Azurin bond lengths

The salient features of A. faecalis pseudoazurin are that (1) it has a Cu-Met bond length shorter than that of either plastocyanin or azurin (see Table III) (2) it has only one NH - S bond, as does plastocyanin and (3) its overall architecture resembles plastocyanin (see Fig. 4), with an extended carboxy terminus folded into two a helices [a preliminary sequence comparison suggested that the folding would resemble plastocyanin (Adman, 1985)]. It retains the exposed hydrophobic face found in azurin and plastocyanin. Just how it interacts with nitrite reductase is still a subject of investigation. It is intriguing that the carboxy-terminal portion folds up onto the face of the molecule where the unique portions of other blue proteins are the flap in azurin, and, as we see below in the multi-copper oxidase, entire domains. 

Unfortunately, bond lengths have not been reported for the copper center of Cbp. Its spectrum (like that of plastocyanin) is much more like that of the A. faecalis pseudoazurin than azurin. Since little variability of the Cu-Sy bond has been seen in the three structures described above, and since the major difference between pseudoazurin and plastocyanin (or azurin) is the length of the Cu-Met S8 bond, this would suggest that the Cu-Met bond is short in this protein, as well. Its EPR is also rhombic, again, like that of pseudoazurin. 

The mononuclear copper site is located in domain 3 and has the four canonical type-1 copper ligands (His, Cys, His, and Met) also found in plastocyanin and azurin. It is coordinated to the NDl atoms of His 445 and 512, the SG atom of the Cys 507, and the SD atom of Met 517 in a distorted trigonal pyramidal geometry. The SD atom is at the long apex (see Fig. 4). Bond lengths of the type-1 copper for both subunits are displayed in Table III. They are compared with oxidized poplar plastocyanin (95) and azurin from Pseudomonas aeruginosa (96). Figure 5 shows an overlay of the type-1 copper site in azurin, plastocyanin, and ascorbate oxidase. 

Table 6. A comparison of copper ligand bond lengths and angles in Pcy, Paz, Acy and Azurin ...

The reorganization energies for ET between the dinuclear Cua center and cytochrome a in cytochrome c oxidase have been estimated to be between 0.15-0.5 eV from ET experiments. This estimate means that the A for the Cua center is even lower than that for the mononuclear cupredoxin center. An electron transfer study of an engineered Cua azurin showed that the calculated A for Cua in the engineered azurin is 0.4eV, which is about half that of the blue copper center in native azurin. " Furthermore, an excited-state distortion analysis of a resonance Raman enhancement profile for Cua also confirmed that the inner-sphere A for Cua is about 50% that of a blue copper center., i7,ioi lower A for Cua can be attributed to the valence delocalization of the dinuclear center the bond length distortions associated with the redox reaction can be spread over two metal ions and thus reduced to 1/2 that of the mononuclear copper or valence-trapped dinuclear centers. 

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