Chlorophyll b—An Essence of Plant Photosynthesis

Chlorophyll (Chl) b is crucial for assembly of the light-harvesting antennae that are required for optimal photosynthetic activity in plants and green algae. Synthesis of its precursor, chlorophyllide (Chlide) b, is catalyzed by Chlide a oxygenase (CAO), which contains a stable tyrosyl radical. Studies with the model organism Chlamydomonas reinhardtii y-1 suggested that protochlorophyllide (Pchlide) a is a substrate for the enzyme in the dark when a ‘cofactor’ is present to form a heterodimer, which apparently decreases the redox potential of Pchlide a. Data described in the literature are consistent with reduction in the redox potential of Chlide a by dimerization, which produces a substrate that allows rapid synthesis and accumulation of Chl b during chloroplast development in oxygenic photosynthetic organisms. In this article, we provide an emerging perspective on CAO’s structure, its assumed radical-mediated catalytic mechanism, and its role in planta.