Ferredoxin thermodynamics

The use of direct electrochemical methods (cyclic voltammetry Pig. 17) has enabled us to measure the thermodynamic parameters of isolated water-soluble fragments of the Rieske proteins of various bci complexes (Table XII)). (55, 92). The values determined for the standard reaction entropy, AS°, for both the mitochondrial and the bacterial Rieske fragments are similar to values obtained for water-soluble cytochromes they are more negative than values measured for other electron transfer proteins (93). Large negative values of AS° have been correlated with a less exposed metal site (93). However, this is opposite to what is observed in Rieske proteins, since the cluster appears to be less exposed in Rieske-type ferredoxins that show less negative values of AS° (see Section V,B). 

Thermodynamic Parameters of the Rieske Fragments from the be. Complexes of Bovine Heart (ISFb) (92) and Paracoccus dentrificans (ISFpd) (140) and of the Rieske-Type Ferredoxin FROM Benzene Dioxygenase (FdBED) (55)... 

For the cytochrome c-plastocyanin complex, the kinetic effects of cross-linking are much more drastic while the rate of the intracomplex transfer is equal to 1000 s in the noncovalent complex where the iron-to-copper distance is expected to be about 18 A, it is estimated to be lower than 0.2 s in the corresponding covalent complex [155]. This result is all the more remarkable in that the spectroscopic and thermodynamic properties of the two redox centers appear weakly affected by the cross-linking process, and suggests that an essential segment of the electron transfer path has been lost in the covalent complex. Another system in which such conformational effects could be studied is the physiological complex between tetraheme cytochrome and ferredoxin I from Desulfovibrio desulfuricans Norway the spectral and redox properties of the hemes and of the iron-sulfur cluster are found essentially identical in the covalent and noncovalent complexes and an intracomplex transfer, whose rate has not yet been measured, takes place in the covalent species [156]. 

Polynuclear Fe-M-S Complexes from "spontaneous self assembly" reactions. Synthetic analog clusters for the Fe2S2 and Fe4S4 centers in the Fe/S proteins (ferredoxins) have been obtained by procedures that are based on the concept of "spontaneous self assembly". The latter (30) assumes that the cores of the Fe/S centers are thermodynamically stable units that should be accessible fiom appropriate reagents even in the absence of a protein environment. 

An obvious application of direct electrochemistry of proteins is the determination of redox potentials. In cases of thermodynamically inaccessible or kinetically reactive species for which traditional potentio-metric methods are inappropriate, direct electrochemistry offers an alternative approach. A good example is the 7Fe ferredoxin from Azo-tobacter chroococcum, which contains one [4Fe-4S] and one [3Fe-4S] cluster. The [4Fe-4S] + potential obtained (7i) by cyclic voltamme-... 

The role of ATP on a molecular level remains one of the great mysteries of the mechanism of nitrogen fixation. As discussed above, the overall thermodynamics of N2 reduction to NH3 by H2 or by its redox surrogate flavodoxin or ferredoxin is favorable. The requirement for ATP hydrolysis must therefore arise from a kinetic necessity. This requirement is fundamentally different from the need for ATP in other biosynthetic or active transport processes, wherein the free energy of hydrolysis of ATP is needed to overcome a thermodynamic limitation. 

P.S. Brereton, et al.. Effect of iron-suKur cluster environment in modulating the thermodynamic properties and biological function of ferredoxin from Pyrococcus furiosus. Biochemistry 1998, 37, 7351-7362. 

Several anaerobic bacteria are now known to contain ferredoxins with potentials that are low enough (e.g. - 0.4 V) to make them candidates for phosphate reduction. Therefore it seems thermodynamically possible that anaerobic microorganisms could synthesise reduced P compounds, for example, PH3. The literature on this appears to be sparse, however there are hints and debates that this may be the case. 

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