Papain myosin fragmentation

Structure of the myosin heads. Myosin and myosin fragments can be isolated in large quantities, but they have been difficult to crystallize. However, Payment and coworkers purified SI heads cleaved from chicken myosin by papain and subjected them to reductive methylation (using a dimethylamine-borane complex see also Eq. 3-34). With most of the lysine side chain amino groups converted to dimethylamine groups, high-quality crystals were obtained, and a structure was determined by X-ray diffraction. ... 

Figure 4.5. Structure of myosin. Myosin comprises both light and heavy chains. The heavy chains may be cleaved by trypsin to generate light meromyosin (LMM) and heavy mero-myosin (HMM). Papain digestion of HMM yields subfragments SI and S2 each SI fragment contains an ATPase site and an actin-binding site. The light chains modify the activity of the ATPase.

Let us first consider the structure of myosin. The results of electron microscopic studies of skeletal-muscle myosin show it to be a two-headed structure linked to a long stalk (Figure 34.2). As we saw in Chapter 33, limited proteolysis can be a powerful tool in probing the activity of large proteins. The treatment of myosin with trypsin and papain results in the formation of four fragments two SI fragments an S2 fragment, also called heavy... 

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