FORMATION OF CARBOXYL GROUPS IN RESPIRATION 225 



free carboxyl to a free carbonyl. A corresponding difference probably 

 exists also in the free energies of hydrogenation, with the consequence 

 that the oxidation-reduction potentials of carboxyl phosphates must be 

 more positive (probably by about 0.2 volt) than those of the correspond- 

 ing free acids; this brings them into the region of +0.3 to +0.4 volt, and 

 makes the reversal of their reactions with pyridine nucleotides or similar 

 catalysts feasible. 



It was shown by Meyerhof, Warburg, and coworkers that, in conse- 

 quence of the phosphorylation of glyceraldehyde, practically all the free 

 energy of its oxidation by pyridine nucleotide is retained in the oxidation 

 product (enol-phosphopyruvate) ; and Lipmann has demonstrated a 

 similar effect of phosphorylation on the transformation of pyruvic acid 

 into carbon dioxide and acetic acid. Thus, both reactions by which 

 carboxyls are created in schemes 9.1 and 9. II can occur without energy 

 dissipation, the oxidation energy being "stored" in the phosphorylated 

 oxidation products. 



The main purpose of this storage is to make possible the utilization 

 of the oxidation energy by the contractile system. This is achieved by 

 a transfer of phosphate from the oxidation product (e. g., enol-phospho- 

 pyruvate) to adenosine diphosphate ; the adenosine triphosphate formed 

 by this transphosphorylation is decomposed back into adenosine di- 

 phosphate and free orthophosphate by interaction with myosin (the 

 protein of the muscle) ; in this process the stored energy is released and 

 converted partly into heat and partly into mechanical work. Since both 

 the oxidation and the transphosphorylation are reversible, the net rate of 

 these processes is regulated by the rate of the terminal, exergonic, 

 myosin-catalyzed dephosphorylation; in this way, the rate of respiration 

 is accelerated or slowed down depending on the amount of work performed 

 by the muscle. 



Only 30-35 kcal out of the 330 kcal combustion energy available in a 

 triose are stored in the three high-energy phosphate molecules created in 

 the two oxidation steps considered above. The other 90% are liberated 

 in subsequent reaction steps, that is, according to scheme 9. II, in the 

 dehydrogenation of succinic, fumaric, and malic acid by their specific 

 dehydrogenases, and in the transfer of 12 hydrogen atoms from the 

 dehydrogenases to oxygen, through the intermediary of alloxazine deriva- 

 tives (yellow enzymes), hemin derivatives (cytochromes), and other 

 reversible oxidation-reduction catalysts. Some of these processes may 

 also be coupled with phosphorylations or transphosphorylations, and 

 their energy made available in this way for muscular work. Indications 

 of this coupling have been found, for instance, in the study of the oxida- 

 tion of succinate to fumarate (which is a step in respiration). According 

 to table 9. IV, the potential of the succinate-fumarate system is ± 0.0 



