Dinuclear site Enzymatic

For the hydrolysis of phosphate esters under mild conditions, metal ions and metal complexes are the most efficient nonenzymatic reagents currently available. However, they do not reach the catalytic efficiency of enzymes, and higher reactivities are desirable in view of applications. To mimic enzymatic dinuclear sites is a strategy to generate more efficient artificial phosphoesterases. 

Cyclic voltammetry has been also used for estimation of the rate constants for oxidation of water-soluble ferrocenes in the presence of HRP (131). There is a perfect match between the data obtained spectrophotometrically and electrochemically (Table IV), which proves that the cyclic voltammetry reveals information on the oxidation of ferrocenes by Compound II. It is interesting to note that an enzyme similar to HRP, viz. cytochrome c peroxidase, which catalyzes the reduction of H202 to water using two equivalents of ferrocytochrome c (133-136), is ca. 100 times more reactive than HRP (131,137). The second-order rate constant equals 1.4 x 106 M-1 s 1 for HOOCFc at pH 6.5 (131). There is no such rate difference in oxidation of [Fe(CN)e]4- by cytochrome c peroxidase and HRP (8). These comparisons should not however create an impression that the enzymatic oxidation of ferrocenes is always fast. The active-R2 subunit of Escherichia coli ribonucleotide reductase, which has dinuclear nonheme iron center in the active site, oxidizes ferrocene carboxylic acid and other water-soluble ferrocenes with a rate constant of... 

The first bona fide MnRNR, isolated from Corynebacterium (formerly Brevibacterium) ammoniagenes, was reported about 10 years ago [122], The MnRNR R1 protein is monomeric (dimeric in E. coli) [123]. A dinuclear Mnm—0—Mnm unit was believed to be present at its active site (cf. oxidized E. coli RNR), and its sensitivity to hydroxyurea (a radical scavenger) suggested a mechanism similar to that of its iron counterpart. However, no tyrosyl radical EPR signal was observed from the MnRNR, casting doubts about the redox role of the manganese center [118], Very recently, however, a stable EPR signal, which is inhibited by hydroxyurea and correlates directly with enzymatic activity, has been detected [124],... 

The structures of type II copper sites have not as yet generated as much interest, but the hetero-dinuclear structures of type IIC copper sites and the unusual protein donor ligands found in type IIB copper sites are noteworthy. The synthetic model approach to gain insight into the structures and functions of these types of copper sites will be also described. The diverse functions of a variety of proteins containing type IIA copper sites inspired many chemists to mimic the functions with synthetic copper complexes, even though the spectroscopic properties of the complexes are not unusual. Results of these studies will be reviewed and different aspects of the reactions relating to enzymatic catalysis will be discussed. 

Iron is oxidized for incorporation into the mineralized core by either a protein enzymatic mechanism involving a putative dinuclear Fe ferroxidase site on the H chain subunit or a mineral surface mechanism. The net stoichiometric reactions for the two kinetic pathways are given by the following equations ... 

In this chapter, the dioxygen activation mechanism at the dinuclear copper-active sites of tyrosinase and catechol oxidase has been surveyed. In both enzymes, a (ji-rfirf -peToxo) dicopper(II) complex has been detected and characterized as a common reactive intermediate by several spectroscopic methods. In spite of longstanding efforts in the enzymological studies, mechanistic details of the enzymatic reactions (phenolase and catecholase activities) still remain ambiguous. On the other hand, recent developments in the model chemistry have provided a great deal of information about the structure and physicochemical properties as well as the reactivity of the peroxo intermediate and have advanced our understanding of the enzymatic reactions. 

In order to generate more structurally relevant biomimetics for dinuclear metallohydrolases much effort has been devoted to the synthesis of asymmetric ligands. These ligands are considered to be more suitable models for the asymmetric coordination environment found in enzymatic systems. Nordlander et al. proposed that asymmetric complexes are not only more appropriate functional models for the active site of phosphoesterase enzymes, but also that they exhibit enhanced catalytic rates compared with their symmetric counterparts [1-3]. A selection of ligands used to generate purple acid phosphatase [1, 4, 5, 6-10], phosphoesterase [11], urease [12, 13], catechol oxidase [14] and manganese catalase biomimetics [15, 16] is displayed in Fig. 7.1. 

---