Chlorophyll Biosynthesis in Higher Plants

Summary

Chlorophyll (Chl) is essential for light harvesting and energy transduction in photosynthesis. The Chl biosynthesis pathway in higher plants is complex and is mediated by more than 17 enzymes. The formation of Chl can be subdivided into four parts: (1) synthesis of 5-aminolevulinic acid (ALA), the precursor of Chl and heme; (2) formation of a pyrrole ring porphobilinogen from the condensation reaction of two molecules of ALA and assembly of four pyrroles leading to the synthesis of the first closed tetrapyrrole having inversion of ring D, i.e., uroporphyrinogen III; (3) synthesis of protoporphyrin IX via several decarboxylation and oxygenation reactions, and (4) insertion of Mg to the protoporphyrin IX (PPIX) moiety steering it to the Mg-branch of tetrapyrrole synthesis leading to the formation of Chl. In higher plants, tetrapyrrole synthesis occurs in plastids, where it is initiated by the reduction of the glutamyl moiety of glutamyl-tRNA to glutamate-1-semialdehyde. The first branch point in the pathway is the methylation of uroporphyrinogen III that directs it toward the synthesis of siroheme, an essential component of nitrite reductase and sulfite reductase, whereas decarboxylation steers it towards PPIX synthesis. A second branch point of the tetrapyrrole biosynthesis pathway is Fe insertion to PPIX leading to the synthesis of hemes. Mg-insertion to the PPIX moiety leads to the synthesis of Mg-protoporphyrins and chlorins. During the day when the ATP levels are high, the magnesium branch of the pathway is favoured, as Mg-chelatase needs ATP for the Mg-PPIX synthesis. In this chapter, we discuss the mechanism of Chl biosynthesis; heterogeneity of the monovinyl and divinyl protochlorophyllide pool; regulation of Chl biosynthesis; the intraplastidic Chl biosynthesis route, and the evolution of Chl biosynthesis.