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1 Introduction Introduction 1 1 Introduction Tetrapyrroles belong to a group of molecules with a common structure. They are synthesized in a branched pathway, in which various end products are formed to different amounts. The most abundant cyclic tetrapyrroles are chlorophyll (Chl) and heme, which are characterized by a chelated magnesium and ferrous ion, respectively. Chlorophyll is involved in light absorption and energy transduction during photosynthesis. Heme is a cofactor of hemoglobin, cytochromes, P450 mixed-function oxygenases, and catalases. Other members of the class of tetrapyrroles include siroheme (the prosthetic group of nitrite and sulphite reductases) and phytochromobilin, the chromophore of phytochrome, which is involved in light perception. Tetrapyrrole biosynthesis has been the subject of numerous studies over several decades. But genetic and biochemical characterization of tetrapyrrole biosynthesis has progressed by using approaches to genetically dissect the tetrapyrrole biosynthetic pathway. Pigment-deficient mutants and antisense technology have proved to be useful for examining the mechanisms of metabolic control or for analyzing biochemically the enzymatic steps which are affected by the mutation or by the antisense RNA expression. Tetrapyrrole intermediates are highly photoreactive. They can easily be excited and transfer the energy or electrons to O2. Then reactive oxygen species (ROS) are produced upon exposure to light and oxygen. Under normal growth conditions the risk of photooxidative damage from intermediates in tetrapyrrole biosynthesis is low. Excessive accumulation of such intermediates is the result of deregulation of tetrapyrrole biosynthesis. Toxic effects of porphyrins are evident in human patients with deficiencies of one of the enzymes of heme biosynthesis. These patients are suffering from metabolic diseases, which are called porphyrias (Moore, 1993). The importance of avoiding accumulation of porphyrin intermediates causing photo-oxidative damage has been demonstrated by the analysis of transgenic tobacco plants with reduced uroporphyrinogen III decarboxylase (UROD) (Mock et al., 1995; Mock and Grimm, 1997; Mock et al., 1998) and reduced coproporphyrinogen oxidase III (CPO) activity (Kruse et al., 1995b; Mock et al., 1998). The deleterious effect of accumulated tetrapyrroles is also evident in plants treated with a variety of herbicides that act via inhibition of protoporphyrinogen oxidase (PPOX) (Duke et al., 1991; Matringe et al., 1989). PPOX is the last common enzyme in chlorophyll and heme biosynthesis. There are two Introduction 2 isozymes in higher plants, one located in plastids and the other in mitochondria (Jacobs and Jacobs, 1977; Matringe et al., 1989; Lermontova et al., 1997). The effect of peroxidizing herbicides on PPOX is light dependent and involves intracellular peroxidation promoted by accumulation of protoporphyrin IX (Proto IX), the product of PPOX action (Matringe and Scalla, 1988; Sandmann et al., 1990; Jacobs et al., 1991; Lee and Duke, 1994). Application of herbicides becomes an universal practice in agriculture. Since diphenyl-ether type (DPEs) herbicides, one group of PPOX inhibiting herbicides, are effective at very low dosage, their mechanism of action is an attractive subject for detailed investigation. For a broad application of herbicides the risk of damage to crop plants has to be limited. Several strategies have been evolved for obtaining resistant plants towards the peroxidizing herbicides directed against PPOX. 1.1 Metabolic pathway of tetrapyrrole biosynthesis A simplified flow diagram of the metabolic pathway for tetrapyrrole biosynthesis is given in Fig.1. The pathway can be subdivided into three sections: 5-aminolevulinate (ALA) synthesis, porphyrin formation from eight molecules of ALA, magnesium or iron ion insertion and modification of the metalloporphyrin to give the end products heme and Chl (Beale and Weinstein, 1990; Smith and Griffith, 1993). Chl synthesis exclusively takes place in chloroplasts, while the last two steps of plant heme synthesis are located in both mitochondria and plastids (Fig. 2). In contrast, in animals, heme synthesis starts in the mitochondria with ALA synthase, continues in the cytoplasm up to the formation of coproporphyrinogen and ends back in the mitochondria with the synthesis of protoheme. The first committed precursor of tetrapyrrole pathway is ALA and there are two alternative pathways of ALA formation. In plants, algae (including the prokaryotic cyanobacteria), and most bacteria, ALA is formed from the C-5 skeleton of glutamic acid, in a pathway requiring three enzymatic reactions and tRNAglu (Smith and Griffith, 1993). In purple bacteria, in yeast and animal mitochondria ALA is formed by condensation of glycine and succinyl-CoA. This is mediated by the pyridoxal phosphate-requiring enzyme ALA synthase (EC 2.3.1.37) (Gibson et al., 1958). Some organisms, such as Euglena gracilis (Weinstein and Beale, 1983) and presumably Scenedesmus (Drechsler et al., 1993) possess both pathways. The C5 pathway in Euglena provides precursor of Chl synthesis, whereas ALA formed by ALA synthase is used for heme. All components of the C5 pathway are localized in the chloroplast stroma. In Introduction 3 the first step, glutamate is ligated to tRNA. This reaction step is catalyzed by glutamyl-tRNA synthetase (EC 6.1.1.17). The same enzyme functions simultaneously in protein and tetrapyrrole biosynthesis. Like aminoacyl-tRNA formation in general, this reaction requires ATP and Mg2+. Next, the tRNA-bound glutamate is converted to a reduced form by glutamyl- tRNA reductase (GluTR) in a reaction that requires NADPH. The product of this reaction has been characterized as glutamate-1-semialdehyde (GSA) (Houen et al., 1983). The final step of the C5 pathway is catalyzed by the enzyme GSA aminotransferase (GSAAT, EC 5.4.3.8), that transfers an amino group of the C2 of GSA to C1 to form the product ALA. The active enzyme forms a homodimer and contains a vitamin B6-derivative, either pyridoxamine phosphate or pyridoxal phosphate. The next part of the pathway to form Proto IX, the last common intermediate between heme and chlorophyll, is probably mechanistically identical in all organisms. Condensation of two molecules of ALA to form the monopyrrole, porphobilinogen, is catalyzed by ALA dehydratase (ALAD, EC 4.2.1.24). Porphobilinogen deaminase (PBGD, EC 4.3.1.8) catalyzes the stepwise addition of four porphobilinogen molecules with the loss of a free amino group at each step, to form the very unstable linear tetrapyrrole 1-hydroxymethylbilan (HMB). The instability of the linear HMB suggests an association between PBGD and the next enzyme in the pathway, Uroporphyrinogen III synthase (UROS, EC 4.2.1.75), to allow a direct transfer of the metabolite. UROS catalyzes the inversion of ring D followed by ring closure of the linear tetrapyrrole to form Uroporphyrinogen III (Urogen III). Urogen III is the last common intermediate that leads to siroheme and Vitamin B12. Uroporphyrinogen decarboxylase (UROD, EC 4.1.1.37) catalyzes the formation of coproporphyrinogen III (Coprogen III) by the sequential removal of carboxyl groups from the four acetate side chains of Urogen III. The next enzyme Coprogen III oxidase (CPO, EC 1.3.3.3) catalyzes the oxidative decarboxylation of the two propionate groups to vinyl groups on rings A and B resulting in formation of protoporphyrinogen IX (Protogen IX). The last common step of the metabolic pathway to heme and chlorophyll is the removal of six electrons from Protogen IX to form protoporphyrin IX (Proto IX), which is catalyzed by Protogen IX oxidase (PPOX EC 1.3.3.4). In plants PPOX exists in two isoforms: plastidic-PPOX I and mitochondrial-PPOX II (Lermontova et al., 1997). Thus, Proto IX is distributed between the plastidic pathway and the mitochondrial heme synthesizing pathway. Insertion of Mg2+ into Proto IX initiates the chlorophyll synthesizing, while insertion of Fe2+ begins the heme/bilin synthesizing branch. Introduction 4 Animals Plants ALAsynthase O NH2 NH2 O O O O NH O O O H2N + 2 O S OH CoA OH OH OH OH Glycine Succinyl-CoA Aminolevulinate Glutamat-1-Semialdehyde Glutamate ALA Glutamate Dehydratase COOH Glutamyl-tRNA- Synthetase H2N COOH Glutamyl-tRNAGLU (4x) HN PBG HOOC A B COOH COOH Glutamyl-tRNA-Reductase NH HN Deaminase Glutamate-1-Semialdehyde HOCH 2 NH HN COOH D C COOH GSA-Aminotransferase HOOC Porphobilinogen 5-Aminolevulinate COOH COOH 5-Aminolevulinate-Dehydratase Porpohobilinogen Hydroxymethylbilane Urogen III synthase Porphobilinogen-Deaminase Hydroxymethylbilane COOH COOH CH3 COOH Uroporphyrinogen III-Synthase CH Uroporphyrinogen III Siroheme 3 COOH HOOC A B COOH NH HN NH HN Urogen III Decarboxylase Uroporphyrinogen III-Decarboxylase NH HN Coproporphyrinogen III NH HN HOOC D C COOH CH3 CH3 Coproporphyrinogen III-Oxidase COOH Protoporphyrinogen IX COOH HOOC HOOC Coproporhyrinogen III Uroporphyrinogen III Coprogen Protoporphyrinogen IX-Oxidase I/II oxidase Protoporphyrin IX Fe-Chelatase Mg-Chelatase Protoheme Mg-Protoporphyrin IX CH2 CH2 CH Methyl-Transferase CH3 CH CH Mg-Protoporphyrin IX-MME CH 3 Phycobilins Heme 3 CH CH 2 CH Phytochrome 3 CH CH NH N 2 Chromophore Protogen NH HN Cyclase oxidase Divinyl-Protochlorophyllide a N HN NH HN CH3 CH3 CH3 CH3 8-Vinyl-Reductase Protochlorophyllide a COOH HOOC COOH HOOC Oxidoreductase Protoporphyrin IX Protoporphyrinogen IX Chlorophyllide a 2+ Fe2+ Mg Chlorophyll-Synthase Chlorophyll a Fe-Chelatase I / II Mg-Chelatase N N SAM N N NADPH/O2 N N Mg Mg Mg CH2 N N N N N N CH CH3 CH3 CH CH2 N N O Fe MeO2C O OH O OH O OH N N HO O MeO O divinyl Pchlide CH CH3 3 Mg-Proto IX Mg-Proto IX Me NADPH HOOC COOH Protoheme N N N N N N Mg Mg hv Mg N N N N N N Phytol NADPH O O O MeO2C MeO2C MeO2C O O OH OH O Chlide Pchlide Chlorophyll a Figure 1: Scheme of the tetrapyrrole biosynthetic pathway (according to Smith and Griffiths, 1993, modified) Introduction 5 The first committed enzyme of the Chl synthesizing branch is the Mg chelatase (MgCh). The plant enzyme consists of three subunits designated CHL I, CHL H and CHL D.
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