Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase explained

Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase
Ec Number:	1.14.13.81
Cas Number:	92353-62-3

Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase, 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

magnesium-protoporphyrin IX 13-monomethyl ester + 3 NADPH + 3 H⁺ + 3 O₂

\rightleftharpoons 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

\rightleftharpoons 13¹-hydroxy-magnesium-protoporphyrin IX 13-monomethyl ester + H₂O

(1b) 13¹-hydroxy-magnesium-protoporphyrin IX 13-monomethyl ester + 2 reduced ferredoxin + O₂

\rightleftharpoons 13¹-oxo-magnesium-protoporphyrin IX 13-monomethyl ester + 2 H₂O

(1c) 13¹-oxo-magnesium-protoporphyrin IX 13-monomethyl ester + 2 reduced ferredoxin + O₂

\rightleftharpoons 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, 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 .^[6]

See also

• Biosynthesis of chlorophylls

Notes and References

1. Bollivar DW, Beale SI . 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 . September 1996 . 12226378 . 157929 . 10.1104/pp.112.1.105 .
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. en. 9. 9. 1157. 10.3390/plants9091157. 32911631. 7570240. free.
3. Willows RD . Biosynthesis of chlorophylls from protoporphyrin IX . Natural Product Reports . 20 . 3 . 327–41 . June 2003 . 12828371 . 10.1039/B110549N .
4. Bollivar DW . Recent advances in chlorophyll biosynthesis . Photosynthesis Research . 90 . 2 . 173–94 . November 2006 . 17370354 . 10.1007/s11120-006-9076-6 . 23808539 .
5. 10.1146/annurev.arplant.57.032905.105448 . Tetrapyrrole Biosynthesis in Higher Plants . 2007 . Tanaka . Ryouichi . Tanaka . Ayumi . Annual Review of Plant Biology . 58 . 321–346 . 17227226 .
6. 10.1111/j.1432-1033.1996.0085u.x . 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 . 1996 . Porra . Robert J. . Schafer . Wolfram . Gad'On . Nasr . Katheder . Ingrid . Drews . Gerhart . Scheer . Hugo . European Journal of Biochemistry . 239 . 1 . 85–92 . 8706723 . free .