Enzymes with Binuclear Active Sites

Ribonucleotide reductase is notable in that its reaction mechanism provides the best-characterized example of the involvement of free radicals in biochemical transformations, once thought to be rare in biological systems. The enzyme in E. coli and most eukaryotes is a dimer, with subunits designated R1 and R2 (Fig. 22-40). The R1 subunit contains two lands of regulatory sites, as described below. The two active sites of the enzyme are formed at the interface between the R1 and R2 subunits. At each active site, R1 contributes two sulfhydryl groups required for activity and R2 contributes a stable tyrosyl radical. The R2 subunit also has a binuclear iron (Fe3+) cofactor that helps generate and stabilize the tyrosyl radicals (Fig. 22-40). The tyrosyl radical is too far from the active site to interact directly with the site, but it generates another radical at the active site that functions in catalysis. 

In the binuclear haem-copper centre of cytochrome oxidases there is no cation radical formed at the active site. Instead the extra positive charge is held by the copper atom as it converts from cuprous (Cu1+) to cupric (Cu2+). In fact there is growing evidence to support the model of Mitchell [56] that it is the protonation steps associated with oxidation state changes in this copper atom (Cub) that provide the link between the electron transfer and proton translocation activities of this enzyme. 

The active site of methionine aminopeptidase contains a binuclear cobalt complex that is required for activity, although a number of divalent metal ions support turnover to varying degrees. X-ray crystallographic studies on the enzyme in complexes with transition state analogs suggests that the binuclear metal cluster serves to stabilize the tetrahedral intermediate in peptide hydrolysis. ... 

The active sites of binuclear non-heme enzymes consist of two Fe atoms, separated by 3 A and bridged by O-atoms derived from hydroxide or carboxylate residues (Fig. Id) (8). The iron centers can adopt 4-, 5-, or 6-coordinate geometries, with the bridging ligands bound via one or two O-atoms. The remaining coordination sites are occupied by His and Asp/Glu residues. 

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