Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase

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
magnesium-protoporphyrin IX 13-monomethyl ester + 3 NADPH + 3 H+ + 3 O2 divinylprotochlorophyllide + 3 NADP+ + 5 H2O (overall reaction)
Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase
Identifiers
EC number1.14.13.81
CAS number92353-62-3
Databases
IntEnzIntEnz view
BRENDABRENDA entry
ExPASyNiceZyme view
KEGGKEGG entry
MetaCycmetabolic pathway
PRIAMprofile
PDB structuresRCSB PDB PDBe PDBsum

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

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. 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.
  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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