Biosynthetic pathways of bacteriochlorophylls working in light-harvesting chlorosomes

Misato Teramuraa, Jiro Haradab, Tadashi Mizoguchia, Hitoshi Tamiakia
aGraduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga, Japan; bDepartment of Medical Biochemistry, Kurume University School of Medicine, Kurume, Fukuoka, Japan

Some groups of photosynthetic bacteria, green sulfur bacteria, filamentous anoxygenic phototrophs, and acidobacteria, have large light-harvesting antenna complexes, called chlorosomes. This antenna system can efficiently collect light energy and allows the bacteria to survive in the environments with very low light intensity. Chlorosomes contain a large amount of bacteriochlorophyll (BChl) c/d/e molecules (dependent on bacterial species) that form well-ordered supramolecule without interaction with any proteins. These BChls possess unique chemical structures (Figure 1, left) to construct chlorosomal self-aggregates; 1) the presence of the 31-hydroxy group which is introduced by hydration of the 3-vinyl group and plays an important role in forming the self-aggregation, and 2) the absence of the sterically bulky 132-methoxycarbonyl group found in all the other chlorophylls which would prevent the formation of the self-aggregates due to the steric hindrance.

In this study, we performed in vitro characterization of the two 3-vinyl hydratases, BchF and BchV[1,2], and the 132-demethoxycarbonylase BciC[3] which were derived from Chlorobaculum tepidum (Fig. 1, right). Based on the results of in vitro enzymatic assays, the biosynthetic pathway of chlorosomal BChl c in Chlorobaculum tepidum is discussed.

Figure: chemical structures of BChl c and modicfications of the substituents by BChF/V and BciC during the biosynthesis.