Fe-S centre

Fig. 5.7. In green sulfur bacteria and in some archaebacteria, a reverse citric acid cycle is used for the assimilation of C02. It must be assumed that this was the original function of the citric acid cycle that only secondarily took over the role as a dissimulatory and oxidative process for the degradation of organic matter. A major enzyme here is 2-oxoglutarate ferredoxin for C02 fixation. Note that it, like several other enzymes in the cycle, uses Fe/S proteins. One is the initial so-called complex I which has eight different Fe/S centres of different kinds but no haem (see also other early electron-transfer chains, e.g. in hydrogenases).

An interesting new experimental approach has been taken in order to separate overlapping EPR spectra as they appear e.g. in the multi Fe/S centre containing complex I. Inversion- and saturation-recovery measurements which allow to measure Ti relaxation times are used in a inversion-recovery filter which is subsequently applied to separate EPR signals on account of their Trdifferences. In addition, this filter can be used in conjunction with high-resolution hyperfine measurements e.g. by ESEEM and thus the separated centres can be characterized in depth.211... 

Three bound Fe-S centres have been proposed to be the next acceptors (see Refs. 29, 48 and 49 for reviews), on the basis of optical and EPR spectroscopy and Mdssbauer studies. The stable, one-electron acceptor of PS I is a soluble Fe-S protein, ferredoxin (Fd) [50], of molecular weight of 10 kDa and = -440 mV. So, PS I transfers electrons against an apparent electrochemical gradient of ca. 0.9 V. 

Absorption spectra of the redox states may be weak and indistinctive so that other characteristics must be monitored. In suitable cases, NMR olfers opportunities to examine thermodynamic and structural aspects of protein redox equilibria. The most widely exploited alternative is EPR, but since room-temperature spectra are not generally observed for metal centres, electrochemical and spectroscopic measurements are made under very different conditions. After establishing equilibrium under ambient conditions, a sample is withdrawn and frozen for examination at cryogenic temperatures. Molybdenum, Type 2 Cu, and— for the most part—Fe-S centres, do not constitute strong or distinctive chromophores and low-temperature EPR is the method of choice. 

Four structural classes of Fe-S centre have been identified to date these are depicted in Fig. 13. All of them feature high-spin tetrahedral Fe(II) or Fe(III) coordinated typically by four sulfur donors. Apart from the monomeric centre found in proteins known as rubredoxins, they are all clusters that contain both protein donors and inorganic bridging (p) sulfido ligands. Most of our knowledge stems from studies made on the small electron-transport proteins known as ferredoxins (Fd s) and from work on model compounds. Figure 14 shows the structure of a ferredoxin isolated from the anaerobe Peptococcus aerogenes [165]. 

Fig. 13. Structures of the four currently established classes of Fe-S centre...

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. 

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