Ligands aspartate carboxylate

The active site structure of peroxidases (Fig. 16-13) is quite highly conserved. As in myoglobin, an imidazole group is the proximal heme ligand, but it is usually hydrogen bonded to an aspartate carboxylate as a catalytic diad (Fig. 16-13).223 In cytochrome c peroxidase... 

The most thoroughly studied class I RNR is that from E. coli, and the remaining discussion focuses on the diiron site in the R2 homodimer of that enzyme, hereafter referred to simply as R2. R2 can be isolated separately from the Rl subunit. The crystal structmes of R2 in both the diferrous and diferric (nomadical) forms revealed the diiron site structures shown in Figure 6 The phenol hydroxyl of tyrosine 122 that forms the radical is hydrogen bonded to an aspartate carboxylate ligand of the diferrous site. 

Recent high-resolution crystal structures of the wild-type E. coli enzyme and its complex with phosphate have enhanced our initial understanding of the enzyme mechanism, which was based on earlier lower-resolution structures of the same forms plus the E-P form of the cadmium-substituted enzyme. In all cases, the zinc ions are separated by about 4.0 A, and there are no protein ligands bridging the two zincs. The magnesium ion is located 4.8 A from the less solvent-exposed zinc (Zn2), to which it is linked by a bridging bidentate aspartate carboxylate, and 7.1 A... 

Metalloenzymes with non-heme di-iron centers in which the two irons are bridged by an oxide (or a hydroxide) and carboxylate ligands (glutamate or aspartate) constitute an important class of enzymes. Two of these enzymes, methane monooxygenase (MMO) and ribonucleotide reductase (RNR) have very similar di-iron active sites, located in the subunits MMOH and R2 respectively. Despite their structural similarity, these metal centers catalyze very different chemical reactions. We have studied the enzymatic mechanisms of these enzymes to understand what determines their catalytic activity [24, 25, 39-41]. 

The active-site model (and the ONIOM model system) includes Fe, one aspartate and two histidine ligands, a water ligand and selected parts of the substrate (see Figure 2-6). The 2-histidine-1-carboxylate ligand theme is shared by several other non-heme iron enzymes [59], For the protein system, we used two different... 

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