Binding affinity of Chl b for the Chl a-binding sites in PSI core complexes

Abstract

Most Chl a in PSI complexes was removed without any loss of P700 by ether treatment, yielding antenna-depleted P700-Chl a protein complexes (CP1s) with a Chl a/P700 ratio of 12. On addition of about 60 molecules of Chl b per P700 with phosphatidylglycerol, about 20 molecules of Chl b per P700 were bound to the complexes. The ratio of the bound Chl b to the added Chl b was about one-third, irrespective of the amount of Chl b added. The same partition ratio was obtained on reconstitution with Chl a, suggesting that the binding affinity of Chl b for the Chl a-binding sites is similar to that of Chl a. The relative quantum efficiency of P700 photooxidation, determined by the increase in its initial rate, increased in proportion to the increase in number of bound Chl b moleculles. The degree of the increase was the same as expected if the bound Chl b had the same antenna activity as the bound Chl a. The bound Chl b emitted fluorescence with a peak at 660 nm, and its yield was as high as the Chl a remaining in the complexes. However, the excitation spectrum of the Chl a fluorescence, detected at 680 nm, was almost the same as the absorption spectrum of the Chl b-bound complexes, indicating efficient energy transfer of the bound Chl b to Chl a. These results suggest that Chl b primarily occupies the Chl a-binding sites close to the reaction center region, acting as an efficient antenna for P700. © The Author 2007. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved.