Abstract
Chlorophylls (Chls, Chl a and Chl b) are tetrapyrrole molecules essential for photosynthetic light harvesting and energy transduction in plants. Once formed, Chls are noncovalently bound to photosynthetic proteins on the thylakoid membrane. In contrast, they are dismantled from photosystems in response to environmental changes or developmental processes; thus, they undergo interconversion, turnover, and degradation. In the last twenty years, fruitful research progress has been achieved on these Chl metabolic processes. The discovery of new metabolic pathways has been accompanied by the identification of enzymes associated with biochemical steps. This article reviews recent progress in the analysis of the Chl cycle, turnover and degradation pathways and the involved enzymes. In addition, open questions regarding these pathways that require further investigation are also suggested.
Highlights
Chlorophyll (Chl), the most abundant pigment existing in the photosynthetic system of land plants and algae, is an indispensable pigment for absorbing light energy and transferring electrons during photosynthesis
It is widely accepted that the identification of the structure of nonfluorescent Chl catabolites (NCCs) is a hallmark study to break through the “biological enigma” existing in the Chl degradation pathway [10]
Chl a exists in both core complexes and light harvesting complexes (LHC), while Chl b exclusively exists in LHC [16]
Summary
Chlorophyll (Chl), the most abundant pigment existing in the photosynthetic system of land plants and algae, is an indispensable pigment for absorbing light energy and transferring electrons during photosynthesis. A Chl turnover pathway is required for PSII repair and protection of young leaves from photodamage [7]. Uncovering the Chl cycle, turnover and degradation pathway is a more recent research topic, and questions remain. It is widely accepted that the identification of the structure of nonfluorescent Chl catabolites (NCCs) is a hallmark study to break through the “biological enigma” existing in the Chl degradation pathway [10]. Recent studies have shown that dioxobilin-type nonfluorescent Chl catabolites (DNCCs), rather than NCCs, are the major degradation products of Chl in some senescent leaves [11]. Three revolutionary discoveries made great contributions to uncovering Chl turnover and degradation pathways. We review the new evolution of the Chl cycle, turnover and degradation pathways, including the biochemical steps, enzymes involved, regulatory mechanisms and remaining questions
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