♦ See referenced article, J. Biol. Chem. 2013, 288, 19330–19341
Most oxygenic photosynthetic organisms contain chlorophyll a as a light-harvesting system. Land plants, green algae, and some cyanobacteria rely on chlorophyll b, which is synthesized by an enzyme called chlorophyllide a oxygenase (CAO). However, the evolutionary relationship between the light-harvesting system and the structural properties of CAO is not clear. In this Paper of the Week, Atsushi Takabayashi at Hokkaido University in Japan and colleagues analyzed the CAO structure in the alga Micromonas. This organism's CAO is derived from two genes, MpCAO1 and MpCAO2, which encode an iron-sulfur domain called the Rieske binding motif and a mononuclear iron-binding motif. By carrying out experiments in Arabidopsis, the investigators demonstrated that both genes were necessary for the production of chlorophyll b. They suggest that evolutionary changes to the structure of CAO led to chlorophyll b being the predominant photosynthetic pigment in Micromonas. They say, “This hypothesis is supported by experiments showing that when the Micromonas CAO was introduced into Arabidopsis, the chlorophyll a/b ratio became low, and chlorophyll b was incorporated into the core antennas of reaction centers, which is similar to light-harvesting systems of Micromonas.”
Prochlorophyte CAOs have only the catalytic domain and form a trimer. CAOs in green algae and land plants have A, B, and C domains and form a trimer. Mamiellales Micromonas CAOs are composed of two proteins with either a Rieske or mononuclear iron-binding motif. Neither protein has an A or a B domain. They interact with each other to form a dimer.