The model structure of the copper-dependent ammonia monooxygenase

Ammonia monooxygenase is a copper-dependent membrane-bound enzyme that catalyzes the first step of nitrification in ammonia-oxidizing bacteria to convert ammonia to hydroxylamine, through the reductive insertion of a dioxygen-derived O atom in an N-H bond. This reaction is analogous to that carried out by particulate methane monooxygenase, which catalyzes the conversion of methane to methanol. The enzymatic activity of ammonia monooxygenase must be modulated to reduce the release of nitrogen-based soil nutrients for crop production into the atmosphere or underground waters, a phenomenon known to significantly decrease the efficiency of primary production as well as increase air and water pollution. The structure of ammonia monooxygenase is not available, rendering the rational design of enzyme inhibitors impossible. This study describes a successful attempt to build a structural model of ammonia monooxygenase, and its accessory proteins AmoD and AmoE, from Nitrosomonas europaea, taking advantage of the high sequence similarity with particulate methane monooxygenase and the homologous PmoD protein, for which crystal structures are instead available. The results obtained not only provide the structural details of the proteins ternary and quaternary structures, but also suggest a location for the copper-containing active site for both ammonia and methane monooxygenases, as well as support a proposed structure of a CuA-analogue dinuclear copper site in AmoD and PmoD.