Electron-transport proteins

The third class of hemoproteins are the cytochromes. They were first discovered by C.A. McMunn, a rural physician in Wolverhampton, who identified their characteristic absorption bands, and by manipulating their oxidation and 

FIGURE 13.13 (a) Chemical structures to the haem groups in cytochromes a, b and c. (b) Axial ligands to the haem groups in cytochromes 

FIGURE 13.14 The proton-motive Q cycle. Electron transfer reactions are numbered and circled. Dashed arrows designate movement of ubiquinol or ubiquinone between centres N and P and of the ISP between cytochrome b and cytochrome cl. Solid black bars indicate sites of inhibition by antimycin, UHDTB, and stigmatellin. From Hunte, Koepke, Lange, Rossmanith, Michel, 2000. Copyright 2000 with 

A functionally similar but structurally much simpler version of the bc complex is found in the plasma membrane of many bacteria, where it participates among other processes in respiration, denitrification, nitrogen fixation, and cyclic photosynthetic electron transfer. 

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The electron transport protein, cytochrome c, found in the mitochondria of all eukaryotic organisms, provides the best-studied example of homology. The polypeptide chain of cytochrome c from most species contains slightly more than 100 amino acids and has a molecular weight of about 12.5 kD. Amino acid sequencing of cytochrome c from more than 40 different species has revealed that there are 28 positions in the polypeptide chain where the same amino acid residues are always found (Figure 5.27). These invariant residues apparently serve roles crucial to the biological function of this protein, and thus substitutions of other amino acids at these positions cannot be tolerated. 

High-potential iron-sulfur proteins (HiPIP) form a family of small (—6-10 kDa) soluble electron transport proteins originally only found in photo synthetic representatives of the proteobacteria (for reviews,... 

Schricker, A. 1997, Soluble electron transport proteins in Heliohacillus mohilis, Diploma Thesis, University of Freiburg/FRG. 

Zhou N-Y, J Al-Dulayymi, MS Baird, PA Williams (2002) Salicylate 5-hydroxylase from Ralstonia sp. strain U2 a monooxygenase with close relationships to and shared electron transport proteins with naphthalene dioxygenase. / Bacfen o/ 184 1547-1555. 

These results may be viewed in the wider context of interactions between potential ligands of multifunctional xenobiotics and metal cations in aquatic environments and the subtle effects of the oxidation level of cations such as Fe. The Fe status of a bacterial culture has an important influence on synthesis of the redox systems of the cell since many of the electron transport proteins contain Fe. This is not generally evaluated systematically, although the degradation of tetrachloromethane by a strain of Pseudomonas sp. under denitrifying conditions clearly illustrated the adverse effect of Fe on the biotransformation of the substrate (Lewis and Crawford 1993 Tatara et al. 1993). This possibility should therefore be taken into account in the application of such organisms to bioremediation programs. 

Fe-S complexes have important functions in today s living systems, in enzymes such as the ferredoxins and oxidoreductases, as well as in electron transport proteins. It is striking that these redox reactions mainly involve elements and compounds such as CO, H2 and N2, which were probably also components of the primeval Earth s atmosphere. Thus, the assumption of an active involvement of Fe-S clusters in a (hypothetical) Fe-S world in processes which finally led to biogenesis appears completely reasonable We now have a background to the theory of the chemoau-totrophic origin of life . 

Cysteine string protein (CSP) Cytochrome b561 Peripheral membrane protein that is paimitoylated on >10 cysteines. May have a role in Ca2+ sensitivity of exocytosis. Electron-transport protein required for intravesicular monooxygenases in subsets of secretory vesicles. Required for dopamine- -hydroxylase and peptide amidase activity. 

Although it is not an electron transport protein, its primary function involves specific electron transfer from cyt c to a heme center, in which binding plays a key kinetic role. It has thus become a model for understanding the structurally more complex interactions between cytoehrome c and its partners in electron transport. This similar cyt c ccp system has several advantages. 

Mossbauer studies of the electron transport protein ferredoxin n from Desulfovibrio gigas have shown that the reduced Fe3S4 cluster contains one trapped valence Fe3+ site and one delocalized Fe2+-Fe3+ pair. The two Fe of the delocalized pair are indistinguishable, and the pair has a dimer spin S12 = 9/2, suggesting ferromagnetic coupling. 

Carter C. W., Jr. (1977). New stereochemical analogies between iron-sulfur electron transport proteins./. Biol. Chem. 252, 7802-7811. 

Iron-sulfur proteins belong to the class of electron-transport proteins [29]. They contain an iron sulfur cluster, e.g. [4Fe-4S], which shuttles between different oxidation states. The structure of the cluster is quite consistent among a series of these proteins, but their redox potentials vary widely. Synthetic models of iron-sulfur proteins have been designed [30] to investigate the factors that determine the reduction potential of the core and to mimic other biologically... 

S-L-x(i i5)-R-(N/F/xF) or M-L-R-(S/N)-F, picked out 138 sequences with 67% showing similarity to known proteins involved in metabolic pathways, electron transport, protein import, protein folding and oxygen scavenging pathways (Carlton et al. 2007). There are undoubtedly variations on these consensus, as have been found during proteomic studies (our unpublished data). 

Such multicomponent enzyme complexes have been created through incorporation of oxidase and electron transport protein subunits from both the TDO (todC 1C2B A) of FI and the BPDO (3/>/>AlA2A3A4) of KF707. The hybrid proteins exhibit substrate specificities similar to those of both parents, but some exhibit altered substrate oxidation rates (Hirose et al., 1994). One hybrid, todC 1 bphA2A3A4,... 

Cylinders and barrels. The twisted P sheets of proteins are often curved to form structures known as P cylinders or p barrels (Fig. 2-16).113 114 Simple cylinders formed by parallel P strands form the backbones of the electron transport protein plastocyanin, the enzyme superoxide dismutase, the oxygen carrier... 

The physiological reducing substrate is a low-molecular-weight (13,000) electron-transport protein, thioredoxin. Thioredoxin has two half-cystine residues that are separated in the polypeptide chain by two other residues. The oxidized form of thioredoxin, with a disulfide bridge between the... 

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