Methane monooxygenase, active site

According to the following evidences, a-sites are most probably di-iron complexes similar to the di-iron active sites of methane monooxygenase ... 

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]. 

Usually, these metalloproteins contain both type 2 and type 3 copper centers, together forming a triangular-shaped trinuclear active site, such as found in laccase (polyphenol oxidase) [38-41] and ascorbate oxidase (3) [42]. Recent evidence for a related arrangement has been reported for the enzyme particulate methane monooxygenase as well [43], but in this case the Cu Cu distance of the type 2 subunit (2.6 A) appears to be unusually short and the third Cu ion is located far from the dinuclear site. 

Scheme 2.6 Examples of reactions catalyzed byenzymesthat carry a dinudear iron active site (a) hydroxylation of methane by soluble methane monooxygenase (sMMO) [7] (b) reduction of ribonucleotides by class I ribonucleotide reductase (RNR)...

Lieberman, R. L. Rosenzweig, A. C. Biological methane oxidation regulation, biochemistry, and active site structure of particulate methane monooxygenase. Crit. Rev. Biochem. Mol. Biol. 2004, 39(3), 147-164. 

Whittington, D.A., Sazinsky, M.H., and Lippard, S J. (2001) X-ray Crystal Structure of Alcohol Products Bound at the Active Site of Soluble Methane Monooxygenase Hydroxylase. J. Am. Chem. Soc, 123, 1794-... 

Yoshizawa, K. (2000) Two-step concerted mechanism for methane hydroxylation on the diiron active site of soluble methane monooxygenase, J. Inorg. Biochem. 78, 23-34. 

However, it is possible to detect a tyrosine radical optically in ribonucleotide reductase, as there is only a relatively weak competing absorption from the binuclear non-haem iron centre [164]. A distinct sharp peak is seen that is not present in proteins that have been treated with the radical scavenger hydroxyurea [165,166] nor is it present in proteins such as haemerythrin or methane monooxygenase, which have similar active-site structures, but lack... 

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