CELLULAR METABOLISM II5 



shows that the velocity constant (^i), is a function of the work 

 ( — AF*) necessary to get the reactants into the transition state. If the 

 concept of the transition state is combined with quantum-mechanical 

 concepts of stabilization by resonance and the dependence of resonance 

 on the number of possible unperturbed forms, we arrive at a rather 

 satisfactory method of dealing with rate problems. 



The activated substrate is oxidized by one or more of oxidation- 

 reduction systems, some sluggish, some electromotively active, which 

 transfer electrons to molecular oxygen. If more than one system is 

 interposed between substrate and oxygen, the electron transfer occurs 

 step by step from the system of more negative potential up to molecular 

 oxygen. 



A study of the oxidation-reduction systems which take part in the 

 transfer of electrons from substrate to oxygen, shows that these systems 

 where the oxidation involves a total two-electron transfer (pyridine 

 nucleotides, flavins) do so in steps of one electron through radical forma- 

 tion as an intermediate step. The climbing over a large energy hill 

 involved in a two-electron oxidation is made easy by the interposition of 

 the intermediate step. Oxidation of foodstuflfs is thus performed by 

 one-electron steps. 



Little is still known about the oxidation-reduction potentials of pyri- 

 dine nucleotides and flavins when in combination with the proteins. 

 Both combine reversibly with proteins, and the dissociation constants 

 of the oxidized and reduced states are sometimes different. However, 

 a model can be provided by a study of the oxidation-reduction potentials 

 of Fe-protoporphyrin and of Fe-protoporphyrin combined reversibly 

 to nitrogenous bases to form the hemochromogens. On addition of co- 

 ordinating bases to the Fe-protoporphyrin there is formation of a series 

 of oxidation-reduction systems which at pH 9.20 go from —0.292 volt 

 in Fe-protoporphyrin to +0.050 volt in nicotine hemochromogen (Fig. 

 3) (5). A similar change of potentials has been observed when flavin 

 is combined to a protein as in Warburg's old yellow ferment, where the 

 potential jumped from —0.208 volt at />H 7.0 for flavin to —0.061 volt 

 for the latter (47). W. M. Clark and his coworkers (22) have given a 

 quantitative treatment of the equilibrium reactions between a nitroge- 

 nous base and Fe-porphyrin and have shown the relation between the 

 change of potential and the dissociation constants of the metallo-porphy- 

 rin-nitrogenous base complex compounds. If association of the Fe"^"^"*" 

 and Fe"*"^ state with the nitrogenous base is equal in strength, there will 

 be no change in potential. If the potential becomes more positive it 



