39 



Bessel Kok 



yielded rates higher than ~800 eq/chl. hour. A peculiarity of 

 these catalysts is their effectiveness in weak as well as in 

 strong light — they increase the quantum yield of the reaction. 



Plastocyanin and cyt. f, if added in substrate amounts in- 

 stead of cyt. c are photo-oxidized themselves with high rates. 

 An interesting feature of these reactions is that the rate is 

 dependent upon the redox state of the substrate. For instance, 

 if cyt. f is added in half-oxidized, half-reduced form, the 

 rate is only half maximal. (In the presence of excess cyt. c, 

 cyt. f is kept in the reduced form and the overall reaction 

 proceeds with optimal rate until the depletion of cyt. c.) 



20 40 60 80 



7o REDUCED CYTOCHROME f 



100 



Figure 3 

 Figure 3 illustrates that in weak as well as in strong 

 light, the rate of cyt. f photo-oxidation is proportional to 

 the ratio cyt, f red. /cyt. f total. To explain this, we 

 assume that external cyt. f equilibrates with a cyt. f mole- 

 cule fixed in the chloroplast matrix in close proximity to 

 P700. A photoact can only be successful if not only P700 but 

 also its associated cyt. f is in the reduced state before the 

 quantum hits. The cytochrome transfers an electron to P700''" 

 immediately after the latter has lost its electron to X, and 

 thus prevents a back reaction between the photoproducts. 

 Such a charge transfer complex between cyt. f and P700 em- 

 bedded in the chloroplast matrix was already shown by Witt 

 et al. (7). In scheme Fig. k we indicated that plasto- 

 cyanin could react in two ways with P700 , directly or via 



