18 Discussion 



Margoliash: Orgel's idea of the importance of the native configuration of the protein 

 of haemoproteins in determining the closeness of attachment of the haem-iron hgands 

 and hence the nature of the complex formed, fits well with the results of our study of 

 the denaturation of cytochrome c. With this haemoprotein it appears that denatur- 

 ation probably does not change the haem iron bound groups but rather has a quanti- 

 tative effect on the haem iron-ligand bonds resulting, as denaturation proceeds, in 

 the gradual disappearance of the specific properties of cytochrome c and its trans- 

 formation into a normal chemical haemochrome (Margoliash, Frohwirt & Wiener, 

 Biochem. J., 71, 559, 1959). 



Mechanism of Oxidative Phosphorylation 



George : Lemberg has raised the question of the kind of mechanism by which the oxida- 

 tion-reduction of cytochrome c can be coupled with phosphorylation, since, for 

 structural reasons, it is difficult to see how an electron mediator can be involved like 

 terephthalic acid or its ester in the oxidation of Cr" by Co"^ 



Arising from our studies of the opening of the crevice in ferricytoclirome c, which 

 happens when the azide and cyanide complexes are formed, Glauser and I have 

 suggested that a conformational change in the protein may be involved as a conse- 

 quence of a switchover to a different bonding group during the oxidation-reduction 

 cycle. 



For example, if the most stable crevice structures for the ferric and ferrous forms, 

 at the pH at which oxidation-reduction occurs, differ in having a primary amino 

 group and a histidine group respectively coordinated to the iron as in A and C. 



I I I red I II 



(A) Prot— Fe"'— NH2 imid > Prot— Fe"— NHg imid (B) 



I 1 I oxid I I I 



(D) Prot— Fe"i— imid NHj < " ' Prot— Fe"— imid NHg (C) 



then upon reduction of ferricytochrome c (A) a metastable, "energy-rich" form of 

 ferrocytochrome c (B) would be produced, reverting to the stable form (C) with a 

 release of energy. Likewise on oxidation of ferrocytochrome c (C) an "energy-rich" 

 form of ferricytochrome c (D) would be produced, reverting to the stable form (A) 

 with a release of energy. The switchover of the crevice group in the reactions 

 (B) -* (C) and (D) -> (A) would entail a conformational change in protein structure 

 which could conceivably be linked in some way to a phosphorylation step (George, 

 P., & Glauser, S. C. Abstracts Third Meeting Biophysical Soc, Pittsburgh, April 

 1959, D4). 



Chance : I should like to ask Orgel for more information on equation (iii) of his paper. 

 At first I thought that you wished to distinguish between electron transfer reactions 

 and the coupling to phosphorylation. However toward the end of your paper you 

 show them to be intimately associated, unless I have misunderstood you. Further, is 

 the iron atom to which ADP is linked a haematin or a non-haematin iron? Would 

 you be willing to indicate to me arguments in favour of one or the other alternative? 



Orgel : I should like to make clear that in this paper I have tried to describe a very general 

 scheme for preserving the energy of oxidation-reduction reactions. I had no particular 

 chemical system in mind. If the Fe+++ is part of a haem compound then the ADP or 

 other acceptor could not be attached to the metal atom but would have to be held in 

 position by attachment to the protein; if the Fe+++ atom is not in a porphyrin ring, 

 then the ADP could be attached to the metal directly. I have no view on the relative 

 likelihood, if any, of the possible alternatives. 



The main idea is that if an electron is extracted from a metal ion which is weakly 

 associated with a ligand then the metal in its new valency may decompose the ligand 

 in such a way as to preserve the redox energy. One illustration is given. 



