BIOLOGICAL ENERGETICS 417 



to the amount of free energy liberated on hydrolysis of the P — X bond. 

 A number of more important ones are collected in Table 16-1. For the 

 low energy group, AF is about — 2000 to — 4000 calories per mole, whereas 

 for the high energy compounds it amounts to around — 11,000 to — 15,000 



Table 16-1 



Free energies of hydrolysis of some phosphoric acid derivatives * 



Compound AF, cal. pK Temp., °C 



Glucose-1-phosphate — 4,750 8.5 38 



Glucose-6-phosphate —3,000 8.5 38 



Fructo?e-6-phosphate —3,350 8.5 38 



Glycerol-l-phosphate —2,200 8.5 38 



ATP (terminal group) —11,500 7.5 20 



Acetyl phosphate —14,500 6.3 37 



Pyruvic acid enol phosphate — 15,900 ? 20 



Creatine phosphate —13,000 7.7 20 



Arginine phosphate — 11,800 7.7 20 



* Reproduced from Avison and Hawkins, "The Role of Phosphoric Esters in Bio- 

 logical Reactions," Quart. Rev., 5, 171 (1951) by permission of the authors and the 

 Chemical Society (London). 



calories. The latter substances are said to contain a high energy phos- 

 phate bond, which is written as "-^P". It is only the chemical energy of 

 such bonds which can be transformed directly into useful work by living 

 organisms, and so far as known, only ATP serves as the immediate source 

 of such energy, both for muscular work and for all other purposes. The 

 metabolic breakdoum of foodstuffs, so far as energy requirements are 

 concerned, is a matter of generating high energy phosphate bonds and 

 of synthesizing ATP. 



Phosphagens 



The presence in CrP of a high energy bond, taken together with the 

 facts already presented, indicates that CrP serves as a '--'P storehouse 

 in muscle. When contractions begin, ATP starts to be used. It would 

 be quickly exhausted except for the Lohmann reaction, which starts func- 

 tioning as soon as some ADP is formed. ATP is thereby resynthesized 

 at the expense of CrP, and the ATP level is kept up until most of the 

 CrP is used, and the muscle becomes exhausted. This situation is reached, 

 however, only during very severe work, because during moderate exer- 

 cise the metabolism of glycogen soon starts, and '^P compounds are pro- 

 duced as fast as needed (see below). When muscular work stops, gly- 

 cogen breakdown continues for a time, ATP is resynthesized, and the 

 Lohmann reaction goes into reverse until the normal amount of CrP 

 characteristic of resting muscle is restored. 



This arrangement gives the muscle a much greater supply of quickly 



