Dinuclear site chain

In the M. trichosporium OB3b system, a third intermediate, T, with kmax at 325 nm (e = 6000 M-1cm 1) was observed in the presence of the substrate nitrobenzene (70). This species was assigned as the product, 4-nitrophenol, bound to the dinuclear iron site, and its absorption was attributed primarily to the 4-nitrophenol moiety. No analogous intermediate was found with the M. capsulatus (Bath) system in the presence of nitrobenzene. For both systems, addition of methane accelerated the rate of disappearance of the optical spectrum of Q (k > 0.065 s-1) without appreciatively affecting its formation rate constant (51, 70). In the absence of substrate, Q decayed slowly (k 0.065 s-1). This decay may be accompanied by oxidation of a protein side chain. 

The next question is, where the protons go to in the active site during the catalytic cycle. For the base, there are too many possibilities to be certain. Groups near to the dinuclear cluster that can accept or exchange protons include the side chains of the amino acids arginine and histidine, thiolate ligands to the cluster, and peptide NH groups. 

The stereochemical control is achieved via a chain end control mechanism, enforced by the steric bulk of the ligand. The dinuclear complexes cannot control the stereochemistry, leading to atactic PLA, due to the lack of a single, hindered lactide coordination site. 

There have been a few reports of first generation coordination complex structural models for the phosphatase enzyme active sites (81,82), whereas there are some examples of ester hydrolysis reactions involving dinuclear metal complexes (83-85). Kim and Wycoff (74) as well as Beese and Steitz (80) have both published somewhat detailed discussions of two-metal ion mechanisms, in connection with enzymes involved in phosphate ester hydrolysis. Compared to fairly simple chemical model systems, the protein active site mechanistic situation is rather more complex, because side-chain residues near the active site are undoubtedly involved in the catalysis, i.e, via acid-base or hydrogenbonding interactions that either facilitate substrate binding, hydroxide nucleophilic attack, or stabilization of transition state(s). Nevertheless, a simple and very likely role of the Lewis-acidic metal ion center is to... 

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 ... 

Each ferritin H-chain subunit possesses a conserved dinuclear iron site known as the ferroxidase center, which catalyzes iron oxidation. Residues E27, E61, E62, H65, and E107 act as metal hgands while residues Y34 and Q144 form hydrogen bonds (the numbering refers to the human H-chain sequence) (see Figure 6). 

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