The Mechanism of the Hill Reaction and Its 

 Relationship to Photophosphorylation* 



BiRGIT VeNNESLAND 



Department of Biochemistry, University of Chicago, 

 Chicago, III., U.S.A. 



Introduction 



The purpose of this paper is an exploration of the interrelationship of 

 some of the chemical reactions brought about by the particulate structures 

 derived from the chloroplasts of higher plants. Such green, chlorophyll- 

 bearing particles have been called grana. This term is used here to mean 

 the insoluble fragments obtained when chloroplasts are disrupted in water. 



Washed grana have been shown to catalyze two rather different types 

 of reactions which both involve a conversion of light energy into chemical 

 energy. One of these is the well-known Hill reaction [i, 2]; that is, the 

 photoreduction of an added oxidant, accompanied by O2 evolution. The 

 other is the equally well-known reaction of photophosphorylation, dis- 

 covered bv Arnon and his associates [3], and by Frenkel [4], working in 

 Lipmann's laboratory. In this latter reaction, orthophosphate and ADP are 

 converted to ATP at the expense of light energv. The question I would like 

 to explore is the nature of the relationship between these two rather 

 different phenomena. I would also like to bring up the problem of the 

 relationship between the reactions catalyzed by washed grana and the 

 reactions of photosynthesis in the intact leaf. Let me say at once that I 

 intend to deal only with selected aspects of these problems, and most 

 particularly with some aspects which appear to have been relatively 

 neglected. Let me say also, in advance, that I do not think I have reached 

 any decisive conclusion about the nature of the relationship between 

 photophosphorylation and the Hill reaction. The following account is a 

 progress report to trace the development of our thinking. 



If one wishes to avoid controversy, the safest w^ay of representing the 

 Hill reaction is in a form which is noncommittal about mechanism. Thus, 

 equations i and 2 show the reaction with quinone and with ferricyanide 



* Supported by a grant from the National Science Foundation. 



