Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase (
EC1.14.13.81), is an
enzyme with
systematic namemagnesium-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
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
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
EC1.21.98.3.[6]