Chlorophyll biosynthesis is a chloroplast-localized branch of tetrapyrrole metabolism that converts glutamate into a magnesium-containing chlorin pigment that is then esterified with a phytyl tail and assembled into photosynthetic complexes. Glutamate is first activated as glutamyl‑tRNA and reduced to glutamate‑1‑semialdehyde, which is transaminated to δ‑aminolevulinic acid; subsequent condensation and modification reactions yield porphobilinogen, then uroporphyrinogen III, coproporphyrinogen III, and finally protoporphyrin IX, the shared branchpoint with heme. In Arabidopsis, commitment to chlorophyll occurs when the multi-subunit magnesium chelatase inserts Mg²⁺ into protoporphyrin IX to form Mg‑protoporphyrin IX, which is then methylated and subjected to oxidative cyclization to generate the fifth isocyclic ring, producing protochlorophyllide intermediates. Protochlorophyllide is reduced to chlorophyllide a by the light‑dependent protochlorophyllide oxidoreductase that is highly abundant in etiolated Arabidopsis cotyledons, making chlorophyll formation strongly light‑regulated in seedlings. In parallel, the plastidial MEP pathway supplies the C20 geranylgeranyl diphosphate that is reduced to phytyl diphosphate; chlorophyll synthase then esterifies chlorophyllide a with this tail to form chlorophyll a, which can be further modified to chlorophyll b in the chloroplast. In Arabidopsis, this pathway is tightly regulated at transcriptional and post-translational levels and coordinated with photosystem assembly, as several chlorophyll intermediates are phototoxic and must be produced in step with the availability of apoproteins and the cellular redox and light environment.

Chlorophyll biosynthesis References