Electron transfer copper nitrite reductase

The blue protein from A. faecalis strain S-6, which was isolated as a requirement for transferring electrons to a copper-containing nitrite reductase, has since been shown to have sequence homology with proteins arbitrarily designated pseudoazurin by Ambler and Tobari (1985), from Achromobacter cycloclastes and from Pseudomonas AMI. [Pseudomonas AMI also produces amicyanin, which is the recipient of electrons from methylamine dehydrogenase, (see below)]. In A. cycloclastes reduced pseudoazurin donates electrons to a copper nitrite reductase (Liu et ai, 1986), as it does in A. faecalis. Ambler and Tobari (1985)... 

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. 

Two major pathways have been shown to exist in nitrite reduction [274]. In the first pathway, nitrite is reduced to NO, while in the second there is a direct conversion of nitrite to NH3 or NH4" ". Two classes of nitrite reductase (NIR), namely the cytochromes cd [274], and the copper nitrite reductase [274], have been identified for the first pathway and two classes of enzyme, namely the siroheme nitrite reductase and cytochrome c nitrite reductase, have been proposed to follow the second pathway. The mechanism of these four enzymes has been recently reviewed [274], and only a brief summary of the electron-transfer reactions of cytochrome cd nitrite reductase will be given here. The initial step in the conversion of NO2 to NO involves a binding of the nitrite ion to the metal of the reduced heme. This first step is followed by the uptake of two protons and the loss of one water molecule to yield an electrophilic ferrous pe +-NO+ species, also formulated as a pe +-NO" complex. The dissociation of NO from this species produces the ferric heme d, which is in turn reduced back to its original state by heme c. Why the eri2yme does not reduce the nitrosyl species, Fe -]s[0 or Fe -NO to its Fe -NO form, prior to dissociation of NO in the heme, has been discussed in the literature [274], and may... 

Fig. 6.9 The catalysts for denitrification. Nitrate is reduced by a molybdenum enzyme while nitrite and oxides of nitrogen are reduced today mainly by copper enzymes. However, there are alternatives, probably earlier iron enzymes. The electron transfer bct complex is common to that in oxidative phosphorylation and similar to the bf complex of photosynthesis, while cytochrome c2 is to be compared with cytochrome c of oxidative phosphorylation. These four processes are linked in energy capture via proton (H+) gradients see Figure 6.8(a) and (b) and the lower parts of Fig. 6.9 which show separately the active site of the all iron NO-reductase, and the active site of cytochrome oxidase (02 reductase).

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. 

Figure 14 Similarity between the putative electron-transfer routes to and from the type 1 copper sites in plastocyanin and nitrite reductase. (Reproduced with permission of Chapman and Hall from J. Sanders-Loehr, in Bioinorganic Chemistry of Copper , ed. K.D. Karlin and Z. Tyeklar, 1993, p. 51) ...

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. 

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... 

While nitrite reductases in many bacteria are haem proteins, some are copper-containing homotrimers which bind three type I and three type II copper centres The type 1 copper centre serves to transfer electrons from donor proteins to the type 2 centre which has been proposed to be the site of substrate binding. 

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