Project Summary Reactions with atmospheric oxygen are required for many life-sustaining processes. The class-III diiron proteins use oxygen to selectively oxidize lipids and to put OH groups into molecules in critical biosynthetic pathways. Class-III diiron enzymes play essential roles in many aspects of lipid synthesis and metabolism and are linked to human health problems including obesity, diabetes, attention-deficit disorder, and neurodegeneration. They are also crucial in the natural bioremediation of oil. There is a dearth of mechanistic information about this family of membrane enzymes, primarily because their membrane-associated nature makes them very difficult to purify and study. Alkane monooxygenase (AlkB) is a member of the class-III integral membrane diiron proteins along with fatty acid desaturases and fatty acid hydroxylases. The amino acid sequence of AlkB indicates that it is not structurally similar to other enzymes with similar functions. Determining its three-dimensional structure is a feat that has eluded scientists for decades. In an important step forward in preliminary work, PI Austin and co-Investigator Feng have solved the first structure of AlkB with a bound substrate and shown that it serves as an excellent model system to understand the catalytic mechanism of class-III diiron proteins. This breakthrough, together with the establishment of a novel assay for rapid functional characterization and the development of a suite of AlkB active AlkB homologs, paves the way to answering key questions about these important metalloenzymes. The PIs will integrate structural, functional, biochemical, computational, and spectroscopic studies to determine the three-dimensional structure of the diiron active site, identify determinants of substrate specificity, learn how AlkB is activated by its partner protein, and probe how the presence of a covalently bound electron-transfer partner, found only in a class of gram positive bacteria, changes the reactivity of this enzyme family. In so doing, they will expand the basic knowledge of strategies to break and make key chemical bonds, which may lead to the development of new synthetic routes to make life-saving and life-extending molecules. Their work will also provide critical insights to efforts to target this family of enzymes for therapeutic purposes.