Iron-sulfur proteins Mossbauer spectra

These spectra, taken at variable temperatures and a small polarizing applied magnetic field, show a temperature-dependent transition for spinach ferredoxin. As the temperature is lowered, the effects of an internal magnetic field on the Mossbauer spectra become more distinct until they result at around 30 °K, in a spectrum which is characteristic of the low temperature data of the plant-type ferredoxins (Fig. 11). We attribute this transition in the spectra to spin-lattice relaxation effects. This conclusion is preferred over a spin-spin mechanism as the transition was identical for both the lyophilized and 10 mM aqueous solution samples. Thus, the variable temperature data for reduced spinach ferredoxin indicate that the electron-spin relaxation time is around 10-7 seconds at 50 °K. The temperature at which this transition in the Mossbauer spectra is half-complete is estimated to be the following spinach ferredoxin, 50 K parsley ferredoxin, 60 °K adrenodoxin, putidaredoxin, Clostridium. and Axotobacter iron-sulfur proteins, 100 °K. 

Detailed studies on and Fb have been hampered by the property that their EPR spectra are not additive. This property has been attributed to a magnetic interaction between reduced F and Fg, indicating that they are very close to each other. The values of F and Fg are -540 and -590 mV respectively in spinach PS I particles. Their values are always in that range, but their relative values vary in different plant species for example, F has a more negative than Fb in barley and in a halophilic alga. The shape and temperature dependence of the EPR spectra of F and Fb are typical of iron-sulfur proteins. They are considered to be 4Fe-4S centers, since after modification by dimethyl sulfoxide their spectrum is characteristic of 4Fe-4S centres and because their Mossbauer spectra are also in agreement with that attribution. The presence of 10-12 Fe-S pairs in each PS I centre is compatible with this assignment (for reviews, see Refs. 25 and 26). 

A third species, Fx, the spectrum of which considerably deviates from that of a ferredoxin, is observed under highly reducing conditions [43,44], From Mossbauer studies it was calculated that Fx is a 4Fe-4S iron-sulfur protein [45], It is still not quite certain, however, whether under physiological conditions Fx really acts as an obligatory electron acceptor. In spite of the above-mentioned uncertainties, EPR is the only technique that is capable of furnishing detailed information on the various iron-sulfur protein acceptors their optical absorbance difference spectra all show a rather uninformative weak band around 430 nm,... 

All these proteins give a typical Mossbauer spectrum in the reduced form identical to that obtained for D.gigas Fdll. Aconitase contains only one type of iron-sulfur center. While ferredoxins are only involved in electron transfer, aconitase has a central metabolic role in the citric acid cycle 37>68h The role of the iron-sulfur center remains to be determined. Glutamate synthase is an important enzyme in... 

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