Hemocyanin geometry

We also have been working for several years with Sepioteuthis lessoniana hemocyanin to clarify the structure of the environment of the coppers. Spectroscopic investigations led us to conclude that the two Cu(II) ions are each in a nonequivalent coordination geometry of very low symmetry (14). 

Figure 7. Structural aspects of purple hemocyanin. Coppers in the circles marked with dots illustrate different coordination geometry from those in the black circles, and shaded parts indicate the slightly deformed active site of oxyhemocyanin.

The structure of the ascorbate oxidase tricopper site is illustrated in Fig. 45. The three copper atoms form an almost equilateral triangle of sides ca. 3.7 A. The Cul and Cu2 atoms are bridged by OH" or O " and make up the EPR-silent type 3 pair each copper atom is coordinated to three histidine residues and the Cu—N(His) distances are all comparable and unexceptional. In contrast to the hemocyanin active site (Section IVD), the copper ions have approximately tetrahedral coordination geometry and are not in identical environments. The third copper ion is coordinated to two histidine residues and to either hydroxide or water. There is no evidence for a fJ-s-OH or fiyO donor at the center of the cluster (and the Cu—Cu distances are too long to support such a bridge). 

While the very rigid and, with respect to the type of donor groups and their geometric disposition (two trans-disposed pyridine donors and two cis-oriented tertiary amines), enforced and inflexible geometry precludes an accurate structural and spectroscopic modeling of copper proteins, it was especially feature (3.) that lead to the isolation and characterization of novel model complexes with hemocyanine- and catechol oxidase activities properties (81, 192, 196, 213). In the latter case, it was possible to isolate and structurally characterize complexes with coordinated catechol model substrates with structural features, which have been proposed to be of relevance in the enzyme catalysis cycle, but have not been observed before in low molecular weight complexes (192, 213). 

Figure 5) and the copper-copper distance is 3.6 A. The geometry of the type-3 copper site in tyrosinase is very similar to that of hemocyanin due to the similar spectroscopic features and has now been derived from the X-ray structure of a bacterial tyrosinase. ... 

There are three reasonable combinations of metal oxidation states for oxidized Type 3 copper that are consistent with spectral and redox data (1) Cu(I) Cu(I) with some other group, e.g., disulfide, functioning as a two-electron acceptor (2) Cu(I)-Cu(III) where Cu(III) is low spin and (3) an antiferromagnetically coupled Cu(II)-Cu(II) dimer. Magnetic susceptibility studies on Rhus vernicifera laccase have established that the two Type 3 copper atoms in this enzyme are present as an antiferromagnetically coupled Cu(II) dimer (4). The Type 3 copper atoms of hemocyanin and tyrosinase appear to be similarly coupled and separated by 3-5 A (5,6,7). Further structural information on the Type 3 copper chromophore is scanty neither the identity of the ligands nor the geometry of the site has been ascertained. There is likewise a paucity of literature on binuclear copper complexes that exhibit structural features expected for Type 3 copper. 

Hemocyanin Tyrosinase Ribonucleotide reductase ° Ferridoxin Andrenodoxin Rubredoxin Blue copper Transferrins and other Fe(III)-tyrosinate proteins " transfer transition dination geometry of metal ligands... 

Importantly, differences in the metal-Ugand bonding to the two tertiary amine donors have been shown to be of importance for the tuning of the spin state of the ferryl complexes (see Section 6.4) [13g] and the redox potentials and catalytic activities of Cu couples [21] the bispidine-derived geometry is particularly well suited for the Jahn-Teller active Cu" ion and has led to a rich Cu coordination chemistry [22] with interesting applications in bioinorganic model chemistry (hemocyanin [23], catecholase[24]), catalytic aziridination [21a,... 

Amino acid analyses of a variety of hemocyanins indicate that a large amount of histidine and methionine per copper pair is present as well as cysteine, although the number involved in disulfide bridges has not been determined. Intuitively, three types of donor atoms are likely to be involved in these protein complexes, namely, oxygen (carboxylate, phenolate, and water), nitrogen (amine, amide anion, and imidazole), and sulfur (thioether and thiolate). Furthermore, copper (II) can adopt square-planar, square-pyramidal, trigonal-bipyramidal, octahedral, and tetrahedral geometries. 

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