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The universal tetrapyrrole precursor, 5-aminolevulinic acid (ALA) is formed by one of two alternative routes. Although these pathways are distinctly different with respect to biosynthetic precursors and intermediates, and the nature of the enzymes and the genes that encode them, there are similarities in their regulatory responses to biosynthetic end products and to environmental and metabolic signals in photosynthetic organisms.

In plants, green algae, archaea and in most bacteria the common precursor of all tetrapyrroles, 5-aminolevulinic acid, is formed by three enzymes. The initial substrate glutamate is converted to glutamyl-tRNA by glutamyltRNA synthetase for use in both protein and tetrapyrrole biosynthesis. During the first committed step an NADPH-dependent glutamyl-tRNA reductase reduces glutamyl-tRNA to form glutamate-1-semialdehyde, which is subsequently transaminated by glutamate-1-semialdehyde-2,1-aminomutase to yield 5-aminolevulinic acid. The enzymatic mechanisms deduced from biochemical investigations and recently solved crystal structures are described for all three enzymes. A potential pathway for metabolic channeling of the reactive aldehyde between glutamyl-tRNA reductase and the aminomutase is outlined.

This chapter comprehensively surveys the conversion of 5-aminolevulinic acid to protochlorophyllide in the Mg-porphyrin-synthesizing branch of tetrapyrrole biosynthesis the formation of protoheme in the iron-chelating branch. This can be considered as the middle and final stages of chlorophyll and heme formation, respectively: the final conversion of protochlorophyllide to chlorophyll is discussed by Rüdiger in Chapter14 (Rüdiger). This chapter reviews the many individual enzymatic steps in these conversions, including enzyme and gene structures and the expression as well as the regulation of these steps.

The last steps of chlorophylls (Chls) and biosynthesis comprise the formation of chlorophyllide (Chlide) from protochlorophyllide (PChlide) , the oxygenation of Chlide to Chlide , the esterification of Chlides to the corresponding Chls, and the reduction of -type pigments to -type pigments. Two separate pathways exist for the biosynthesis of Chls, a light-dependent and a light-independent (dark) pathway. The decisive step is the hydrogenation of PChlide to Chlide , catalyzed either by a light-dependent PChlide oxidoreductase (POR) or by a light-independent PChlide oxidoreductase (DPOR, D for dark). The conversion of - to -type pigments and conversely of - to -type pigments, the ‘chlorophyll cycle,’ presumably allows the plants to adjust the Chl ratio to the environment: the reduction of Chl to Chl precedes the degradation of the -type pigment. This chapter describes the last steps of Chl biosynthesis with emphasis on the enzymes that catalyze the individual steps: included are discussions of the corresponding genes and recombinant enzymes.

The photosynthetic green sulfur bacteria synthesize a complex mixture of bacteriochlorophylls and chlorophylls. Depending on the strain, the dominant species is bacteriochlorophyll (BChl) , , or , which serves as the major light-harvesting pigment in the chlorosome antenna. Each of these BChl species occurs as a mixture of homologs differing in stereochemistry, methylation, and esterifying alcohol. In addition, BChl is present in various protein-based antenna complexes and in the reaction centers. A third chlorophyll (Chl) species, Chl esterified with Δ2, 6-phytadienol, functions as the primary electron acceptor in the reaction center.

In eukaryotic cells, tetrapyrrole intermediates and end-products play a crucial role as intra-organellar regulators within the chloroplast or mitochondrion, which are sites of heme and chlorophyll biosynthesis, by exerting a feedback control on the enzyme activities of their own biosynthetic pathway. It is proposed that some tetrapyrrole metabolites also act as inter-organellar signals communicating with the nucleus to coordinate nuclear and organellar activities by regulating nuclear gene expression in response to the physiological and developmental state of the mitochondrion or plastid.

Although chlorophyll synthesis in Spring and its degradation in Autumn are undoubtedly the most colorful manifestations of life on Earth, chlorophyll catabolism remained an enigma until about fifteen years ago. Contrary to expectation, chlorophyll breakdown in vascular plants rapidly leads to colorless degradation products and only fleetingly involves colored intermediates, which result from enzymatic oxidative opening of the chlorophyll macrocycle. This key oxygenolytic step in higher plants is rapidly followed by an enzymatic reduction to form short-lived fluorescent catabolites. These latter tetrapyrroles isomerize rapidly in an acid-catalyzed chemical step to colorless tetrapyrrolic catabolites. The colorless and non-fluorescent bilanones finally accumulate in the vacuoles of the degreened plant tissues. This chapter outlines the structural features of chlorophyll catabolites from natural sources and the biochemistry of chlorophyll breakdown.

Photosynthesis evolved very early on the Earth, but after respiration, and probably after the metabolic processes for methanogenesis and sulfur oxidation. This occurred in ancestors of anoxygenic photosynthetic bacteria. An ancestral reaction center of Photosystem I and II (RCI/II) type of photosynthesis arose in which a five membrane-spanning helix (MSH) protein bound two molecules of chlorophyll (Chl)/bacteriochlorophyll (BChl) in a special pair and had a Chl/quinone primary acceptor, and this protein fused, early on, with a six MSH antenna protein. Logic suggests that the earliest photopigments were protoporphyrin IX, followed by Mg protochlorophyllide , followed by Chl/BChl. It is not clear whether Chl or BChl came first. The evolution of the modern RCI type occurred later but it is not clear under what selection pressure it arose, possibly when ferric salts and sulfur compounds became more available in the Proterozoic Eon.

In anoxygenic photosynthetic bacteria, the primary photochemical process in the conversion of light energy involves the transfer of an electron from a primary electron donor, a bacteriochlorophyll dimer, to a series of electron acceptors in the reaction center. The effects of the surrounding protein on the properties of the bacteriochlorophyll dimer are discussed for three types of interactions: magnesium coordination to histidines, electrostatic interactions with charged amino acid residues, and hydrogen-bonding to the side chains of amino acid residues. Alterations of these interactions through mutagenesis are found to be correlated to changes in the oxidation/reduction midpoint potential, electron spin density, and optical spectra. The correlation is explained using models of the electronic structure of the dimer. The use of mutants in which the energies of different electron transfer states have been manipulated has provided the opportunity to explore the relationship between driving force and rate, the factors controlling the asymmetry of electron transfer, and new electron transfer reactions involving tyrosine residues.

Chlorosomes, the oblong light-harvesting bodies of green photosynthetic bacteria, are attached to the inner side of the cytoplasmic membrane. Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) was used to study isotopically labeled chlorosomes and in vitro model compounds. Using uniform isotope labeling of chlorophyll molecules, 2D and 3D MAS NMR dipolar correlation spectroscopy was performed in high magnetic field. The chemical shifts provided invaluable information about the structure via ring current effects, while long range correlations were generated that lead to intermolecular distance constraints. Novel methodology was developed and implemented using a Chl /HO aggregate, and a structural arrangement of bilayers of Chl sheets with interdigitating tails was resolved. Application of this technology to chlorosomes and BChl aggregates provided unambiguous evidence that self-organization of BChl is the principal structural factor in establishing the rod elements in chlorosomes. This confirms that proteins do not play an essential role in the light harvesting function, which is of fundamental biological interest. Finally, MAS NMR leads to a bilayer model for the tubular supra-structure of sheets of BChl c in the chlorosomes of .