In our laboratory we are investigating
microbial pathways of short-chain hydrocarbon oxidation and the biochemical,
mechanistic,
and spectroscopic properties of the enzymes involved in these pathways.
The metabolism of aliphatic hydrocarbons by aerobic bacteria involves their
activation to alcohols (in the case of alkanes) or epoxides (in the case
of alkenes). The oxygenases catalyzing the initial transformations of alkanes
and alkenes have been shown in some instances to have potential as catalysts
for stereospecific epoxide synthesis and/or chlorinated hydrocarbon degradation.
The products of hydrocarbon oxidation are further metabolized by diverse
reactions that include oxidation, hydration, isomerization, or carboxylation.
Our research has led to the identification of new microbial transformations
and enzymes of significant biotechnological and environmental interest (Figure
1).
One ongoing research project concerns the physiology and biochemistry underlying
the microbial oxidation of alkenes. For these studies we are using two soil bacteria,
Xanthobacter autotrophicus and Rhodococcus rhodochrous, each
of which is able
to
grow using ethylene, propylene, or butylene as their carbon and energy source.
The metabolism of alkenes is initiated by a monooxygenase that inserts O2 into
the olefin bond, forming the corresponding epoxides in a stereospecific manner.
The epoxides are further metabolized by the concerted action of four previously
unidentified enzymes that open the epoxide ring and carboxylate a reaction intermediate,
forming beta-keto acids as products (Figure 2). This process involves the usage
of coenzyme M (2-mercaptoethanesulfonic acid), a cofactor previously thought
to be used only in the reductive formation of methane by methanogenic Archaea.
A major goal of this project is the biochemical and mechanistic characterization
of the enzymes of alkene and epoxide metabolism. We are also characterizing the
genes involved in alkene and epoxide metabolism and studying how the expression
of the alkene-oxidizing enzymes is regulated at the molecular level.
A second ongoing research project in our laboratory concerns bacterial acetone
metabolism. Acetone is a toxic molecule that is synthesized industrially and
formed biologically during bacterial fermentation and mammalian starvation. A
number of bacteria are able to grow with acetone as a source of carbon and energy.
In addition, acetone is formed as an intermediate in the metabolism of propane
and isopropanol by some bacteria. Bacterial pathways of acetone metabolism and
the biochemical properties of acetone-metabolizing enzymes are poorly understood.
We are attempting to advance the state of knowledge of these areas by studying
the pathway of acetone metabolism and the properties of the acetone-metabolizing
enzyme(s) of Xanthobacter autotrophicus, Rhodococcus rhodochrous, Rhodobacter
capsulatus, and other acetone-utilizing microorganisms.
We are currently expanding our studies of microbial alkene metabolism to look
at pathways and enzymes of isoprene (2-methyl-1,3-butadiene) and ethylene metabolism.
Ethylene and isoprene are produced in large quantities (collectively, more than
two hundred million tons per year) by plants, and are metabolized by a variety
of microorganisms. In spite of the importance of isoprene and ethylene as nonmethane
hydrocarbon components of the atmosphere, there is little known of catabolic
pathways for their oxidation. We are currently examining pathways of ethylene
and isoprene metabolism in several Gram-positive and Gram-negative isolates.
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