is likely to operate as a mechanical battery where different types of energy sources are stored, and then converted back into contractile power.