BOTANY, VOLUME 2 - PLANT PHYSIOLOGY - 2007

6. METABOLIC PHYSIOLOGY

6.15. Synthesis of Tetrapyrroles

Both cyclic tetrapyrroles (Porphyrins) and non-cyclic tetrapyrroles fulfill various Functions in plants. Chlorophylls and bacteriochlorophylls absorb light energy during Photosynthesis (see 6.4.2); heme is an essential component of Cytochromes, catalases, and peroxidases, and is found in the leghemoglobin of ROOT nodules. Cytochromes act as electron carriers, for example, in cellular Respiration (see 6.10.3) or photosynthesis (see 6.4.2); cytochrome P450 is a component of Monooxygenases (see 7.6.3.2); the heme of catalases is responsible for neutralizing reactive oxygen species in the form of H₂O₂ within Peroxisomes and glyoxysomes (see 6.5.6; 6.12). Peroxidases perform numerous functions in oxidative reactions: for instance, they serve to detoxify xenobiotics (potential poisons) and are important for Lignin formation (see 6.17.2). Siroheme, as a constituent of sulfite reductase (see 6.7) and nitrite reductase (see 6.6.1), also functions as an electron carrier. The leghemoglobin in the root nodules of legumes (see 9.2.1) serves to store molecular oxygen during atmospheric Nitrogen Fixation.

Unlike iron-containing Hemes and magnesium-containing chlorophylls, the tetrapyrrole ring of vitamin B₁₂ (cyanocobalamin) contains cobalt as its central atom. Vitamin B₁₂ is synthesized by only a few species of Bacteria. It is neither synthesized by plants, which do not utilize it, nor by animals, which require it. Animals and humans obtain vitamin B₁₂ through animal-derived foods or via intestinal microflora (in herbivores!). Human requirement for vitamin B₁₂ is only a few micrograms per day, but strict vegetarian diets can lead to vitamin deficiency symptoms (such as anemia).

Non-cyclic tetrapyrroles serve as the chromophore groups of phycobiliproteins, which are accessory Photosynthetic Pigments in cyanobacteria and red Algae (see 6.4.2; 6.4.3). Phytochromobilin, structurally similar to phycocyanobilin and phycoerythrobilin, acts as the chromophore for plant red-light receptors (phytochromes — see 7.7.2.4).

In green plants, The Biosynthesis of the porphyrin system takes place in Plastids, which subsequently also initiate the synthesis of chlorophylls, heme, and siroheme. Mitochondrial heme synthesis relies on precursors formed within the plastids. It is hypothesized that plastids export heme for use as a prosthetic group in heme-containing Enzymes located in other cellular compartments. The synthesis of tetrapyrroles is shown in Fig. 6.112 in as much detail as currently understood.

Class="center">Fig. 6.112. Compartmentation and regulation of tetrapyrrole METABOLISM in plants. Light activation of protochlorophyllide oxidoreductase (POR A) is typical of angiosperms, which turn green only in the light.

The complex biochemical transformations of tetrapyrroles can be outlined here only in broad terms (Fig. 6.113). The basic building block of the tetrapyrrole system, porphobilinogen, is formed by the Condensation of two molecules of δ-aminolevulinic acid. In plants, cyanobacteria, and many other bacteria, this acid originates from glutamate (in animals, Yeasts, and certain bacteria, from succinyl-CoA and Glycine), which is reduced to 1-semialdehyde and subsequently converted into δ-aminolevulinic acid via intramolecular thiamine pyrophosphate-dependent Transamination. Interestingly, the activating preliminary step for the reduction of the carboxyl group involves not a phosphoric acid residue, but rather glutamyl-tRNA (which also serves as a glutamate donor during plastid Translation); the reduction takes place at the aminoacyl-tRNA stage.

Fig. 6.113. Biosynthesis of tetrapyrroles from glutamate. Compounds at stages prior to uroporphyrinogen are true tetrapyrroles; protoporphyrin IX is the common precursor of chlorophyll and heme (see Fig. 6.112), as well as non-cyclic tetrapyrroles, which arise from protoheme via ring Cleavage.

Four porphobilinogen molecules undergo deamination and condense into a non-cyclic molecule of hydroxymethylbilane (an intermediate stage), which is then cyclized into the first cyclic tetrapyrrole—uroporphyrinogen III—through the action of uroporphyrinogen synthase with the elimination of Water. Through several intermediate steps, protoporphyrin IX is synthesized, which is converted into protoheme by ferrochelatase or into protochlorophyllide a by magnesium chelatase. The reduction of the D-ring converts protochlorophyllide a into chlorophyllide a, onto which chlorophyll synthase (also known as prenyltransferase) transfers a phytol residue (for The Structure of chlorophyll, see Fig. 6.44). Chlorophyll b is formed from chlorophyll a or chlorophyllide a, though the exact details remain unknown. The synthesis of siroheme branches off as early as the uroporphyrinogen III stage. Non-cyclic tetrapyrroles are formed through ring cleavage at a preliminary stage of porphyrin synthesis originating from protoheme (see Fig. 6.112).

Most green algae, gymnosperms, photosynthetic bacteria, and cyanobacteria synthesize chlorophyll both in the light and in the dark, whereas angiosperms do so exclusively in the light. In the latter, protochlorophyllide reductase is light-regulated.

The regulation of tetrapyrrole synthesis According to the plant's metabolic demands operates as follows (see Fig. 6.112). Specifically, the end products protochlorophyllide and protoheme inhibit the synthesis of δ-aminolevulinic acid, while magnesium chelatase is inhibited by protochlorophyllide and chlorophyllide. The production of δ-aminolevulinic acid is light-activated via Phytochrome. These regulatory mechanisms prevent the excessive dark accumulation of photoreactive protochlorophyllide molecules.