Electron transferases

Other electron transferases include the rubredoxin and ferredoxin iron-sulfur proteins, so named because they contain iron-sulfur clusters of various sizes. Rubredoxins are found in anaerobic bacteria and contain iron ligated to four cysteine sulfurs. Ferredoxins are found in plant chloroplasts and mammalian tissue and contain spin-coupled [2Fe-2S] clusters. Further discussion of rubredoxin and ferredoxin proteins can be found in Chapters 6 and 7 of reference 15, and cytochromes will be extensively discussed in Chapter 7 of this text. 

Because of coupling (see Chapter 7) there are relationships between the thermodynamic properties of reactions in some of the EC classes. All oxidoreductase reactions can be considered to be coupled reactions because each one can be divided into two, or in a some cases, three half reactions that do not share atoms but are connected by formal electrons. Transferase reactions can each be considered to result from the coupling of two oxidoreductase reactions or two hydrolase reactions. Fifteen examples are discussed in reference (6). Each of the coupled reactions contributes its A, G ° and A, A h to the coupled reaction. Hydrolase reactions and isomerase reactions are never coupled reactions. Some lyase reactions are coupled. Ligase reactions are all coupled by definition because they join together two reactions with the hydrolysis of a pyrophosphate bond in ATP or a similar triphosphate. A spectacular example of coupling is provided by EC 6.3.5.4 because there are seven reactants. This never happens in chemistry. 

The two-domain, structural motif in FNR represents a common structural feature in a large class of enzymes that catalyze electron transfer between a nicotinamide dinucleotide molecule and a one-electron carrier. Beside the photosynthetic electron-transfer enzyme, others non-photosynthetic ones include flavodoxin reductase, sulfite reductase, nitrate reductase, cytochrome reductase, and NADPH-cyto-chrome P450 reductase. FNR belongs to the group of so-called dehydrogenases-electron transferases, i.e., flavoproteins that catalyze electron transfer from two, one-electron donor molecules to a single two-electron acceptor molecule. 

An alternative to the application of mediators is the direct transfer of electrons between the prosthetic group of the enzyme and the amperometric electrode (Fig. 19). In this heterogenous reaction the electrode acts as an electron transferase. 

There are four classesof electron transferases, each of which contains many members that exhibit important structural differences flavodoxins, blue copper proteins, iron-sulfur proteins, and cytochromes. 

The protein environment thus exerts a powerful influence over the cluster reduction potentials. This observation applies to all classes of electron transferases—the factors that are critical determinants of cofactor reduction potentials are poorly understood at present but are thought to include the low dielectric constants of protein interiors ( 4 for proteins vs. —78 for H2O), electrostatic effects due to nearby charged amino-acid residues, hydrogen bonding, and geometric constraints imposed by the protein. 

Heterogeneous electron transfer reactions have been realized with more than 50 different proteins, mainly electron transferases, and also substrateconverting oxidoreductases. At bare metal electrodes irreversible adsorption accompanied by denaturation prevents a fast electron transfer to the protein molecules. Adsorption of modifiers that promote an appropriate orientation of the protein results in a facilitated direct electron transfer with different redox enzymes, for example, cytochromes and ferre-doxins. 

Electron-transferases Transferases Kinases Transaminases Indicated species Cosubstrates NAD(P)H O2/H2 Mediators Products Products ... 

T. Matsue, H. Yamada, H. Chang, I. Uchida, K. Nagata, and K. Tomita, Electron transferase activity of diaphorase (NADH acceptor oxidoreductase) from Bacillus stearothermophilus, Biochim. Bio-phys. Acta 1038 29 (1990). 

Hence, the term electron transferases as contrasted with oxidases appears preferable over dehydrogenases for the blue radical flavoproteins (80). 

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