Ligands glutamate carboxylate

Fig. 4. Proposed catalytic cycle for the hydroxylation of methane by MMO. The reductase and B components may interact with the hydroxylase in one or more steps of the cycle. Protons are shown in the step in which intermediate Q is generated, but other possibilities exist (see Fig. 3 and the text). The curved line represents a bridging glutamate carboxylate ligand.

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 dinuclear active site of urease (1) has been studied in great detail23-29 and has inspired manifold model studies—hence a separate section, Section 6.3.4.12.7, is dedicated to the coordination chemistry related to urease. E. coli Glx I is the first example of a Ni-dependent isomerase and contains a single Ni11 ion coordinated by two histidines, two axial carboxylates of glutamic acid, and two water molecules (2).30-32 It is not active with Zn bound, which is believed to result from the inability of the Zn-substituted enzyme to bind a second aqua ligand and to adopt a six-coordinate structure. 

The predominance of aspartate and glutamate residues in the Ca -coordination spheres of proteins was quantified in Section III,D. They constitute 29% and 18%, respectively, of the Ca " -coordinating ligands from the proteins presented in Table II. An analysis of the hydrogenbonding networks around the Ca -binding sites of these proteins (Color Plate 2 and Fig. 7) indicates several reasons for the prevalence of these negatively charged amino acids. The side-chain carboxylate moiety of... 

---