286 BIOPHYSICAL STUDIES ON NERVE AND MUSCLE 



filaments, actin in the thin ones. The Z-lines are the outer edges of areas 

 which bisect the myofibril, and have been shown to be the medium through 

 which the stimulus, or order to contract, is carried from the surface mem- 

 brane of the fiber (the sarcolemma) into the myofibril. The sarcolemma car- 

 ries it electrochemically (like nerve) along the fiber. 



Muscle consists of 18 to 20 per cent protein, by weight. About 60 per cent 

 of this protein is a condensation product of several "myosins" with actin, a 

 very complex molecule whose complete physical structure is very sensitive to 

 the ionic content and pH of the medium. It interchanges between a globu- 

 lar, almost spherical, hard G-actin, to a fibrous, stiff F-actin. Only myosin 

 has the ATPase activity and can accept the free energy of hydrolysis of ATP. 

 But the myosin of muscle is itself made up of smaller parts: 



Rapidly extractable from minced muscle in salt solutions is myosin-A 

 (called "myosin" or "/-myosin" in some books). Electrophoresis causes 

 separation of myosin-/! into three fractions: one heavy (//) meromyosin, 

 and two light (L) meromyosins. Only the //-meromyosin retains the ATP- 

 ase activity, Extractable only slowly, or in other media, are myosin-/? 

 ("natural actomyosin" or "^-myosin") and tropomyosin, which differ in 

 physical properties from myosin-^4. Rejected by the extraction procedures 

 is the globular G-actin, which, in the presence of ATP and dilute salts, 

 slowly converts to the much more viscous, fibrous F-actin. The chemical 

 composition is not simple. Thus there is some evidence that tropomyosin + 

 G-actin + another protein constitute myosin- A. Some physical characteris- 

 tics of myosin and actin are gathered in Table 10-3. 



The muscle proteins are rich in polar residues such as — P0 3 " 3 , — OH, 

 — CONH — , and — COOH. These polar residues seem to be intimately 

 connected with the process of contraction. Myosin's partner in the con- 

 tractile reaction is ATP. To ATP, the fact that the catalytic enzyme, myo- 

 sin, contracts during the hydrolysis, or splitting of ATP into ADP + P, is 

 quite incidental. To the living system, however, the fact is vital! Dephos- 

 phorylation occurs during or immediately after the contraction process. 



Hydrolysis of ATP as a free energy-producing reaction is not confined to 

 myosin as a catalyst, as we saw in Chapter 7. It provides the energy which 

 drives many living processes. The following scheme represents the splitting 

 reaction and its auxiliary reactions: 



H 2 + ATP 4 ^ ADP 2 + HP0 4 " 2 



+ K 2 }[ + 



//+ //+ //+ 



K, 11 + K 3 11 



ATP- 3 ADP 3 H 2 P0 4 



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