View source for Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase

{{Short description|Class of enzymes}}
{{Infobox enzyme
| Name       = Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase
| EC_number  = 1.14.13.81
| CAS_number = 92353-62-3
| GO_code    =
| image      = 
| width      = 
| caption    =  
}}
'''Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase''' ({{EC number|1.14.13.81}}), is an [[enzyme]] with [[List of enzymes|systematic name]] ''magnesium-protoporphyrin-IX 13-monomethyl ester, ferredoxin:oxygen oxidoreductase (hydroxylating)''.<ref>{{cite journal | vauthors = Bollivar DW, Beale SI | title = The Chlorophyll Biosynthetic Enzyme Mg-Protoporphyrin IX Monomethyl Ester (Oxidative) Cyclase (Characterization and Partial Purification from Chlamydomonas reinhardtii and Synechocystis sp. PCC 6803) | journal = Plant Physiology | volume = 112 | issue = 1 | pages = 105–114 | date = September 1996 | pmid = 12226378 | pmc = 157929 | doi = 10.1104/pp.112.1.105 }}</ref> In plants this enzyme [[catalysis|catalyses]] the following overall [[chemical reaction]]
[[File: Oxidative cyclase to chlorin.svg|thumb|centre |600px|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<sup>+</sup> + 3 O<sub>2</sub> <math>\rightleftharpoons</math> divinylprotochlorophyllide + 3 NADP<sup>+</sup> + 5 H<sub>2</sub>O (overall reaction)
Recent evidence<ref name="Barley">{{cite journal| vauthors = Stuart D, Sandström M, Youssef HM, Zakhrabekova S, Jensen PE, Bollivar DW, Hansson M |date=2020-09-08|title=Aerobic Barley Mg-protoporphyrin IX Monomethyl Ester Cyclase is Powered by Electrons from Ferredoxin|journal=Plants|language=en|volume=9|issue=9|pages=1157|doi=10.3390/plants9091157|pmid=32911631|pmc=7570240|doi-access=free}}</ref> 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 + O<sub>2</sub> <math>\rightleftharpoons</math> 13<sup>1</sup>-hydroxy-magnesium-protoporphyrin IX 13-monomethyl ester + H<sub>2</sub>O
:(1b) 13<sup>1</sup>-hydroxy-magnesium-protoporphyrin IX 13-monomethyl ester + 2 reduced ferredoxin + O<sub>2</sub> <math>\rightleftharpoons</math> 13<sup>1</sup>-oxo-magnesium-protoporphyrin IX 13-monomethyl ester + 2 H<sub>2</sub>O
:(1c) 13<sup>1</sup>-oxo-magnesium-protoporphyrin IX 13-monomethyl ester + 2 reduced ferredoxin + O<sub>2</sub> <math>\rightleftharpoons</math> divinylprotochlorophyllide + 2 H<sub>2</sub>O

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,<ref name=Barley /> which is part of the [[biosynthetic pathway]] to [[chlorophyll]]s.<ref name="Review" >{{cite journal | vauthors = Willows RD | title = Biosynthesis of chlorophylls from protoporphyrin IX | journal = Natural Product Reports | volume = 20 | issue = 3 | pages = 327–41 | date = June 2003 | pmid = 12828371 | doi = 10.1039/B110549N }}</ref><ref>{{cite journal | vauthors = Bollivar DW | title = Recent advances in chlorophyll biosynthesis | journal = Photosynthesis Research | volume = 90 | issue = 2 | pages = 173–94 | date = November 2006 | pmid = 17370354 | doi = 10.1007/s11120-006-9076-6 | s2cid = 23808539 }}</ref><ref>{{cite journal |doi=10.1146/annurev.arplant.57.032905.105448  |title=Tetrapyrrole Biosynthesis in Higher Plants |year=2007 |last1=Tanaka |first1=Ryouichi |last2=Tanaka |first2=Ayumi |journal=Annual Review of Plant Biology |volume=58 |pages=321–346 |pmid=17227226 }}</ref>
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 number|1.21.98.3}}.<ref>{{cite journal |doi=10.1111/j.1432-1033.1996.0085u.x |title=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 |year=1996 |last1=Porra |first1=Robert J. |last2=Schafer |first2=Wolfram |last3=Gad'On |first3=Nasr |last4=Katheder |first4=Ingrid |last5=Drews |first5=Gerhart |last6=Scheer |first6=Hugo |journal=European Journal of Biochemistry |volume=239 |issue=1 |pages=85–92 |pmid=8706723 |doi-access=free }}</ref>

== See also ==
* [[Chlorophyllide|Biosynthesis of chlorophylls]]

== References ==
{{reflist}}

{{Dioxygenases}}
{{Enzymes}}
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[[Category:EC 1.14.13]]