The role of chlorophyll b in photosynthesis: hypothesis.
BACKGROUND
The physico-chemical properties of chlorophylls b and c have been known for decades. Yet the mechanisms by which these secondary chlorophylls support assembly and accumulation of light-harvesting complexes in vivo have not been resolved.


METHODS
Biosynthetic modifications that introduce electronegative groups on the periphery of the chlorophyll molecule withdraw electrons from the pyrrole nitrogens and thus reduce their basicity. Consequently, the tendency of the central Mg to form coordination bonds with electron pairs in exogenous ligands, a reflection of its Lewis acid properties, is increased. Our hypothesis states that the stronger coordination bonds between the Mg atom in chlorophyll b and chlorophyll c and amino acid sidechain ligands in chlorophyll a/b- and a/c-binding apoproteins, respectively, enhance their import into the chloroplast and assembly of light-harvesting complexes.


METHODS
Several apoproteins of light-harvesting complexes, in particular, the major protein Lhcb1, are not detectable in leaves of chlorophyll b-less plants. A direct test of the hypothesis--with positive selection--is expression, in mutant plants that synthesize only chlorophyll a, of forms of Lhcb1 in which weak ligands are replaced with stronger Lewis bases.


CONCLUSIONS
The mechanistic explanation for the effects of deficiencies in chlorophyll b or c points to the need for further research on manipulation of coordination bonds between these chlorophylls and chlorophyll-binding proteins. Understanding these interactions will possibly lead to engineering plants to expand their light-harvesting antenna and ultimately their productivity.
Eggink
L L
LL
Department of Plant Biology Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, AZ, USA. leggink@asu.edu
Park
H
H
Hoober
J K
JK
D000595:Amino Acid Sequence; D002734:Chlorophyll / Q000378:metabolism* / Q000502:physiology; D045342:Light-Harvesting Protein Complexes / Q000378:metabolism; D008954:Models, Biological; D008969:Molecular Sequence Data; D010788:Photosynthesis / Q000502:physiology*; D045332:Photosystem II Protein Complex / Q000378:metabolism; D011485:Protein Binding; D020524:Thylakoids / Q000378:metabolism
D045342:Light-Harvesting Protein Complexes; D045332:Photosystem II Protein Complex; D002734:Chlorophyll