Determination of relative chlorophyll binding affinities in the major light-harvesting chlorophyll a/b complex.
The major light-harvesting complex (LHCIIb) of photosystem II can be reconstituted in vitro from its recombinant apoprotein in the presence of a mixture of carotenoids and chlorophylls a and b. By varying the chlorophyll a/b ratio in the reconstitution mixture, the relative amounts of chlorophyll a and chlorophyll b bound to LHCIIb can be changed. We have analyzed the chlorophyll stoichiometry in recombinant wild type and mutant LHCIIb reconstituted at different chlorophyll a/b ratios in order to assess relative affinities of the chlorophyll-binding sites. This approach reveals five sites that exclusively bind chlorophyll b. Another site exhibits a slight preference of chlorophyll b over chlorophyll a. The remaining six sites are filled preferentially with chlorophyll a but also tolerate chlorophyll b when this is offered at a large excess. Three of these chlorophyll a-affine sites could be assigned to distinct positions defined by the three-dimensional LHCIIb structure. Exclusive chlorophyll b sites complemented by chlorophyll a sites that are selective only to a certain extent are consistent with the observation that chlorophyll b but not chlorophyll a is essential for reconstituting stable LHCIIb. These data offer an explanation why a rather constant chlorophyll a/b ratio is observed in native LHCIIb despite the apparent promiscuity of some binding sites.
Hobe
Stephan
S
Institut für Allgemeine Botanik, Johannes-Gutenberg-Universität Mainz, Müllerweg 6, D-55099 Mainz, Germany.
Fey
Holger
H
Rogl
Hans
H
Paulsen
Harald
H
D019943:Amino Acid Substitution; D001665:Binding Sites; D002734:Chlorophyll / Q000378:metabolism*; D007700:Kinetics; D045342:Light-Harvesting Protein Complexes; D016297:Mutagenesis, Site-Directed; D018532:Pisum sativum / Q000378:metabolism; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000378:metabolism*; D045332:Photosystem II Protein Complex; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism
D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D011994:Recombinant Proteins; D002734:Chlorophyll