Flavodoxin and

In large subunit enzymes (PVC and HPII), a short segment of about 30 residues links the a-helical domain to the C-terminal domain (Fig. 8). The latter segment is a conspicuous addition to the small subunit containing about 150 residues folded into a structure that resembles flavodoxin. For example, there is a root mean square deviation of 3.0 A between flavodoxin and approximately 100 residues of the C-terminal domains of either HPII or PVC. This can be compared to the 1.8 A root mean square deviation for 134 centers between the C-terminal domains of HPII and PVC. Unlike the N-terminal end, the final C-terminal residue Ala753 is visible in the structure of HPII. The C-terminal domain contains extensive secondary structure in the form of four a-helices (al5-18) and eight fi-strands (fi9-16). Despite the obvious structural similarity to flavodoxin, there is no evidence of nucleotide binding in the domain and its function remains a mystery. 

Fig. 19-5), reduced flavodoxin, and perhaps other sources playing a role. In at least one species, the ultimate source of electrons to reduce ferredoxin is pyruvate (Fig. 22-2). 

Flavodoxins and Electron-Transferring Flavoproteins Stephen G. Mayhew and Martha L. Ludwig... 

Fig. 16. Structures of different types of ligands acting as two-electron redox relays in natural and artificial systems Flavins such as (11) are the essential constituents of flavodoxines and flavoproteins 137). The a-ketoglutarate anion (a-KG, 12) is a typical example of a sacrificial redox mediator which decomposes during catalysis (72). Synthetic chelates such as bis-arylimino-acenaphthene (BIAN, 13) have been proposed for the development of bio-inspired multielectron transfer photosensitizers 138).

Doucette, G. J., Erdener, D. L., Peleato, M. L., Hartman, J. J., Anderson, D. M. (1996). Quantitative analysis of iron-stress related proteins in Thalassiosira weissfiogii, measurement of flavodoxin and ferredoxin using HPLC. Mar. Ecol. Prog. Ser. 130, 269—276. 

Redox catalysis is the catalysis of redox reactions and constitutes a broad area of chemistry embracing biochemistry (cytochromes, iron-sulfur proteins, copper proteins, flavodoxins and quinones), photochemical processes (energy conversion), electrochemistry (modified electrodes, organic synthesis) and chemical processes (Wacker-type reactions). It has been reviewed altogether relatively recently [2]. We will essentially review here the redox catalysis by electron reservoir complexes and give a few examples of the use of ferrocenium derivatives. 

It should be noted that some anaerobic eubacteria (e.g. Escherichia coli, Clostridium acidurici, Chlorobium limicold) possess 2-oxoacid oxidoreductases, either in addition to, or instead of, the dehydrogenase complexes (ref. [37] and references therein). As in the archaebacteria, electrons are transferred to ferredoxin or flavodoxin and the enzymes are small oligomeric proteins. The evolutionary relationships of these enzymes converting pyruvate to acetyl-CoA will be discussed in section 6.2. 

J. B. Matthew, P. C. Weber, F. R. Salemme and F. M. Richards, Electrostatic orientation during electron transfer between flavodoxin and cytochrome-C, Nature, 301 (1983) 169-171. 

For the reductive cleavage of SAM into methionine and the deoxyadenosyl radical (DOA ), an electron has to be transferred from a reduced [Fe-S] cluster. The reducing system consisting of NADPH, flavodoxine, and flavodoxine reductase has been identified in E. This allowed to set up an in vitro assay, which besides the... 

Fio. 6. The hydrogen bonding pattern of the -pleated sheet region in (a) dogfish apo-LDH, (b) lobster holo-GAPDH, (c) horse LADH, (d) adenylate kinase, (e) Clostridium MP flavodoxin, and (f) BPN subtilisin. Residues pointing into the hydrophobic cavities between the sheet and helices have been heavily circled. 

Adsorbed layers of cytochrome c3 have also been used to facilitate electron transfer of molecules that otherwise would not yield high electron-transfer rates. Facilitated electron transfer has been observed for flavodoxin and ferredoxin on cytochrome c3 modified basal plane pyrolytic graphite (26). 

Figure 13.2. A proposal for a simplified classification of CYPs with reference to either use of a ferredoxin or alternative electron transport mechanism. Class la— typical bacterial system supported by ferredoxin and ferredoxin reductase, for example, CYPlOl Class Ib—Rhodococcus sp. CYP fusion protein containing a ferredoxin domain Class Ic—Methylococcus capsulatus CYPS 1-ferredoxin fusion . Class 11a— typical eukaryotic CYP/NADPH-cytochrome P450 reductase system Class lib—a fusion protein of a CYP and flavoprotein reductase, for example, P450gM 3 Class IIc—P450 j containing separate flavodoxin and flavodoxin reductase partners . Class fll—standalone functional CYPs, for example, P450jj. ...

Hall DA, Vander Kooi CW et al (2001) Mapping the interactions between flavodoxin and its physiological partners flavodoxin reductase and cobalamin-dependent methionine synthase. Proc Natl Acad Sci USA 98(17) 9521-9526... 

The electron transfer, as observed in vitro between these two proteins, is believed to occur through the formation of a speciflc complex between the two proteins, being the interaction mainly electrostatic in nature. The nature and properties of the protein-protein complex (stoichiometry, interaction sites, association constants) were probed. Integration and correlation of the experimental results obtained from magnetic resonance studies on protein-protein titrations with the available structural and biochemical data is presented. A structural model for a hypothetical ternary complex, formed between one molecule of flavodoxin and two molecules of cytochrome C3, is proposed using the available X-ray structures of the isolated proteins and, when required, model structures predicted by homology modeling. 

H-NMR and molecular modeling approaches were employed to probe and characterise the different flavodoxin - cytochrome C3 complexes and to develop plausible stoichiometry and structural models of them. X-ray determined atomic co-ordinates are available for some of the studied proteins, but when required, homology modeling was used to predict some of the needed structures D, salexigens flavodoxin and D. desulfuricans ATCC 27774 cytochrome C3) (see Palma et ai, 1994). 

In a recent set of experiments, where the titration of D, gigas cytochrome C3 with D. salexigens flavodoxin reconstituted with labelled FMN was followed by C-NMR, we showed that the cytochrome moiety pocesses one unique site for binding flavodoxin [Palma, P.N., Caldeira, J., Ascenso. J.R., Bacher, A., LeGall, J., Moura, I., Wampler, J., Moura, J.J.G., unpublished results]. These results thus suggests that cytochrome C3 have, in fact, one interaction site for flavodoxin and that the formation of the complex affects the resonances of more than one heme protons. 

To build the model, we started with the previously proposed model for a binary complex between D. vulgaris cytochrome C3 and flavodoxin [Stewart et al., 1988], in which heme 4 and the FMN groups are facing each other, in a close proximity and co-planar orientation. This seems, in fact, to be one of the best interaction conformations, not only in terms of topological complementarity, but also in terms of favourable energy of the electrostatic interaction, as was also proposed before, for the interactions between Clostridium pasteurianum flavodoxin and different c-type cytochromes [Weber and Tollin, 1985]. In addition, it also suggests a suitable physical model for the observed ionic strength dependence of the electron transfer between the two proteins. 

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