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Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase

Class of enzymes From Wikipedia, the free encyclopedia

Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase
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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

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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)
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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]

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