Nitrite reductase copper-binding sites

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,... 

Intriguingly, the blue copper sites, especiaUy those with a carbonyl oxygen at the axial coordination position, display high affinity for Zn + ions. Mutants in which the Met is replaced by Gin or Glu preferentiaUy bind Zn + when expressed in heterologous systems, e.g., Escherichia coli. Examples include azurin, amicyanin, nitrite reductase, and possibly also plastocyanin (Diederix et al., 2000 Hibino et al., 1995 Murphy et al., 1995 Nar et al., 1992a Romero et al., 1993). In the case of azurin it has been shown that both wild-type and the Met—Gin mutant have the same affinity for both Zn +and Cu + (Romero ci a/., 1993). In addition, EXAFS studies showed that some preparations of blue copper proteins purihed from their natural sources also contain small fractions of Zn derivatives (DeBeer George, personal communication). 

Dodd, F. E., van Beeumen, J., Eady, R. R., and Hasnain, S. S., 1998, X-ray structure of a blue copper nitrite reductase in two crystal forms. The nature of the copper sites, mode of substrate binding and recognition by redox partner, J. Mol. Biol. 282 3699382. 

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. 

Direct reaction of NO with enzymes has been shown for cytochrome c oxidase (cyt c oxidase). The reaction of NO with the binuclear metal centre of cyt c oxidase apparently leads to the formation of nitrite at the active site [123] the mechanism of which was described as the opposite of nitrite reduction to NO by non-haem nitrite reductases [124]. The inhibition was caused by the binding of NO to the reduced copper centre of the enzyme rather than the expected reaction with Fe. ... 

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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