MANUEL F. MORALES AND JEAN BOTTS 



of enzyme action in the case of myosin ATPase, then it follows 

 that the adsorption of ATP in the course of the enzymatic act 

 is a mechanism for (algebraically) reducing the local charge on 

 the myosin. To predict the dimensional consequence of ATP 

 adsorption we must introduce the third participant in the reac- 

 tion — an adsorbable cation, in all probability Mg2+ [vide infra). 

 Quantitative amino acid analyses (2,43) indicate that, except 

 for some possibly very specialized regions of the myosin mole- 

 cule, its net charge in salt-free neutral solution is negative. 

 Under such conditions the preferential adsorption of ATP should 

 expand or extend the structure. Contraction, on the other 

 hand, should be possible if the deformable regions of myosin are 

 positively charged prior to ATP adsorption. Electrophoretic 

 studies (66) of myosin early indicated that positivity of the pro- 

 tein as a whole (and so presumably of some of its subregions) can 

 be achieved by adsorption of Mg2+; more recent research 

 {vide infra) indicates that at least some magnesium is held by the 

 protein with great tenacity. It has therefore seemed to us 

 reasonable to construct our simplest contractile system by sup- 

 posing that in such a system, deformable regions of the myosin 

 molecule, made positive and extended through Mg"'"''" adsorp- 

 tion, suffer a reduction in charge — hence a mechanical collapse — 

 through subsequent adsorption of ATP^~. 



Although we have mentioned that myosin is an ATPase, we 

 have used the fact only to indicate that ATP must be specifically 

 adsorbable to the myosin. No use has been made of the fact 

 that ATP goes on to be decomposed into adenosinediphosphate 

 (ADP) and orthophosphate (P) ; indeed in this model the sub- 

 sequent A TP cleavage is unnecessary to the mechanism of deformation. 

 In experimental models (e.g., dissolved myosin particles), how- 

 ever, the subsequent reaction generally occurs, and ADP and P 

 accumulate in the solution until virtually all the ATP has been 

 decomposed. In at least some of these situations it can be shown 

 that ADP + P are bound to myosin much less than is ATP, and 

 upon exhaustion of ATP tlie deformation (in these cases an ex- 

 pansion rather than a contraction) of the myosin particles is 



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