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The role of chlorophyll b in photosynthesis: Hypothesis

Laura L Eggink, Hyoungshin Park and J Kenneth Hoober*

BMC Plant Biology 2001, 1:2  doi:10.1186/1471-2229-1-2

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Addendum to clarify specificity of coordination

J Kenneth Hoober   (2003-01-15 16:44)  Departmen of Plant Biology, Arizona State Unveristy email

Addendum to Eggink, L.L., Park, H. and Hoober, J.K. 2001. The role of chlorophyll b in photosynthesis: Hypothesis. BMC Plant Biology 1:2.

This paper proposed that specificity of binding of chlorophyll (Chl) a and b in the light-harvesting complexes (LHCs) of thylakoid membranes in chloroplasts is a consequence of the stronger Lewis acid property of the Mg in Chl b. Also, the stronger coordination bonds formed between Chl b and apoproteins would facilitate assembly and enhance stability of LHCs. Oxidation of the 7-methyl group of Chl a to the electronegative formyl group causes redistribution of the tetrapyrrole macrocycle π electrons toward the periphery of the molecule. As a result, the central Mg in Chl b has a more positive point charge than that in Chl a and would be expected to coordinate more strongly with ‘hard’, electronegative Lewis bases. Chl a readily forms coordination bonds with the imidazole group of His or a charge-compensated carboxyl group of Glu in ion-pair with the guanidinium group of Arg (Eggink, L.L., and Hoober, J.K. 2000. Chlorophyll binding to peptide maquettes containing a retention motif. J. Biol. Chem. 275: 9087-9090). Surprisingly, the level of binding of Chl b to peptides containing these ligands was much less than that of Chl a. By consideration of electronegativity equalization, Noy et al. demonstrated migration of negative charge from imidazole to the metal during coordination with [Ni]-bacteriochlorophyll a (Noy, D., Yerushalmi, R., Brumfeld, V., Ashur, I., Scheer, H., Baldridge, K.K. and Scherz, A. 2000. Optical absorption and computational studies of [Ni]-bacteriochlorophyll-a. New insight into charge distribution between metal and ligands. J. Am. Chem. Soc. 122: 3937-3944). These data suggest that interaction of the more positive Mg center of Chl b with His, and possibly the Glu-Arg pair, is weakened by partial positive charge on these ligands resulting from electron density migration to the metal. Binding of Chl b may also be inhibited by strong interaction with water (Ballschmiter, K., Cotton, T.M., Strain, H.H., and Katz, J.J. 1969. Chlorophyll-water interactions: Hydration, dehydration and hydrates of chlorophyll. Biochim. Biophys. Acta 180: 347-359). We would then expect that Chl b would favor coordination with ligands containing electronegative oxygen, with carboxyl and amide groups most favored to displace a water molecule. This proposal is in general supported by the ligands selected by each of the Chls during assembly of the light-harvesting complex. The stronger Lewis acid property of Chl b possibly facilitates retention of apoproteins of light-harvesting complexes during import into the chloroplast.

Also, in this paper Chl(ide) a oxygenase was incorrectly referred to as ‘Chl(ide) a oxidase’.

Competing interests

None declared

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