16 Discussion 



would be expected for an endothermic ionization process. Hence, if the measurements 

 were carried out at low temperatures (liquid air or liquid hydrogen) as is normally 

 the case, without suitable control experiments it is not certain how much hydroxide 

 was present. Thirdly, unless ferrihaemoglobin is carefully freed from ammonium salts 

 there is a marked tendency for the ammonia complex to be formed in alkaline solution : 

 so, if the experiments were made with either single crystals or a microcrystalline paste 

 from an ammonium sulphate mother liquor, in the absence of suitable controls a 

 contribution to the observed signal from the ammonia complex cannot be disregarded. 

 In the case of measurements at low temperatures this would be more serious since 

 there is good reason to believe that the formation of the complex would be exothermic. 

 The experiments reported in our present paper are not subject to these uncertainties, 

 and they provide equally direct evidence for the existence of a thermal mixture. 



Electron Transport 



Lemberg: The interesting theory of oxidative phosphorylation involving a quinone- 

 hydroquinone system (Todd and others) appears less likely in the phosphorylation step 

 connected with the oxidation of cytochrome c, although Glahn and Nielsen {Nature, 

 Lond., 183, 1578 (1959)) have recently suggested that this step involves binding of the 

 phosphate to the formyl group of haem a. Orgel's explanation still leaves us with 

 the difficulty that we do not know conjugated systems which might take up phosphate 

 groups, except perhaps the histidine imidazoles bound to haem iron. The electron 

 transport through a respiratory chain of several cytochromes makes it appear sterically 

 unlikely that electron transport through the imidazoles as postulated by Theorell can 

 be a sufficient explanation; similar difficulties exist with regard to haem-haem inter- 

 actions in haemoglobin. I therefore ask whether the physicochemists consider it 

 impossible that electron transfer may occur through "aliphatic" portions of a protein, 

 or possibly through a chain of water molecules bound in the protein. 



Winfield: The example of electron transfer given by Orgel (terephthalic acid complex) 

 is one which can readily be demonstrated experimentally. But may there not be some 

 kinds of conduction which are important in biology and yet not readily demonstrable? 

 If one were able to remove an electron from one end of a paraffin chain simultaneously 

 with addition of an electron at the other end, would not the resulting electron move- 

 ment along the chain take place with negligible activation energy? It seems possible 

 that conducting chains of this kind could be interposed between conjugated con- 

 ducting groups of the type described by Orgel. In other words, conduction of electrons 

 between the prosthetic groups of adjacent enzymes {in vivo) may not require a path 

 which is conjugated throughout its length. 



In the passage of electrons through a series of cytochromes in the living cell, I 

 think that the individual enzymes are joined by metal bridges or hydrogen bonds. 

 If the metal atom were calcium, one might expect that the electrons would pass across 

 the bridge either not at all or with no pause. But with a metal atom such as iron or 

 copper acting as bridge, I think that the electron would reside for a finite time in the 

 metal ion and that there would be an activation energy required to move an electron 

 across such a bridge. The pause might well be of biological significance. A small 

 activation energy for the transfer of electrons between an interconnected series of 

 cytochromes would restrict "hunting" in a system which would otherwise be uncon- 

 trollably sensitive to transient fluctuations in the environment. In addition the metal 

 bridge could provide for by-passing part of the electron flow along paths which 

 branch from the main respiratory chain. 



Lockwood: In proposing models for electron transport two essentially diff"erent ones 

 have been given. There is one in which electrons can be put in at one end of the chain 

 and taken out at the other and that can be repeated an indefinite number of times. 

 The comparison to a piece of copper wire is convenient. I take it that the model 

 given in reaction (1) of Orgel's paper is an example of this type of conductor. In 

 other models that have been proposed an electron can be put in at one end of the 

 chain and taken out at the other but this produces an alteration of the configuration 

 and the process cannot be repeated till the electron transport has been reversed. An 



