Alkane oxidizing metalloenzymes, with a special emphasis on alkane monooxygenase or AlkB
The goal of this work is to understand the structure and function of metal-containing enzymes (metalloenzymes) that catalyze the hydroxylation of alkanes (simple organic molecules made up of only carbon and hydrogen). The majority of our work in this area has focused on studying alkane monooxygenase (AlkB), a metalloenzyme that catalyzes the oxidation of most of the alkanes with a chain length between 5 and 22 carbons in the environment. We have developed tools that have allowed us to also study the mechanisms of other metalloenzymes that catalyze similar reactions and, in so doing, offer insights into general trends in alkane oxidation and molecular oxygen activation
The compelling question driving much of our recent work in this area has been “How does the structure of AlkB dictate its function?” AlkB is found in a number of organisms that catalyze the transformation of alkanes, the main components of oil. Over the past 15 years, the work my colleagues and I have done has provided information about how this transformation occurs in two members of the AlkB family, the AlkB from Pseudomonas putida GPo1 and the AlkB from the marine organism Alcanivorax borkumensis AP1. We still need to know much more about the enzyme’s three-dimensional structure to understand the critical connection between structure and function. That's a difficult challenge, since in functioning cells AlkB spans the inner cell membrane; such molecules are notoriously difficult to study. My lab is currently trying to crystallize AlkB for X-ray analysis. We have also been trying to characterize AlkB’s electronic structure using UV-Vis spectroscopy and magnetic circular dichroism spectroscopy. We have also begun exploring AlkBs from other organisms to try to understand the structural and functional breadth of this family of enzymes.
AlkB appears to be fundamentally different from other oxygen-activating enzymes, although more structurally information is needed to confirm or reject that assertion. Data from the labs of others suggests that the diiron active site is coordinated by nitrogen-rich ligands. The other diiron O2-activating enzymes (e.g. soluble methane monooxygenase sMMO, toluene monooxygenase TMO) contain an oxygen-rich coordination environment. Published work suggests that AlkB has a long substrate channel that positions the terminal end of substrate towards the enzyme’s active site where hydroxylation occurs. Our own data also indicates that the channel dimensions have a profound effect on reactivity. To try to get additional information about AlkB structure, we have developed a protocol for purifying AlkB that yields relatively pure, active protein. More recently, we have begun to affinity tag, clone and purify AlkB from a wider range of sources, thus expanding the range of AlkBs whose structure and function we can explore.
Lead neurochemistry and metallothioneins
The goal of this project is to begin to understand a bit more about the role that metal ions play in the brain and to understand the cellular function of metallothionein-3. Our initial efforts focused on determine whether metallothionein-3 (MT3) binds to lead, and if it does, with what affinity? MT3 is a central nervous system specific isoform of metallothionein found in humans. Its role in modulating the neurochemistry of lead has not been previously explored.
Metallothioneins are a class of molecules that are thought to function as heavy metal sponges, soaking up certain dangerous heavy metals so that they do less harm to organisms. Could metallothionein-3 be playing a role in the complex neurochemistry associated with childhood lead exposure? My work will not answer that question definitively, but it should tell us something about what is chemically possible in the interaction between lead and metallothionein. That would then narrow the range of possibilities of what could be happening in biological systems.
We have published several papers that indicates that lead binds to MT-3 more tightly than zinc and that removal of either zinc or lead from MT-3 results in a triphasic process with important implications for the potential function of MT-3 in vivo. We are now examining the factors that lead to the expression of MT-3. We’ve published work showing that unlike other mammalian metallothioneins, MT-3 does not respond to zinc ion concentrations. We are also studying whether the structure of MT-3 changes when it is bound to different metal ions and whether these structural changes impact protein-protein interactions.
Heterogeneous catalysts to ameliorate human impacts on the environment
Several different heterogeneous catalyst development projects have been undertaken in my lab. Two were targeted at catalyzing the decomposition of pesticides or pollutants and one was targeted at developing a green catalyst that could use oxygen to selective hydroxylate substrates. The focus of the current project is the development of catalysts that can assist in efforts to convert wood waste into fuel.
Most recently, my work in this area was done in collaboration with colleagues at the University of Maine at Orono, as part of a large DOE funded grant to develop infrastructure to transform Maine woodwaste into transportation fuels and chemicals. My lab has focused on designing, synthesizing, characterizing, and testing catalysts that can be used to upgrade the oil and chemicals produced in our pyrolysis reactors into usable fuels and chemicals that can emerge from the fast pyrolysis technology. In fast pyrolysis wood waste is heated rapidly in the absence of oxygen to form an oil. The resulting oil is too acidic and oxygenated to use as a transportation fuel, or even to blend, so our task has been to develop catalysts that can be used to catalytically remove oxygen. The goal of my work has been to develop catalysts that are effective at this catalytic hydrodeoxygenation and to understand how they work so we can have a rational framework that would ultimately allow us to design and synthesize less inexpensive catalysts using earth abundant and non-toxic materials.
Recently we have been focusing on a class of catalysts whose catalytic efficiency has been remarkable – small Ru particles on TiO2. In this work, we are using phenol as a model for the phenolic compounds in pyrolysis oil, which are among the most intractable. The goal is to design catalysts that selectively convert phenol to benzene because this represents the most efficient use of hydrogen to remove oxygen. (Ultimately the oils contain phenolic compounds, not phenol, so we would not be making benzene in our fuels.) A recent publication reports results done in collaboration with computational scientist Lars Grabow from the University of Houston posits a novel hypothesis as to how these catalysts work. We propose that interface sites on TiO2 but adjacent to Ru nanoparticles, serve as amphoteric catalyst, shuttling protons and preserving reducing equivalents from hydrogen to the substrate. Current efforts in the group are focused on designing new materials to test this hypothesis.