A chlorosome is a large and efficient light-harvesting antenna system found in some photosynthetic bacteria. This system comprises self-aggregates of bacteriochlorophyll (BChl) c, d, or e possessing a chiral 1-hydroxyethyl group at the 3-position, which plays a key role in the formation of the supramolecule. Biosynthesis of chlorosomal pigments involves stereoselective conversion of 3-vinyl group to 3-(1-hydroxyethyl) group facilitated by a 3-vinyl hydratase. This 3-vinyl hydration also occurs in BChl a biosynthesis, followed by oxidation that introduces an acetyl group at the 3-position. Herein, we present in vitro enzymatic assays of paralogous 3-vinyl hydratases derived from green sulfur bacteria, Chlorobaculum tepidum and Chlorobaculum limnaeum, the filamentous anoxygenic phototroph Chloroflexus aurantiacus, and the chloracidobacterium Chloracidobacterium thermophilum. All the hydratases showed hydration activities. The biosynthetic pathway of BChl a and other chlorosomal pigments is discussed considering the substrate specificity and stereoselectivity of the present hydratases.
Keywords: Bacteriochlorophyll; Chloracidobacterium; Chlorosome; Filamentous anoxygenic phototroph; Green sulfur bacterium; Hydratase.