Oxygen transfer enzymes molybdenum centres

The molybdenum in these enzymes is bound by a special organic pterin cofactor, and is not held directly by the sidechains of proteins. The pterin cofactor actually is a dithiolate complex. The molybdenum in the enzymes is not re-oxidised directly by molecular dioxygen and the ancillary flavin and Fe/ S centres have to do with the way in which dioxygen is activated oxygen transfer by molybdenum enzymes is of oxygen atoms from water and not from dioxygen. 

Example of a non-classical oxo-transfer reaction is rare where a relatively stable complex mimicking the role of enzyme-substrate adduct could be isolated. However, the complex [MoO(3,5-DBCat)2(Opy)] may be treated as a complex of similar nature. This complex in solution responds to intramolecular reaction where the coordinated pyO gets deoxygenated and released as simple pyridine. The oxygen is now attached to the molybdenum centre. 

A current overall picture of the reaction mechanism of xanthine oxidase, which differs substantially from one proposed earlier (87) is as follows. The enzyme is presumed to have two independent catalytic units, though this has not so far been proved rigorously. Reducing substrates are bound at molybdenum and reduce this from Mo(VI) both to Mo(V) and to Mo (IV). Reducing equivalents are then transferred by intramolecular reactions from molybdenum to iron-sulphur and also, either directly or via this, to flavin. Oxidizing substrates as a class, seem capable of reacting with all three types of centre in the enzyme. Thus, oxygen reacts predominantly with flavin, phenazine methosulphate... 

All known molybdenum- and tungsten-containing enzymes catalyse reduction-oxidation reactions. The oxidation state of the metal centre can vary between iv, v and vi, hence one- and two-electron transfer reaction steps are possible. In Nature two different ways exist to control the catalytic power and the oxidation state of the metal centre of molybdenum enzymes. One is a mononuclear metal centre, which consists of sulfur and oxygen atoms as coordination sphere around molybdenum and the other is the multinuclear metal centre in which the molybdenum is part of an iron-sulfur cluster, which is only known for bacterial nitroge-nase enzymes. ... 

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