CRYSTAL-STRUCTURES OF THE METHANE MONOOXYGENASE HYDROXYLASE FROM METHYLOCOCCUS-CAPSULATUS (BATH) - IMPLICATIONS FOR SUBSTRATE GATING AND COMPONENT INTERACTIONS

The crystal structure of the nonheme iron-containing hydroxylase compo nent of methane monooxygenase hydroxylase (MMOH) from Methylococcus ca psulatus (Bath) has been solved in two crystal forms, one of which was refined to 1.7 Angstrom resolution. The enzyme is composed of two cop ies each of three subunits (alpha(2) beta(2) gamma(2)), and all three subunits are almost completely alpha-helical, with the exception of tw o beta hairpin structures in the alpha subunit. The active site of eac h alpha subunit contains one dinuclear iron center, housed in a four-h elix bundle, The two iron atoms are octahedrally coordinated by 2 hist idine and 4 glutamic acid residues as well as by a bridging hydroxide ion, a terminal water molecule, and at 4 degrees C, a bridging acetate ion, which is replaced at -160 degrees C with a bridging water molecu le, Comparison of the results for two crystal forms demonstrates overa ll conservation and relative orientation of the domain structures, The most prominent structural difference identified between the two cryst al forms is in an altered side chain conformation for Leu 110 at the a ctive site cavity, We suggest that this residue serves as one componen t of a hydrophobic gate controlling access of substrates to and produc ts from the active site, The leucine gate may be responsible for the e ffect of the B protein component on the reactivity of the reduced hydr oxylase with dioxygen, A potential reductase binding site has been ass igned based on an analysis of crystal packing in the two forms and cor roborated by inhibition studies with a synthetic peptide corresponding to the proposed docking position. (C) 1997 Wiley-Liss, Inc.