Whenever you see green color out in nature, you are likely to look at chlorophyll. This is the pigment used by all plants to do photosynthesis. There are two versions, chlorophyll a and chlorophyll b. These are structurally very similar to one another but have different colors, blue-green and yellowish green, respectively. Both pigments fulfill different jobs during photosynthesis and therefore are bound very selectively by the proteins of the photosynthesis apparatus in plants. How these plant proteins recognize the two chlorophylls, despite their small structural differences, and thus are able to bind them selectively, has been largely unknown so far.
Researchers have partially solved this riddle. The team used the so-called Water-soluble Chlorophyll Protein of cauliflower and Virginia pepperweed as a model protein. This protein possesses only a single chlorophyll binding site per protein molecule and is able to bind both chlorophyll versions.
Upon variation of the amino acids near the chlorophyll binding site, the preference of the protein for one chlorophyll or the other changed. In one case, exchanging a single amino acid altered the relative binding strengths by a factor of 40. Researchers obtained crystal structures of water-soluble chlorophyll protein variant binding either Chl a or Chl b. The Chl binding sites in these structures were compared with those in the major light-harvesting complex (LHCII) of the photosynthetic apparatus in plants to search for similar structural features involved in Chl a/b binding specificity.
"This does not explain everything about Chl a/b binding specificity in the photosynthetic apparatus," said the senior author, "but our results yield useful hypotheses that now can be tested with photosynthesis proteins. In the longer run, this may help to improve light harvesting in new photovoltaic devices or in artificial photosynthesis."
How plants bind their green pigment chlorophyll
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