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Metalloenzymes

Metalloenzymes catalyze a wide variety of reactions ranging from electron transfer to the insertion of oxygen into carbon-hydrogen bonds. In performing these action the metal can be alone, in a cluster, or associated with a porphyrin.

Work in the Ohlendorf lab has been in two classes of metalloenzymes. The first class, dioxygenases , use an isolated iron or manganese atom to catalyze the cleavage of aromatic rings with the incorporation of both atoms of molecular oxygen. This reaction is the key step in the degradation pathway for many aromatic compounds found in the environment. The second class, monooxygenases , use a binuclear Fe-O-Fe cluster to insert one atom from molecular oxygen into an organic substrate.

Iron coordination sphere of protocatechuate 3,4-dioxygenase (3,4-PCD).

The degradation of aromatic compounds in the environment converge on a small number of catechol derivatives. These key aromatic intermediates are cleaved into aromatic compounds by dioxygenases. Dioxygenases can be classified by the location of their cleavage site.

Intradiol dioxygenases use Fe(III) to cleave between the vicinal hydroxyls. Extradiol dioxygenases cleave outside the hydroxyls. When R is not a hydrogen, the two nonequivalent sites lead to proximal and distal cleavage. Proximal extradiol dioxygenases use Fe(II) or Mn(II) to cleave the aromatic ring.

The Ohlendorf lab has determined the structure of methane monooxygenase hydroxylase (MMOH) from Methylosinus trichosporium OB3b. This 249,000 dalton dimer consists of three polypeptide chains that fold into the largely helical structure shown below. MMOH catalyzes the insertion of an atom of oxygen in a wide variety of carbon-hydrogen bonds. In name methane monooxygenase is used to capture 99.5% of the 10⁹ tons of methane produce in oceans, lakes and wetlands annually by anaerobic fermentation and return it to biomass. MMOH is the catalytically competent subunit of methane monooxygenase.
Work on MMOH is in collaboration with Dr. John Lipscomb of the Dept. of Biochemistry of the University of Minnesota.

Schematic illustrating the secondary structure elements of the MMOH monomer. The biologically-active form of the molecule is the dimer.

Selected Publications

• Elango, N., R. Radhakrishnan, et al. (1997). "Crystal structure of the hydroxylase component of methane monooxygenase from Methylosinus trichosporium OB3b." Protein Sci 6(3): 556-68.
• Froland, W. A., D. H. Dyer, et al. (1994). "Preliminary crystallographic analysis of methane mono-oxygenase hydroxylase from Methylosinus trichosporium OB3b." J Mol Biol 236(1): 379-81.