Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase

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Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase
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EC no. 1.14.13.81
CAS no. 92353-62-3
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Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase (EC 1.14.13.81), is an enzyme with systematic name magnesium-protoporphyrin-IX 13-monomethyl ester, ferredoxin:oxygen oxidoreductase (hydroxylating). [1] In plants this enzyme catalyses the following overall chemical reaction

The chlorin ring system forms as the esterified propionate sidechain is cyclised on to the porphyrin ring of protoporphyrin IX to form divinylprotochlorophyllide Oxidative cyclase to chlorin.svg
The chlorin ring system forms as the esterified propionate sidechain is cyclised on to the porphyrin ring of protoporphyrin IX to form divinylprotochlorophyllide
magnesium-protoporphyrin IX 13-monomethyl ester + 3 NADPH + 3 H+ + 3 O2 divinylprotochlorophyllide + 3 NADP+ + 5 H2O (overall reaction)

Recent evidence [2] shows that the necessary electrons which cycle the enzyme from oxidised to reduced form come from ferredoxin. In green tissue, ferredoxin can receive these electrons directly from photosystem I so that NADPH need not be involved. However, in the dark, ferredoxin can also be reduced via Ferredoxin—NADP(+) reductase, allowing the reaction to proceed in that case. It is therefore more accurate to show the individual steps as follows:

(1a) magnesium-protoporphyrin IX 13-monomethyl ester + 2 reduced ferredoxin + O2 131-hydroxy-magnesium-protoporphyrin IX 13-monomethyl ester + H2O
(1b) 131-hydroxy-magnesium-protoporphyrin IX 13-monomethyl ester + 2 reduced ferredoxin + O2 131-oxo-magnesium-protoporphyrin IX 13-monomethyl ester + 2 H2O
(1c) 131-oxo-magnesium-protoporphyrin IX 13-monomethyl ester + 2 reduced ferredoxin + O2 divinylprotochlorophyllide + 2 H2O

This enzyme requires Fe(II) for activity. In barley the cyclase protein is named XanL and is encoded by the Xantha-l gene. An associated protein, Ycf54, seems to be required for proper maturation of the XanL enzyme, [2] which is part of the biosynthetic pathway to chlorophylls. [3] [4] [5] In anaerobic organisms such as Rhodobacter sphaeroides the same overall transformation occurs but the oxygen incorporated into magnesium-protoporphyrin IX 13-monomethyl ester comes from water in the reaction EC 1.21.98.3. [6]

See also

Related Research Articles

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Chlorophyll <i>a</i> Chemical compound

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Chlorophyll <i>b</i> Chemical compound

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<span class="mw-page-title-main">Magnesium protoporphyrin IX methyltransferase</span>

In enzymology, a magnesium protoporphyrin IX methyltransferase is an enzyme that catalyzes the chemical reaction

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In enzymology, divinyl chlorophyllide a 8-vinyl-reductase (EC 1.3.1.75) is an enzyme that catalyzes the chemical reaction

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<span class="mw-page-title-main">Protochlorophyllide reductase</span>

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<span class="mw-page-title-main">Magnesium chelatase</span> Enzyme

Magnesium-chelatase is a three-component enzyme (EC 6.6.1.1) that catalyses the insertion of Mg2+ into protoporphyrin IX. This is the first unique step in the synthesis of chlorophyll and bacteriochlorophyll. As a result, it is thought that Mg-chelatase has an important role in channeling intermediates into the (bacterio)chlorophyll branch in response to conditions suitable for photosynthetic growth:

<span class="mw-page-title-main">Chlorophyll synthase</span> Class of enzymes

In enzymology, chlorophyll synthase is an enzyme that catalyzes the chemical reaction

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<span class="mw-page-title-main">Chlorophyllide-a oxygenase</span> Class of enzymes

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<span class="mw-page-title-main">Chlorophyllide</span> Chemical compound

Chlorophyllide a and Chlorophyllide b are the biosynthetic precursors of chlorophyll a and chlorophyll b respectively. Their propionic acid groups are converted to phytyl esters by the enzyme chlorophyll synthase in the final step of the pathway. Thus the main interest in these chemical compounds has been in the study of chlorophyll biosynthesis in plants, algae and cyanobacteria. Chlorophyllide a is also an intermediate in the biosynthesis of bacteriochlorophylls.

<span class="mw-page-title-main">Chlorophyllide a reductase</span> Enzyme

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References

  1. ↑ Bollivar DW, Beale SI (September 1996). "The Chlorophyll Biosynthetic Enzyme Mg-Protoporphyrin IX Monomethyl Ester (Oxidative) Cyclase (Characterization and Partial Purification from Chlamydomonas reinhardtii and Synechocystis sp. PCC 6803)". Plant Physiology. 112 (1): 105–114. doi:10.1104/pp.112.1.105. PMC   157929 . PMID   12226378.
  2. 1 2 Stuart D, Sandström M, Youssef HM, Zakhrabekova S, Jensen PE, Bollivar DW, Hansson M (2020-09-08). "Aerobic Barley Mg-protoporphyrin IX Monomethyl Ester Cyclase is Powered by Electrons from Ferredoxin". Plants. 9 (9): 1157. doi: 10.3390/plants9091157 . PMC   7570240 . PMID   32911631.
  3. ↑ Willows RD (June 2003). "Biosynthesis of chlorophylls from protoporphyrin IX". Natural Product Reports. 20 (3): 327–41. doi:10.1039/B110549N. PMID   12828371.
  4. ↑ Bollivar DW (November 2006). "Recent advances in chlorophyll biosynthesis". Photosynthesis Research. 90 (2): 173–94. doi:10.1007/s11120-006-9076-6. PMID   17370354. S2CID   23808539.
  5. ↑ Tanaka, Ryouichi; Tanaka, Ayumi (2007). "Tetrapyrrole Biosynthesis in Higher Plants". Annual Review of Plant Biology. 58: 321–346. doi:10.1146/annurev.arplant.57.032905.105448. PMID   17227226.
  6. ↑ Porra, Robert J.; Schafer, Wolfram; Gad'On, Nasr; Katheder, Ingrid; Drews, Gerhart; Scheer, Hugo (1996). "Origin of the Two Carbonyl Oxygens of Bacteriochlorophyll a. Demonstration of two Different Pathways for the Formation of Ring e in Rhodobacter sphaeroides and Roseobacter denitrificans, and a Common Hydratase Mechanism for 3-acetyl Group Formation". European Journal of Biochemistry. 239 (1): 85–92. doi: 10.1111/j.1432-1033.1996.0085u.x . PMID   8706723.
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