If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Cofactors, Prosthetic Groups, Electron Carriers Biosynthesis → Porphyrin Compounds Biosynthesis → Chlorophyll Biosynthesis → Chlorophyllide a Biosynthesis|
Some taxa known to possess this pathway include : Allochromatium vinosum, Chlorobaculum tepidum, Rhodobacter capsulatus, Rhodobacter sphaeroides, Rhodopseudomonas palustris, Rhodospirillum rubrum, Rubrivivax gelatinosus
Expected Taxonomic Range:
Chlorophyll is a green photosynthetic pigment found in photosynthetic plants, algae, and bacteria. Its name is derived from ancient Greek: chloros = green and phyllon = leaf. Chlorophyll absorbs mostly in the blue and, to a lesser extent, red portions of the electromagnetic spectrum, hence its intense green color. There are several variants of chlorophyll ( chlorophyll a, chlorophyll a', chlorophyll b, chlorophyll c1, chlorophyll c2, and chlorophyll d). Non-oxygenic bacteria possess a related pigment called bacteriochlorophyll, which also exists in several variants ( bacteriochlorophyll a, bacteriochlorophyll b, bacteriochlorophyll c, bacteriochlorophyll d, bacteriochlorophyll e and bacteriochlorophyll g). This pathway describes the formation of chlorophyllide a, an important intermediate in the biosynthesis of all chlorophylls.
About This Pathway
The biosynthetic pathway of bacteriochlorophylls begins with the condensation of eight molecules of 5-aminolevulinate to form uroporphyrinogen-III, the first cyclic tetrapyrrole compound in the pathway (see tetrapyrrole biosynthesis I (from glutamate)). This compound is then sequentially converted to protoporphyrin IX, which is the branch-point intermediate to hemes and chlorophylls.
The incorporation of Mg2+ into protoporphyrin IX occurs at the next step, resulting in the formation of Mg-protoporphyrin (MgP). A methyl transferase converts MgP into magnesium-protoporphyrin IX 13-monomethyl ester (MgPMME), which is the substrate for a cyclase reaction, which creates a fifth ring (ring E), a common feature of all chlorophylls. In anaerobic phototrophs the cyclase is a hydratase, incorporating an oxygen atom from a water molecule, by a complex process catalyzed by the cobalamin-dpendent enzyme anaerobic magnesium-protoporphyrin IX monomethyl ester cyclase. After reduction of a divinyl group to a monovinyl (or ethyl) group (a reaction that may happen at different stages along the pathway), protochlorophyllide a is finally reduced to chlorophyllide a. In this pathway, which is found in the anoxygenic phototrophs, this final reduction is carried by the enzyme light-independent protochlorophyllide reductase, permitting the organisms to synthesize bacteriochlorophyll in the dark.
This biosynthetic pathway, which occurs in all anaerobic photosynthetic organisms, has been described through genetic analysis of bacterial mutants and in vitro reconstitution of individual reactions [Suzuki97, Beale99, Ouchane04, Bollivar06].
Aerobic phototrophs possess a very similar pathway, with one exception being that the cyclase found in these organisms uses molecular oxygen as the oxygen source. A variant of the pathway found in aerobic phototrophs is described in chlorophyllide a biosynthesis III (aerobic, light independent).
Superpathways: superpathway of bacteriochlorophyll a biosynthesis
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Choquet92: Choquet Y, Rahire M, Girard-Bascou J, Erickson J, Rochaix JD (1992). "A chloroplast gene is required for the light-independent accumulation of chlorophyll in Chlamydomonas reinhardtii." EMBO J 11(5);1697-704. PMID: 1374710
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Gibson95a: Gibson LC, Willows RD, Kannangara CG, von Wettstein D, Hunter CN (1995). "Magnesium-protoporphyrin chelatase of Rhodobacter sphaeroides: reconstitution of activity by combining the products of the bchH, -I, and -D genes expressed in Escherichia coli." Proc Natl Acad Sci U S A 92(6);1941-4. PMID: 7892204
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