Dinuclear site activation

As a part of their efforts to model dinuclear copper active sites, Reed and co-workers reported, using alkoxo-based dinucleating ligands, a few very interesting systems from the viewpoint of magnetostructural correlations (379) (TBP Cu-Cu 3.325A 2.1 278cm ), (380) (TBP ... 

The minimal functional module in [NiFe] hydrogenases always contains the NiFe(CN)2(CO) site plus the proximal [4Fe-4S] cluster. The active site in [Fe] hydrogenases consists of the Fe-Fe site linked to a [4Fe-4S] cluster. Oxidation of the hydride is either an action of the dinuclear site alone, or a concerted action of this site plus the proximal cluster. 

Usually, these metalloproteins contain both type 2 and type 3 copper centers, together forming a triangular-shaped trinuclear active site, such as found in laccase (polyphenol oxidase) [38-41] and ascorbate oxidase (3) [42]. Recent evidence for a related arrangement has been reported for the enzyme particulate methane monooxygenase as well [43], but in this case the Cu Cu distance of the type 2 subunit (2.6 A) appears to be unusually short and the third Cu ion is located far from the dinuclear site. 

A common theme in this catalysis has been the proposal of dinuclear catalyst active sites, often termed bimetallic catalysis. This section will review various dinuclear and dimeric complexes which have shown activity for copolymerization. 

An additional crucial piece of information emerges from the alloxan-thine study (24). Thus, it was shown that one alloxanthine binds to the enzyme per active molybdenum site. This result clearly implies that the molybdenum site is mononuclear. If a dinuclear site were involved, then it would be unlikely to require two alloxanthine molecules for inhibition and would be expected to be at least partially inhibited with one alloxan-thine/two molybdenum. Also, a difference in binding constant would be expected for the second compared with the first bound alloxanthine, but none is found. This result, coupled with the lack of evidence for Mo(V)-Mo(V) spin-spin interactions in the EPR spectra, clearly implicates a mononuclear site, and it would seem that xanthine oxidase possesses two full catalytic units, each containing one molybdenum, one flavin, and two Fe2S2 units (20). Other molybdenum oxidases also contain paired prosthetic groups and subunits, and it is likely that they each have two catalytic units per molecule. 

The pathway followed during these reactions appears different from that of the Gif system, because use of the same complexes under strict Gif conditions (pyridine-acetic acid-Zn powder) gives a different product ratio the same reactivity and selectivity is obtained for all monomeric and dimeric complexes (15-18). The reactivity observed with 2 is reminiscent of the chemistry of the dinuclear ferrous active site found in MMO, although their mechanistic similarities must still be demonstrated (19, 20). 

The catalases catalyze the disproportionation of hydrogen peroxide (equations). Most catalases contain the iron-protoporphyrin IX prosthetic group see Iron Heme Proteins, Peroxidases, Catalases Catalase-peroxidases). However, some bacteria are able to synthesize catalases that are not inhibited even by millimolar concentrations of azide and cyanide, suggesting that some catalases are nonheme enzymes it is now known that these enzymes possess a dinuclear Mn active site. 

Two zinc enzymes with dinuclear active sites have been characterized, promoting interest in dinuclear zinc model systems. Phospholipase C from Bacillus cereus 103) contains three zinc atoms per subunit. An X-ray crystal structure determination at good resolution (1.5 A) revealed that two of these constitute a dinuclear site with a Zn—Zn distance of 3.3 A. The metal atoms are symmetrically bridged by an aspartate residue and by OH or H2O. Each zinc atom has approximate trigonal bipyramidal geometry with the ligation shown in Fig. 22. The third zinc atom is quite close to the bridged pair. 

FIGURE 15.1 The dinuclear Ni active site of urease. Ni atoms are shown in green, metal-bound water as red spheres the carbamylated Lys is K217. (From Mulrooney Hausinger, 2003. Copyright 2003 with permission from Elsevier.)... 

Iron chelators (type 2) are of medical interest as agents that can block cell proliferation. To what extent this is due to interference with RNR is not well understood. However, recent experiments on the isolated proteins show that an agent such as desferrioxamine can destroy the radical and chelates iron from the active form of the HSVl protein 111), whereas it has no direct effect on the tyrosyl radical of mouse R2 112). However, it seems that the iron in the dinuclear sites without neighboring tyrosyl radical is accessible to desferrioxamine complex-ation in mouse as well as HSVl R2, again suggesting a structural difference between iron sites with and without a neighboring radical. 

The nature of the Ngose reaction is described with respect to electron donation, energy requirement, and reduction characteristics, with particular analysis of the seven classes of substrates reducible by N20se, a complex of a Mo-Fe and Fe protein. Chemical and physical characteristics of Fe protein and crystalline Mo-Fe protein are summarized. The two-site mechanism of electron activation and substrate complexation is further developed. Reduction may occur at a biological dinuclear site of Mo and Fe in which N2 is reduced to NH3 via enzyme-bound diimide and hydrazine. Unsolved problems of electron donors, ATP function, H2 evolution and electron donation, substrate reduction, N20se characteristics and mechanism, and metal roles are tabulated, Potential utilities of N2 fixation research include in-creased protein production and new chemistry of nitrogen. 

The ferroxidase center, important for rapid oxidation of Fe to Fe, was discovered relatively recently in the history of research into the metal sites in ferritins. Ferroxidase activity within H subunits appears to occur at a dinuclear site situated within a four-helix bundle and resembling the dinuclear centers found in ribonucleotide reductase, methane monooxygenase, fatty acid desaturases, and ruberythrin (Chapter 8.11). In bacterioferritins, for which protein crystal structures have been reported for ferritin from Escherichia col and Rhodobacter capsulatus the overall motif of a shell of 24 subunits with relative masses of about 18,500 Da is preserved but there are also 12 protoporphyrin IX heme groups present with unknown function which might have a role in connecting the dimer units and are buried within the shell between identical subunits related by twofold symmetry. In these bacterioferritins the subunits are all identical and contain both ftrroxidase and nucleation sites. 

Dioxygen activation is also accomplished at the dinuclear copper-active sites in tyrosinases and catechol oxidases. Tyrosinases (EC 1.14.18.1) are widely distributed throughout bacteria, fungi, plants, and animals, catalyzing the ortho-hydroxylation of phenols to catechols (phenolase activity. Equation (1)) and the oxidation of catechols to o-quinones (catecholase activity. Equation (2)). 

---