Two Metal Sites

Probably all adenylyl cyclases are inhibited competitively by substrate analogs, which bind at the site and to the enzyme configuration with which cation-ATP binds (cf Fig. 4). One of the best competitive inhibitors is (3-L-2, 3 -dideoxy adenosine-5 -triphosphate ( 3-L-2, 3 -dd-5 -ATP Table 4) [4], which allowed the identification of the two metal sites within the catalytic active site (cf Fig. 4) [3]. This ligand has also been labeled with 32P in the (3-phosphate and is a useful ligand for reversible, binding displacement assays of adenylyl cyclases [4]. The two inhibitors, 2, 5 -dd-3 -ATP and 3-L-2, 3 -dd-5 -ATP, are comparably potent... 

Two metal sites were identified in the structure solved of the chimeiic AC5Ci AC2C2 adenylyl cyclase in complex with 3-L-2, 3 -dd-5 -ATP (cf. Figs. 5 and 7) One is... 

Figure 1. Relationship among the crystal structures of Re3B and Mo2lrB2. Full circles are atoms in 1/2 open circles are atoms in 0 the two metal sites in Re3B (8f and 4b) are differentiated by larger and smaller circles smallest circles are B atoms. The structure of Mo2lrB2 is generated by a shift of every second prism row of Rc3B (vector c/4).

Most of the mixed-valence systems mentioned by Robin and Day and by Hush were in the solid state. The problem of creating discrete chemical systems for which experiments could be carried out either in solution or in the solid state was first attacked experimentally by Creutz and Taube (6). Their approach was to link together the two metal sites through a ligand bridge, which led to dimers and oligomers. 

Figure 6. Illustration of exchange interactions in homovalent system consisting of two metal sites A and B. The system contains two electrons. The six distinct microstates are indicated on Ae left. The antiferromagnetic contribution results from mixing an excited state ionic configuration wiA Ae ground state singlet.

The information that there is a high probability that the metal lies in the carboxyl plane, but with possible deviations for specific metals, provides a mechanism for searching for metal-binding positions in proteins. This method was used in a study of the enzyme xylose isomerase from Streptomyces rubiginosus (Carrell et al., 1989). Two metal sites were located. One metal-binding site involves three carboxylates (aspartate and glutamate), histidine, and water, and the other involves four carboxylate... 

A combined crystallographic and u3CdNMR study of the complex [Cd(bipy)2(N03)2]-iH20 in the solid state and in solution has been described the crystal structure reveals two crystallographically and chemically distinct octahedral environments for the metal. In one case, the metal has two monodentate O-donor nitrate ions coordinated (Cd—N, 2.33-2.43 A Cd—O, 2.41-2.435 A), and in the other it is coordinated to one monodentate nitrate ion and to one water molecule (Cd—N, 2.34-2.39 A Cd—0N02, 2.43 A Cd—OH2, 2.246 A). The solid state U3CdNMR investigations also indicated the presence of two metal sites.51... 

The organometallic chemistry of alkynylcyclopropanes involves primarily the formation and reactions of carbon-metal er-bonds. Metals come essentially from the main group elements, with lithium playing a major role. The two metallation sites are the cyclopropyl and the acetylenic positions, which are expected to differ considerably in their acidity values (t-butylacetylene, pKa = 25230, cyclopropane, pKa = 46183) but less in the reactivity of their metal conjugated bases towards electrophiles. 

Intervalence charge transfer Electron transfer (thermal or photoinduced) between two metal sites differing only in oxidation state. Quite often such electron transfer reverses the oxidation states of the sites. The term is frequently extended to the case of metal-to-metal charge transfer between non-equivalent metal centers. 

The amino acid residue that bridges the two-metal sites is shown in italic boldface. When the symbol H2O is given this may represent from one to three metal-bound water molecules. R is the distance between the metal atoms. When Trp is listed as a ligand, the a-amino and a amide carbonyl are the ligands. See footnote of Table 1 for the definitions of other terms. 

Fig. 27. NMR shift of the H/metal signal on EuroPt-1 (peak fi in Fig. 12) for different coverages (a) or under different pressures (b). Compare the knee in Fig. 27a with that in Fig. 26b. The lines are a fit to a three-site rapid-exchange model. At low coverage, only one site on the metal is occupied (horizontal part in Fig. 27a). At intermediate values there are two metal sites with different shifts (curved part in Fig. 27a). and at high pressure there is a third contribution from hydrogen in the gas phase (right-hand part of F ig. 27b). [Reproduced with permission from Chesters el al (48). Copyright 1996 Royal Society of C hcmistry.j...

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