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PubMed1992

Evidence that the type 2 copper centers are the site of nitrite reduction by Achromobacter cycloclastes nitrite reductase.

Methods have been developed for selective depletion and reconstitution of the Type 2 Cu (non-blue) sites in the nitrite reductase from A. cycloclastes, resulting in preparations ranging from 0.5 to 2.6 Type Cu per trimer; the Type 1 Cu content is invariant at 3.0 per trimer. The activity of the enzyme is directly proportional to the Type 2 content as measured by direct metal determination or by analysis of the EPR spectra. These results indicate that an earlier report that the A. cycloclastes enzyme contains only Type 1 Cu sites is incorrect, and that the Type 2 Cu centers constitute the site at which NO2- is reduced. Furthermore, they suggest that other Cu nitrite reductases that are reported to contain only Type 1 Cu sites and exhibit relatively low activity may actually be largely Type 2 Cu-depleted forms of the enzymes.

Alcaligenes↗
PubMed2002

Effect of histidine 6 protonation on the active site structure and electron-transfer capabilities of pseudoazurin from Achromobacter cycloclastes.

The paramagnetic (1)H NMR spectrum of Cu(II) pseudoazurin [PACu(II)] contains eight directly observed hyperfine-shifted resonances which we have assigned using saturation transfer experiments on a 1:1 mixture of PACu(I) and PACu(II). The spectrum exhibits a number of similarities to those of other cupredoxins, but differences are found concerning the Cu-S(Met) interaction. The spectrum is dependent on pH* in the range 8.5-4.5 (pK(a)* 6.4), and a conformational change involving movement of the copper ion away from the Met toward the equatorial ligands, as a consequence of protonation of the surface His6 residue, is identified. Corresponding changes are also seen in the UV/vis spectrum. The protonation/deprotonation equilibrium of His6 influences the reduction potential of the protein in the same pH range. The self-exchange rate constant of PACu at pH* 6.0 (25 degrees C) is considerably smaller (1.1 x 10(3) M(-1) s(-1)) than the value obtained at pH* 7.6 (3.7 x 10(3) M(-1) s(-1)). The effect on the self-exchange reactivity is mainly due to an alteration in the reorganization energy of the copper site brought about by the structural change resulting from His6 protonation.

Alcaligenes↗