Structural studies of bacterial multicomponent monooxygenases : insights into substrate specificity, diiron center tuning and component interactions

Abstract

(cont.) α-subunit cavities. The presence of 6-bromohexan-l-ol induces one of the active site helices to adopt a [pi] conformation. Together, these findings suggest modes by which molecules may move through the MMOH cavities and how both substrates and MMOB may influence the structure of the active site pocket.

Bacterial multicomponent monooxygenases (BMMs) are capable of oxidizing a variety of hydrocarbon substrates at a non-heme carboxylate-bridged diiron center housed within a 200-250 kDa hydroxyase protein. Chapter 1 introduces the members of the BMM family as well as several related diiron proteins with functional relevance to BMMs. The structures of the individual components and the diiron centers are discussed in relation to their catalytic function and the tuning of the metal centers. The structure of the toluene /o-xylene monooxygenase hydroxylase (ToMOH) is presented in chapter 2. The dinuclear iron center is virtually identical to that in the methane monooxygenase hydroxylase (MMOH), yet several novel features, such as a 40 [angstrom] channel, may explain the differences in the substrate specificity between BMM subfamily members. A structural basis for the regiospecificities of toluene monooxygenase and phenol hydroxylases is discussed In Chapter 3 are described metal reconstitution studies of MMOH to probe the ligand geometries of the diiron center and the possible effects on the structure by the coupling protein, MMOB, and the orfY gene product, MMOD. The structures of Mn(II) and Co(II) reconstituted MMOH are identical to that of the diferrous protein. MMOB and MMOD make the addition and removal of iron from MMOH more difficult, suggesting that these proteins serve to block solvent and/or small molecule access to the active site by binding to the four-helix bundle housing the diiron center. Product movement to and from the diiron centers of BMMs is essential for catalytic function. In chapter 4 the crystal structures of MMOH with several bound products are reported. The binding of these products alter the positioning of several side chains in the MMOH