Reductases, copper proteins, electron transfer

Most proteins in the PS-11 complex are membrane spanning, but the three extrinsic proteins that are involved in oxygen evolution are located on the lumenal side of the thylakoid membrane. Although most proteins in the PS-1 complex are also membrane-spanning, a few are located toward the stromal side of the thylakoid membrane, e.g., the iron-sulfur proteins that contain FeS-A and FeS-B and the Fd-docking protein. On the other hand, some mobile electron-transfer proteins are present on one side or the other of the membrane, with Fd present on the stromal side and near the ferredoxin-NADP" -reductase (FNR) and the FNR-binding protein, and the copper-protein electron carrier, plastocyanin (PCy), present on the lumenal side, close to the PS-1 primary donor P700. 

It is interesting to speculate why nitrite reductase has its type I coppers in domains 1, whereas in hCP the mononuclear copper binding sites are retained in the domains 2,4, and 6 where they are comparatively buried in the protein. One possible reason can be related to the difference in functions of the two proteins. NR has to interact with a relatively large pseudo-azurin macromolecule in order for electron transfer to take place,... 

Several copper enzymes will be discussed in detail in subsequent sections of this chapter. Information about major classes of copper enzymes, most of which will not be discussed, is collected in Table 5.1 as adapted from Chapter 14 of reference 49. Table 1 of reference 4 describes additional copper proteins such as the blue copper electron transfer proteins stellacyanin, amicyanin, auracyanin, rusticyanin, and so on. Nitrite reductase contains both normal and blue copper enzymes and facilitates the important biological reaction NO) — NO. Solomon s Chemical Reviews article4 contains extensive information on ligand field theory in relation to ground-state electronic properties of copper complexes and the application of... 

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

Copper-deficient cells of Ps. perfectomarinus give N20 rather than dinitrogen from nitrite. This has led to the interesting suggestion that the dinitrogen monoxide reductase is a copper protein,15361537 but it appears that the properties of the copper protein differ from those of the dinitrogen monoxide reductase.1538 The copper protein may well lie on the electron-transfer pathway to the N20 reductase. 

The constrained nature of the copper center in BCB domains reduces its reorganization energy, which is considered an important feature for their function in long-range electron transfer processes. They are capable of tunneling electrons, usually over 10- to 12-A distances, intramolecu-larly within the same protein (in the case of multicopper oxidases and nitrite reductases) or intermolecularly between a donor and an acceptor protein (in the case of cupredoxins) in a thermodynamically favorable environment. 

Nitrite reductases catalyze both of the reactions below the physiological electron donors are either c-type cytochromes or small blue-copper proteins (eqnations 1 and 2). h28 xhe Type 1 center acts as an electron-accepting site, which then transfers the electron to the Type 2 copper where snbstrate binding and rednction occur. 

The electron carriers in the respiratory assembly of the inner mitochondrial membrane are quinones, flavins, iron-sulfur complexes, heme groups of cytochromes, and copper ions. Electrons from NADH are transferred to the FMN prosthetic group of NADH-Q oxidoreductase (Complex I), the first of four complexes. This oxidoreductase also contains Fe-S centers. The electrons emerge in QH2, the reduced form of ubiquinone (Q). The citric acid cycle enzyme succinate dehydrogenase is a component of the succinate-Q reductase complex (Complex II), which donates electrons from FADH2 to Q to form QH2.This highly mobile hydrophobic carrier transfers its electrons to Q-cytochrome c oxidoreductase (Complex III), a complex that contains cytochromes h and c j and an Fe-S center. This complex reduces cytochrome c, a water-soluble peripheral membrane protein. Cytochrome c, like Q, is a mobile carrier of electrons, which it then transfers to cytochrome c oxidase (Complex IV). This complex contains cytochromes a and a 3 and three copper ions. A heme iron ion and a copper ion in this oxidase transfer electrons to O2, the ultimate acceptor, to form H2O. 

A number of other Cu electron transfer proteins which contain type-1 Cu centres (azurin, cemloplasmin, laccase, nitrite reductase, msticyanin, and stellacyanin) are known. They aU have three coordination positions contributed by 2 His and one Cys, similar to the copper coordination chemistry in plastocyanin — yet they span... 

Robust voltammetry and in situ STM to molecular resolution have been achieved when the Au(lll)-electrode surfaces are modified by linker molecules, Fig. 8-10, prior to protein adsorption. Comprehensive voltammetric data are available for horse heart cyt and P. aeruginosa The latter protein, which we address in the next Section, has in a sense emerged as a paradigm for nanoscale bioelectrochemistry. We address first briefly two other proteins, viz. the electron transfer iron-sulfur protein Pyrococcus furiosus ferredoxin and the redox metalloenz5mie Achromobacter xylosoxidans copper nitrite reductase. 

The physiological electron-donor of nitrite reductase is the type 1 copper protein pseudoazurin, which transfers a single electron to the type 1 copper center of nitrite reductase, from where it is transferred to the type 2 copper... 

Structure (of nitrite reductase from Alcaligenes faecalis). The subunits of nitrite reductase contain two domains. Each domain consists of a /1-barrel structure, similar to that in the small blue proteins [272]. The type 1 copper center is embedded in one of these /1-barrels and is coordinated by the ligands His 95 (domain 1), Cys 136, His 145, and Met 150 (domain 2) [26]. The type 2 copper center is coordinated by a water molecule and three histidine ligands His 100 and His 135 (subunit x) as well as His 306 (subunit y). It is situated at the interface of two subunits [272], The water molecule is displaced from the type 2 copper ion upon nitrite binding (Fig. 34). Although both copper ions are coordinated by neighboring residues, they are located approximately 12.5 A apart, which prevents a direct electron transfer [272]. 

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