Substitution in Metalloproteins

The rate constants and k represent rate constants for a surface reaction and have units m mol s and s respectively. The accelerative effects are about 10 -10 fold. They indicate that both reactants are bound at the surface layer of the micelle (surfactant-water interface) and the enhanced rates are caused by enhanced reactant concentration here and there are no other significant effects. Similar behavior is observed in an inverse micelle, where the water phase is now dispersed as micro-droplets in the organic phase. With this arrangement, it is possible to study anion interchange in the tetrahedral complexes C0CI4 or CoCl2(SCN)2 by temperature-jump. A dissociative mechanism is favored, but the interpretation is complicated by uncertainty in the nature of the species present in the water-surfactant boundary, a general problem in this medium. 

If the polyelectrolyte can coordinate strongly to a metal ion, marked deceleration effects can be noted, as, for example, in the reactions of Ni + and Co + with pad in the presence of polyphosphates. Modifications of equilibria constants in these micelles must also be recognized as contributing to rate change, e. g., ligand pK or keto-enol equilibria may be altered. 

Undoubtedly the most complicated mileau for a substitution process is that of a protein. However, the principles developed in this chapter for substitution in metal complexes also apply to metalloproteins. Allowance for a role for the protein, particularly near the site, must always be made. The formation and dissociation of a metalloprotein (PM) may be represented in an undoubtedly simplified form as  

Multisites in proteins are not uncommon. The removal of metal ions from such centers is likely to be involved. This is in fact illustrated by the iron removal from serotransferrin, see also Sec. 2.6. This protein is bilobal and each lobe contains an iron-binding site. These are 35 A apart and it is believed that direct interaction between the sites is absent. The two Fe s, designated a and b, are different and their removal is biphasic, although not markedly so. 

Simultaneous rate equations are complex, but solvable. Simplifications are possible e. g. A l = 4 and 2 = 3 if the sites are non-cooperative. Strong binding ligands such as edta or synthetic sidereophores effect iron removal and the two rate constants associated with the biphasic Fe removal are both curved towards saturation when plotted against [ligand]. 

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Five proteins containing molybdenum are known nitrate reductase, nit-rogenase, xanthine oxidase, aldehyde oxidase and sulphite oxidase. They also contain iron, and the first four are best classified as multi-enzyme systems. Early studies on xanthine oxidase used a number of important ESR techniques, particularly rapid freeze kinetic methods and isotopic substitution in metalloproteins. This work has been reviewed [38, 39], Nitrogenase is the subject of considerable recent interest since it contains detectable iron-sulphur centres but as there is some disagreement at present concerning the interpretations of the results readers are referred to the original literature [40-42]. 

Table 1 Trans-substitution of metal ligands in metalloproteins.

Finally, several other metal-substituted zinc metalloprotein derivatives have been prepared, including those of VO, Fe(II), Co(III), Pt(II), and HgCl2. Although these systems add little directly to our understanding of the relationship between structure and function of the enzymes, nonetheless they represent new bioinorganic compounds and are of interest in themselves, or can add information on the coordinating capabilities, and reactivity in general, of the residues present in the active cavity. 

A detailed analysis of Ni11 complexes with mew-substituted porphyrins bearing zero, one, two, or four /-butyl groups revealed that both the out-of-plane and in-plane distortion depend on the perturbation symmetry of the peripheral substituents (number and position of substitutents), and their orientation.1775 These results have implications for understanding the role of nonplanar distortions in the function of metalloproteins containing nonplanar porphyrins.1776... 

In general, photolysis induces substitutional and redox-related changes, whereas pulse radiolysis primarily promotes redox chemistry. Indeed one of the unique features of the latter method is to induce unambiguous one electron reduction of multi-reducible centers. Metalloproteins can be rapidly reduced to metastable conformational states and subsequent changes monitored. 

The substitution process permeates the whole realm of coordination chemistry. It is frequently the first step in a redox reaction and in the dimerization or polymerization of a metal ion, the details of which in many cases are still rather scanty (e.g. for Cr(III) ). An understanding of the kinetics of substitution can be important for defining the best conditions for a preparative or analytical procedure. Substitution pervades the behavior of metal or metal-activated enzymes. The production of apoprotein (demetalloprotein and the regeneration of the protein, as well as the interaction of substrates and inhibitors with metalloproteins are important examples. ... 

Redox reactions usually lead, however, to a marked change in the species, as reactions 4-6 indicate. Important reactions involve the oxidation of organic and metalloprotein substrates (reactions 5 and 6) by oxidizing complex ions. Here the substrate often has ligand properties, and the first step in the overall process appears to be complex formation between the metal and substrate species. Redox reactions will often then be phenomenologically associated with substitution. After complex formation, the redox reaction can occur in a variety of ways, of which a direct intramolecular electron transfer within the adduct is the most obvious. 

As with any metalloprotein, the chemical and physical properties of the metal ion in cytochromes are determined by the both the primary and secondary coordination spheres (58-60). The primary coordination sphere has two components, the heme macrocycle and the axial ligands, which directly affect the bound metal ion. The pyrrole nitrogen donors of the heme macrocycle that are influenced by the substitutents on the heme periphery establish the base heme properties. These properties are directly modulated by the number and type of axial ligands derived from the protein amino acids. Typical heme proteins utilize histidine, methionine, tyrosinate, and cysteinate ligands to affect five or six coordination at the metal center. 

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