288 MARTIN D. KAMEN 



quite reasonable to suggest participation of an SH-compound or 

 grouping. 



At any rate, back reduction of haem by reduced chlorophyll would be 

 slowed because both reactants would be expected to have reached their 

 ground states after the primary deactivation by electron transfer, e.g. 



Chi,,, + Haem-^,„d ^^— >ChlH + Haem°-,„, 



Hence, some activation energy would be required to initiate the back 

 reaction, despite the great energy difference of some 2 e.V. tending to 

 drive it. The presence of a specific enzyme which would give the reduced 

 chlorophyll the alternative of a reduction process requiring little or no 

 excitation energy compared with the uncatalyzed back reduction of the 

 oxidized haem could represent one of the stereochemical requirements for 

 stabilizing the reduced product in the presence of the oxidizing system 

 created by the electron transfer. 



With the electron now located in some molecule at the reducing end 

 of a " respiratory" chain, electron migration through the flavins, quinones, 

 and various haem enzymes to the terminal oxidant, created by the initial 

 photochemical electron transfer, would complete the cycle. As we will hear 

 in the other papers, this type of electron transport coupled to the quantum 

 excitation process is generally assumed to be the basic mechanism for 

 photophosphorylation. An impressive, if not conclusive, accumulation of 

 data is at hand to support this notion. Some of these data undoubtedly will 

 be presented at this session. 



An alternative scheme presented by Hill and Bendall [52] suggests that 

 the phosphorylation step is coupled to a flow of electrons against the 

 potential gradient between "tie points" on the respiratory chain repre- 

 sented by the haem proteins, in this case, cytochrome 6g and cytochrome/. 

 The cytochromes are assumed to be involved in back reactions which 

 restore the system to its original state before photo-excitation. Hill and 

 Bendall consider this type of mechanism necessary because of the fact 

 demonstrated by Arnon et al. [53] that photophosphorylation increases, 

 rather than decreases, the yield of molecular oxygen in the chloroplast 

 reaction. However, there are alternative explanations for this phenomenom, 

 which do not require the concept of "reductive" phosphorylation. Thus, 

 if phosphorylation occurs, as we have discussed, below the nucleotide 

 level, rather than between nucleotide and haem, then the consumption of 

 oxygen precursor assumed by Hill and Bendall as obligatory for photo- 

 phosphorylation, will not occur. Rather the assumption required is that 

 coupling of phosphate esterification to reduction of pyridine nucleotide 

 increases the amount of pyridine nucleotide reduced and hence of oxygen 

 precursor formed. 



There are a few points I think need brief discussion relating to the 

 generalization of haem proteins as H-donors in the fundamental photo- 



