8 : 4/ Muscles 



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energy. When this happens, the molecule ATP is split into adenosine 

 diphosphate, ADP, and inorganic phosphate (£). Symbolically, one may 

 write this as 



ATP - ADP + ® + energy 



(Readers without previous knowledge of biochemistry should not allow them- 

 selves to be dismayed by this jargon of letters like ATP and ADP. Many people 

 who use them don't know the structural formula represented by these symbols; 

 all one needs to know is the stoichiometric formula written above. The physical 

 forms of ATP and ADP must be very important for their actions, but no one yet 

 has succeeded in relating these concepts. The molecule ATP is made up of one 

 molecule of the purine, adenine; one molecule of the pentose sugar, ribose; and 

 three phosphate groups joined by pyrophosphate bonds. Its structural formula is 

 shown below, but the reader unfamiliar with biochemistry is advised to stick to 

 the symbol ATP rather than trying to remember the structure. 



H— N— H 



H 



/^ 



O 



t 



-o — P — o 



H 



Adenine plus ribose forms the molecule adenosine. Adenosine plus one phosphate 

 condenses to adenylic acid or adenosine monophosphate, AMP. The latter plus another 

 phosphate condenses to ADP. Energy is released when ATP is split to ADP and 

 when ADP is split to AMP and (?). ) 



There seems to be no doubt, from a large variety of experiments, 

 that ATP is the source of energy used in muscular contraction. Just 

 how this occurs is not clear. For instance, ATP might be split before 

 the muscle contracts or just as it contracts. An alternative possibility 

 is that the muscle proteins store energy which is used during the twitch 

 and then is slowly built up again from ATP during recovery. ATP 

 might form a complex with the muscle proteins. Recent experiments 

 indicate that several intermediates must exist between ATP and the 

 contractile proteins. 



Some of the evidence for the direct interaction of ATP comes from 

 experiments with purified myosin and with actin and myosin. If 

 solutions of these proteins in the globular form are mixed with ATP, 

 they form fibers. In the fibrillar form, ATP causes actin-myosin fibers 

 to contract. Moreover, ATP is split in this process because the protein 

 myosin acts as an enzyme catalyzing the splitting of ATP into ADP plus 



