METABOLIC PROCESS PATTERNS 



Unfortunately, all schemes following the well-established rule of assigning 

 to oxidation-reduction potentials values increasingly positive toward oxygen depict 

 respiratory processes ambiguously. In respiration, the chemical potential gradually 

 diminishes, of course, toward oxygen, being eventually spent with oxygen reduction, 

 when the listed potential attains the most positive value. 



As a characteristic of the process pattern, a separabihty of two 

 main flow lines appears. The line representing catabolism of the 

 substrate glides along on an approximately equipotential path; and 

 at right angles to it the hydrogens which have been released by de- 

 hydrogenation journey to oxygen. So far, the greatest progress has 

 been made in the field of substrate catabolism in which workable 

 schemes have emerged. Schemes like that of the citric acid cycle, 

 however, do not supply information about the chemical pathways of 

 respiratory transformations of energy beyond the stage of hydrogen 

 donation. The pathway of the substrate supplies merely the level 

 from which electrons are emitted, loaded with potential energy and 

 ready to be used. We learn, thus, from our map that the potential 

 difference between oxygen and each of the six dehydrogenation steps 

 which sum up to complete oxidation of a triose averages very closely 

 to the value of the oxyhydrogen potential. In other words, carbo- 

 hydrate reacts grossly like a mixture of carbon dioxide and molecular 

 hydrogen: 



(CHOH-HsO), = (C02-2H2), (1) 



At first approximation, it seems justified therefore to consider 

 the respiratory process as a repeating series of rather uniform process 



" For isocitric and ketoglutaric acids, tentatively, the potentials of hydroxybutyric (4) and 

 pyruvic acid (11), respectively, were used here. For other values of the oxidation-reduction potentials, 

 cj . Green (4). 



ft Reference potential: hydrogen electrode at /)H 7; c} . page 141. 



c The terms, water system and. phosphate system, refer to the hydrated and the corresponding 

 phosphorylated double bonds, as, for example, in phosphoglyceraldehyde hydrate and phosphoglycer- 

 aldehyde phosphate (9). The difference of the oxidation-reduction potentials between the water series 

 and the phosphate series is approximately constant and equal to the volt equivalent of the energy-rich 

 phosphate. For acetyl phosphate, recently, a bond energy of approximately 15 kcal. was calculated 

 (10, 121 which corresponds to roughly 0.3 v. This value is appropriate for calculations primarily con- 

 cerned with bond generation. Th# average energy is somewhat lower, 12 kcal. or 0.25 v., which value 

 is preferred for turnover calculations. In cases in which more or less arbitrarily the actually reacting 

 dehydrogenation system is assumed to be of the phosphate type, the connecting line is drawn through 

 the phosphate system. In these cases, a vertical line in the graph connects the potential points of water 

 and phosphate system, indicating the energy transformation. Tht wide empty area above the line con- 

 necting the potentials indicates the large part of the energy which remains here unaccounted /or {cJ. Fig. 2). 



<* Calculated from the combvistion heat of one-half mule of glucose, 343 kcal. The agreement 

 between the rather roughly approximated voltage and the combustion heat is noteworthy. To be 

 accurate, 0.25 v., the equivalent of the nonoxidative phosphate bond in phosphopyruvate, should be 

 added to the sum of the oxidation-reduction potentials. 



