Molybdopterin hydroxylases

One of the means of separating contributions within either center is based on relaxation. In simple EPR data acquisition, one finds that FeSI appears already around 77 K, whereas FeSII starts to grow in at much lower temperatures, typically below 30 K. This is true for all different samples of molybdopterin hydroxylases and implies differing relaxation behavior, mainly in spin lattice relaxation time Tj. Therefore, in Davies ENDOR, one can use the time separation of the detection pulses (see Fig. 4) in order to distinguish between the two FeS centers in their ENDOR response. This is shown in Figure 12 for Mop as an example. Recording a is for a 400-ns time delay between the detection pulses, trace b is for 1200 ns. At... 

In the group of molybdopterin hydroxylases, there is the complication in terms of spectral overlap of two different [2Fe2S] clusters, FeSI and FeSII, which was already discussed for the proton interactions above. The main question involved with the study of Fe interactions is whether the two sites can be distinguished, i.e.. 

Nicotinic acid hydroxylase from Clostridium barkerii catalyzes reaction (55), the hydroxylation of a pyridine group, and has similarities to xanthine dehydrogenase. Nicotinic acid hydroxylase is a 300 000 molecular weight flavoprotein containing iron-sulfur and FAD centres, selenium1034 and a molybdopterin cofactor.1035 Formate dehydrogenase contains selenium as selenocysteine,1036 but this does not appear to be the case for nicotinic acid hydroxylase. The possibility that the selenium is incorporated into the molybdopterin cannot be excluded at present. 

Nicotinic acid hydroxylase. Nicotinic acid hydroxylase consists of four dissimilar subunits and occurs in forms with different molecular masses. There are 5-7 Fe, one FAD, and one molybdenum molybdopterin per 160 kDa protein. Unusually, nicotinic acid hydroxylase as isolated from Clostridium barkeri contains Mo(V) rather than Mo(VI). By EPR spectroscopy of the enzyme containing either natural-abundance Se or Se (/ = Gladyshev and co-workers showed that Se is directly coordinated, although in an unidentified form to Mo. Furthermore, a flavin radical and two [2Fe-2S] clusters could be observed with EPR spectroscopy. ... 

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