Research
Interests
The Role and Reactivity of Ketyl Radical
Anions in Anaerobic Bacteria
In collaboration with Prof.
Dr. Wolfgang Buckel
(Laboratorium
für Mikrobiologie, Philipps-Universität Marburg D-35032 Marburg, Germany)
The
dehydration or a,b elimination
of water from biomolecules is a very common enzymatic reaction. Almost all
dehydratases catalyze the removal of an hydroxy group in the b position of an electron withdrawing
carboxylate, thioester, or carbonyl group:
The
C-Ha bond of such
compounds is activated (pKa ≈ 20) and can be relatively easily
deprotonated by a basic residue of an enzyme. Several anaerobic bacteria,
however, are able to catalyze a dehydration reaction in which it is the
hydroxyl group that is in the a-position so that the C-Hb bond to be cleaved is not at all
activated (pKa ≈ 40). An example of such a reaction is that
catalyzed by the two-component 2-hydroxyglutaryl-CoA dehydratase system found
in some intestinal bacteria. A possible reaction mechanism for this
transformation is found below [1]:
In
this scenario, a ketyl radical anion species (2) is generated by one electron reduction of the substrate
2-hydroxyglutaryl-CoA (1). Upon loss
of hydroxide, the C-Hb bond
of the enoxy radical (3) is expected to be activated, allowing
the formation of yet another ketyl radical anion intermediate (4). Finally, loss of the initiating
electron yields the reaction product (5).
As part of a recent collaboration with Professor Buckel, we were able to
demonstrate that the enoxy radical intermediate (3), if generated, is indeed in possession of an activated C-Hb bond (pKa ≈ 14) [2].
However, the likelihood of the initial one electron reduction, as well as the
properties and reactivity patterns of the ketyl radical anions (2 and 4) in the above mechanism, have yet to be established. Furthermore, there are several other known
enzymes that apparently make use of a similar mechanism [1], the details of
which require clarification.
In
this light, I am currently using computational procedures to characterize the
ketyl radical anions that appear so crucial in the unusual dehydration
reactions catalyzed by anaerobic bacteria. This characterization takes place in
the gas phase, in aqueous solution, and in the enzymatic environment and is
therefore be best accomplished by a combination of quantum and classical
mechanical techniques.
[1] Buckel, W.; Golding, B. T. FEMS Microbiol. Rev. 1999,
22, 523
[2] Smith, D. M.; Buckel, W.; Zipse, H. Angew.
Chem. Int. Ed. Eng. 2003, 42, 1867