Structures and Mechanisms of Two Enzymes that Produce Hydrocarbon Fuels from Abundant Metabolites

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With this award the "Chemistry of Life Processes" program of the "Division of Chemistry" is supporting work at the Pennsylvania State University to study two enzymes that catalyze key steps in the conversion of fatty acids (abundant and common cellular metabolites) to hydrocarbons (alkanes and alkenes). These can then be used as drop-in biofuels for combustion engines. The research supported is impacting development of renewable energy, because the use of microorganisms is a promising strategy for biofuel production. This process is particularly environmentally friendly, because it is ultimately carbon neutral and driven by solar energy. The project is studying how the enzymes described above function with an eye to importing them into genetically engineered microorganisms which can produce biofuels. Work supported by this award is also offering students and postdoctoral scholars opportunities to be broadly trained in biophysical methods. It also supports a biennial workshop, led by Penn State metallobiochemists and supported by colleagues from other institutions, that trains students and postdoctoral scholars in unique methods used in this research field. Genetically manipulable microorganisms that could (i) produce fatty acids from sunlight and carbon dioxide (either directly by using photosynthetic cyanobacteria, or indirectly by feeding the engineered microorganisms sugars obtained from fast-growing and high-yielding plants such as sugar cane) and (ii) convert the fatty acids into "drop-in" biofuels are actively pursued as vehicles for biogenesis of renewable fuels. Two recently discovered enzyme pathways for the conversion of fatty acids into linear alkane or alkene hydrocarbons involve enzymes that harbor iron-containing cofactors at their active site. The first example involves the two-step alkane biosynthesis pathway in cyanobacteria that converts fatty acids via fatty aldehydes to alk(a/e)nes. The second step in this pathway is catalyzed by the enzyme aldehyde-deformylating oxygenase (ADO), a member of the ferritin-like diiron-carboxylate oxidases and oxygenases. In the previous funding cycle we studied the ADO reaction as a novel, cryptically redox oxygenation reaction. Further mechanistic studies revealed potential inefficiencies and vulnerabilities that could limit the usefulness of this pathway for biofuel production. We propose to study the reaction of ADO in greater depth, with the expectation that further mechanistic insight will guide the rational design of biofuel-producing microorganisms. The second example involves a recently discovered iron-enzyme, UndA, that utilizes O2 to decarboxylate fatty acids to carbon dioxide and terminal alkenes. Our preliminary work suggests that UndA is also a diiron enzyme, contrary to the published assumption that it is a mononuclear iron enzyme. We propose to identify the correct cofactor and determine the molecular mechanism in detail. These studies will be an important contribution to ongoing research efforts that aim at utilizing this metabolic pathway for biofuel production.