Biochemistry of chlorophyll biosynthesis in photosynthetic prokaryotes

Abstract

Chlorophylls (Chls) are tetrapyrrole pigments that are essential for photosynthesis, which supports almost all organisms on the planet. Thus, elucidation of the molecular mechanisms of Chl biosynthesis is a major biological challenge. Nearly 100 different Chls with differing ring structures and substituents are represented by Chls a, b, c, d, and f and bacteriochlorophylls a, b, c, d, e, and g. Phototrophic prokaryotes perform photosynthesis using specific sets of Chls that capture available light under the conditions of their natural habitats. For example, cyanobacteria grow photosynthetically in the top layers of water columns using Chl a, whereas purple bacteria perform anoxygenic photosynthesis using bacteriochlorophyll a in the deeper layers of the water columns. Extensive gene searches have been performed in photosynthetic prokaryotes since the 1990s, and the largely complete scheme of Chl biosynthetic pathways includes a core pathway that is conserved among all photosynthetic organisms and comprises diverse reactions for the production of a variety of Chls. With this framework of biosynthetic pathways, further studies of Chl biosynthesis are directed at understanding the physiological and biochemical aspects. The physiological aspects include elucidation of regulatory networks that are integrated with other cellular processes, and the biochemical aspects include elucidation of three-dimensional structures of Chl biosynthetic enzymes to understand molecular mechanisms. In this chapter, we describe the current investigations of molecular mechanisms of enzymes in the Mg branch focusing on the latter aspects.