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Laura L Eggink, Hyoungshin Park and J Kenneth Hoober*
Corresponding author: J Kenneth Hoober firstname.lastname@example.org
BMC Plant Biology 2001, 1:2 doi:10.1186/1471-2229-1-2
J Kenneth Hoober
(2003-01-15 16:44) Departmen of Plant Biology, Arizona State Unveristy
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)
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