Some metalloproteins

This purpose of this book is to teach, so the structural methods have been illustrated with simple examples. In this chapter of case histories, we look at more complicated systems, which require the use of information from several techniques, but again we have mainly chosen fairly small molecules so that the key issues can be easily comprehended. But the same methods are used throughout inorganic chemistry, and there are some incredibly complicated problems to be solved. 

Metalloproteins are a diverse and fascinating group of compounds, some of them of extreme importance to sustain life. They have one or more metal atoms at the active site, surrounded by the rest of the protein, which holds the reagents in the right place(s) for reaction to be catalyzed. In the two examples that we discuss here there must be a whole series of reactions, so there are several metal atoms, and the stmcture is not static but changes during the sequence of reactions. 

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This discussion of copper-containing enzymes has focused on structure and function information for Type I blue copper proteins azurin and plastocyanin, Type III hemocyanin, and Type II superoxide dismutase s structure and mechanism of activity. Information on spectral properties for some metalloproteins and their model compounds has been included in Tables 5.2, 5.3, and 5.7. One model system for Type I copper proteins39 and one for Type II centers40 have been discussed. Many others can be found in the literature. A more complete discussion, including mechanistic detail, about hemocyanin and tyrosinase model systems has been included. Models for the blue copper oxidases laccase and ascorbate oxidases have not been discussed. Students are referred to the references listed in the reference section for discussion of some other model systems. Many more are to be found in literature searches.50... 

These masked enolate complexes also react smoothly with carbonyl compounds, in reactions surprisingly reminiscent of the reaction of dimedone with formaldehyde (Fig. 5-13). The dinuclear products are of some interest as models for the dinuclear sites of some metalloproteins. 

Compounds that combine crown ethers with porphyrins have been developed in order to mimic the binuclear metal binding sites of some metalloproteins. 

Zinc was demonstrated to be an essential trace element for eukaryotes in 1869 it has subsequently been shown to be essential for all life forms. It generally plays either a catalytic or a structnral role in some metalloproteins that interact with DNA, zinc appears to play both roles. It is an essential constitnent of many enzyme systems, as first demonstrated, for carbonic anhydrase, in 1940. Several hundred zinc metalloenzymes and metaUoproteins have now been recognized, in a wide range of organisms. It generally functions at or near the active site, particnlarly in catalysis of... 

Although we have focused on individual structural units in rubredoxins, ferredoxins and HIPIPs, we should note that some metalloproteins contain more than one Fe S unit. For example, the ferredoxin isolated from Azotobacter vinelandii contains both [4Fe S] and [3Fe-4S] units, with the closest Fe-"-Fe separation between units being i930 pm. 

In addition to DNA, which is known to form a variety of knots, i these structures are found in simpler synthetic molecules. Recently, virtual knots have been recognized in the architecture of some metalloproteins, once the disulfide bridges and metal bonds have been taken into account.i ... 

The following sections show the structure and functions of some metalloproteins and metalloenzymes in more detail in order to explain the interaction of the metal centers with the macromolecular protein environments. 

For substitution of diamagnetic Mg ions by paramagnetic Mn ions, the perturbation of the system is ahnost not perceptible, as evidenced by some metalloproteins that function with both of these ions. 

Nonheme ferrous centers in some metalloproteins react reversibly with NO forming nitrosyl complexes with S = 312 characterized by the g values of about 4.0 and 2.0 [51]. The EPR spectrum of the nitrosylated NorR (abacterial NO-responsive transcription factor, the enhancer binding protein) is typical of a d high-spin Fe NO", where the S = 5/2 iron is antiferromagnetically coupled to the NO (Fig. 5, [52]). This is confirmed by the X-ray, resonance Raman, MCD, Mossbauer spectroscopies, and DFT calculations. Similar structures were proposed for the classical complexes, [Fe(NO)(l-isopropyl-4,7-(4-ferf-butyl-2mercaptobenzyl)-l,4, 7-triazacyclononane)], [53], Fe(EDTA)NO [54—56], the brown-ring compound, Fe(H20)5N0 [57], and for the Fe(N/V ,N -trimethyl-l,4,7-triazacyclononane) (N3)2N0 [54]. Interestingly, for the latter a spin equilibrium between the valence tautomers 5=1/2 and 3/2 in the solid state was observed. 

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