The 17‐Propionate Function of (Bacterio)chlorophylls: Biological Implication of Their Long Esterifying Chains in Photosynthetic Systems^†

Abstract

Molecular structures of (bacterio)chlorophylls [=(B)Chls] in photosynthetic apparatus are surveyed, and a diversity of the ester groups of the 17-propionate substituent is particularly focused on in this review. In oxygenic photosynthetic species including green plants and algae, the ester of Chl molecules is limited to a phytyl group. Geranylgeranyl and farnesyl groups in addition to phytyl are observed in (B)Chl molecules inside photosynthetic proteins of anoxygenic bacteria. In main lightharvesting antennas of green bacteria (chlorosomes), a greater variety of ester groups including long straight chains are used in the composite BChl molecules. This diversity is ascribable to the fact that chlorosomal BChls self-aggregate to form a core part of chlorosomes without any specific interaction of oligopeptides. Biological significance of the long chains is discussed in photosynthetic apparatus, especially in chlorosomes.

Chelation ability of spironaphthoxazine with metal ions in silica gel

“…Organic metal chelate complexes attract much interest as functional materials because they play an important role in energy collection and conversion in plants [1,2]. Such materials are used for artificial photosynthesis and photoelectric conversion [3−5].…”

Structural determination of dihydro‐ and tetrahydrogeranylgeranyl groups at the 17‐propionate of bacteriochlorophylls‐a

“…1(b)) in anoxygenic photosynthetic bacteria have a phytyl group as their ester group R in Fig. 1 [1,2]. Such (B)Chl molecules are biosynthesized in photosynthetic organisms and function in light-harvesting and reaction center complexes.…”

“…1) possessing a carboxy group at the 17 2 -position are esterified with various long hydrocarbon chains [4][5][6]. In oxygenic phototrophs, chlorophyllide-a is esterified with geranylgeranyl (GG) diphosphate by a Chl synthase (ChlG) to give geranylgeranylated Chl-a, and successive hydrogenation of three of its four C‚C double bonds by a GG reductase (ChlP) yields phytylated Chl-a via the intermediates possessing a dihydrogeranylgeranyl (DHGG) 1 and tetrahydrogeranylgeranyl (THGG) 1 ester (see Scheme 1). In the alternative pathway, GG diphosphate is first reduced and the resulting phytyl (Phy) diphosphate reacts with chlorophyllide-a by a Chl synthase to give phytylated Chl-a.…”

Risk Management by Organisms of the Phototoxicity of Chlorophylls

“…The resulting β-ketocarboxylic acid spontaneously loses CO 2 to produce pPhesa/b. Its 13 2 -carbanion attacks the 17 2 -carbonyl group in a molecule to yield cPPB-aE/bE. The intramolecular mixed Claisen condensation would be an enzymatic process.…”

“…2 However, in their free (unregulated) state, chlorophylls are phototoxic to living organisms. This dual character derives from their relatively long-lived photoexcited states, which ensure the transfer of excitation energy between the molecules in the light-dependent reactions of photosynthesis and potentially generate highly toxic reactive oxygen species, including singlet oxygen.…”