Ligands tyrosinase

Several diverse metal centres are involved in the catalysis of monooxygenation or hydroxylation reactions. The most important of these is cytochrome P-450, a hemoprotein with a cysteine residue as an axial ligand. Tyrosinase involves a coupled binuclear copper site, while dopamine jS-hydroxylase is also a copper protein but probably involves four binuclear copper sites, which are different from the tyrosinase sites. Putidamonooxin involves an iron-sulfur protein and a non-heme iron. In all cases a peroxo complex appears to be the active species. 

Copper(II) complexes with phenoxo ligands have attracted great interest, in order to develop basic coordination chemistry for their possible use as models for tyrosinase activity (dimeric complexes) and fungal enzyme galactose oxidase (GO) (monomeric complexes). The latter enzyme catalyzes the two-electron oxidation of primary alcohols with dioxygen to yield aldehyde and... 

Evidence tom a variety of sources indicates that the active site of tyrosinase is very similar to that of hemocyanin, a dioxygen-binding protein found in molluscs and arthropods (15,16). This type of active site contains two copper ions, which are cuprous in the deoxy state, and which reversibly bind dioxygen, forming the oxy form of the enzyme or protein in which a peroxy ligand bridges between two cupric ions. 

The veiy different reactivities of hemocyanin and tyrosinase toward oxidizable substrates se n to be due to the presence of a substrate binding site in the latter. It appears, therefore, that the oxygenation of the phenol substrate occurs either by reaction of copper-bound pooxide or hydrc ieroxide with the ortho position of the copper-bound phenol (16) or that the 0-0 bond of the peroxide ligand is cleaved... 

The first of two conversions catalyzed by the metalloenzyme tyrosinase is the selective ortho-hydroxylation of a phenol moiety, yielding catechol. Few synthetic systems exist that are capable of selectively performing the same monooxygenase reaction [172-178]. The group of Casella has studied a series of dicopper complexes based on 2,6-disubstituted benzene hgands providing two N3 compartments [179-181] (26a,b) as well as the asymmetrically substituted tripodal amine-based ligand (27) [182]. 

Stack and co-workers recently reported a related jx-rf / -peroxodi-copper(II) complex 28 with a bulky bidentate amine ligand capable of hydroxylating phenolates at - 80 °C. At - 120 °C, a bis(yu,-oxo)dicopper(III) phenolate complex 29 with a fully cleaved 0-0 bond was spectroscopically detected (Scheme 13) [190]. These observations imply an alternative mechanism for the catalytic hydroxylation of phenols, as carried out by the tyrosinase metalloenzyme, in which 0-0 bond scission precedes C - 0 bond formation. Hence, the hydroxylation of 2,4-di-tert-butylphenolate would proceed via an electrophilic aromatic substitution reaction. 

Fukuzumi and co-workers described spectroscopic evidence for a ix-rf- ] -peroxo-(Cu )2 species stabilized with a fcidentate nitrogen ligand, but no (catalytic) oxidation behavior towards catechol was noted (a related trinu-clear copper species converted 2,4-di-ferf-butylphenol stoichiometrically towards the biphenol derivative) [224], Stack et al. have described a similar ] -peroxo-(Cu )2 species (28, vide supra) that could be considered a structural and functional model for tyrosinase-activity, as it efficiently reacted with catechol, benzyl alcohol and benzylamine to yield quinone (95%), benzaldehyde (80%) and benzonitrile (70%) [172,173]. This dinuclear per-0X0 species is generated by association of two monomeric copper centers, in contrast to the systems based on dinucleating Ugand scaffolds described above. 

Many metalloproteins contain more than one metal center. Tyrosinase, for example, has a dinuclear Type 3 (T3) copper active site which, in its oxidized form, comprises two Cu(II) centers each held by three histidine groups with a p-r 2 r 2 peroxido bridging ligand (Fig. 22). 

Figure 92 (a) Structural mechanism for the hydroxylation of monophenolic substrates by oxytyrosinase (b) reaction coordinate diagram for associative ligand substitution at the copper site of tyrosinase... 

Copper ligands of the recombinant tyrosinase from A. oryzae expressed in Saccharomyces cerevisiae or Escherichia coli were identified by site-directed mutagenesis [145],... 

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