Journal
PHOTOSYNTHESIS RESEARCH
Volume 74, Issue 1, Pages 1-10Publisher
KLUWER ACADEMIC PUBL
DOI: 10.1023/A:1020816128794
Keywords
bacterichlorophyll a; chirality; chlorophyll a; chlorophyll a '; coordination; P700
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Reported crystallographic data and calculated molecular models indicated that chlorophyll (Chl) a and bacteriochlorophyll (BChl) a tend to bind the fifth ligand on the side of the macrocycle where the C13(2) -(R) methoxycarbonyl moiety protrudes (denoting the 'back' side). The crystal structures of 34 photosynthetic proteins possessing (B) Chl cofactors revealed that most of Chl a and BChl a (and b) are coordinated by any peptidyl residue (e.g., histydyl-imidazolyl group), peptidyl backbone or water from the 'back' side. Almost all the cofactors that bind a water molecule as the fifth ligand in these proteins have a 'back' configuration. Theoretical model calculations for methyl chlorophyllide a (MeChlid a) and methyl bacteriochlorophyllide a (MeBChlid a) bound to an imidazole molecule indicated that the 'back' side is energetically favored for the ligand binding. These results are consistent with the fact that ethyl chlorophyllide a (EtChlid a) dihydrate crystal consists of the 'back' complex. The modeling also showed that both removal and stereochemical inverse of the C13(2)-methoxycarbonyl group affect the relative stability between the 'back' and 'face' complexes. The effect of the C13(2)-moiety on the choice of the macrocycle side for the ligand binding is discussed in relation to the function of P700.
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