Chapter Six - The biochemistry, physiology, and evolution of the chlorophyll cycle.

The chlorophyll cycle is a metabolic pathway in plants and green algae that interconverts chlorophyll a and chlorophyll b. The chlorophyll a-to-b conversion is catalyzed by a Rieske-type oxygenase, chlorophyll(ide) a oxygenase, while the chlorophyll b-to-a conversion is catalyzed in two steps by chlorophyll b reductase (a short-chain dehydrogenase) and 7-hydroxymethyl chlorophyll a reductase (a flavoprotein). The level of chlorophyll(ide) a oxygenase is regulated in a feedback loop by the accumulation of chlorophyll b in plants. In contrast, chlorophyll b reductase levels are regulated by a pool of light-harvesting complexes (LHC) that are energetically uncoupled to the core complexes of photosystems. Experimental evidence suggests that LHC levels in plants are regulated by chlorophyll b biosynthesis. LHC levels, however, appear to be independent of chlorophyll b in green algae. Instead, Prasinophytes, a group of green algae, have a chlorophyllide a oxygenase protein possessing structural alterations that differ dramatically from the rest of the green algae, which appear to boost chlorophyll b biosynthesis in these organisms. Hypothetical scenarios for the evolution of the chlorophyll cycle are presented. [ABSTRACT FROM AUTHOR]