E. coli Dimethyl Sulfoxide Reductase


Structure and Function:

E. coli Dimethyl Sulfoxide Reductase (DMSO reductase), a redox enzyme, is a membrane-bound oxidoreductase which catalyzes the two-electron oxidation of menaquinol in the membrane coupled with the two-electron reduction of dimethyl sulfoxide (DMSO) to dimethyl sulfide (DMS).  The enzyme consists of three subunits: the catalytic subunit (DmsA) containing a mononuclear molybdenum ion and a molybdopterin guanosine dinucleotide cofactor (MGD)1 (see below), the electron transfer subunit (DmsB) containing four [4Fe-4S] clusters1,5, and a membrane anchor subunit (DmsC)6 (enzyme schematic shown above).


In this Laboratory:

DMSO reductase has been found to turn over both dimethyl sulfoxide (DMSO) and trimethylamine N-oxide (TMAO) when adsorbed at an edge-plane graphite surface (protein film voltammetry).  There is evidence of a ‘switch’ in the voltammetry, which is very well-defined at pH 9.0- the rate of DMSO reduction decreases as the driving force is raised.  DMSO reductase does not appreciably oxidize DMS under the conditions employed. The potential of the DMSO/DMS couple is +160 mV (i.e. higher than the observed catalytic potential), so this is not unexpected. However, trimethylphosphine (PMe3) was found to be a suitable substrate for oxidation (more oxyphilic substrate; lower redox potential).


We observe that, under the experimental conditions outlined, DMSO reductase operates within narrow potential limits. Catalytic currents show a decrease outside these limits: in spite of the greater thermodynamic driving force the reactions proceed more slowly. This effect has been observed in other redox enzymes such as succinate dehydrogenase, which exhibits such a ‘tunnel-diode effect’ in the reductive direction.7 DMSO reductase is significant in that it has both an upper and lower limit, and thus appears to exhibit a potential optimum.

Although there has been much research into the principle of ideal pH conditions for enzymes (elucidating proton equilbria), the area of ideal driving potential is not well investigated. Optimal potential ranges are readily demonstrated by protein film voltammetry, and are useful because they give information about the role of different oxidation states of the redox cofactors. For example, our recent work has shown that the boundaries of the potential window in DMSO reductase correspond to one-electron processes and coincide closely with the range of stability of Mo (V) as mesured by EPR. This suggests that crucial steps in turnover occur whilst molybdenum is in the (V) oxidation state. 8

This work has been completed as a collaboration with Joel Weiner in the Department of Biochemistry at the University of Alberta.

References:

1.  Cammack, R.; Weiner, J. H.; Biochemistry 1990, 29, 8410-8416.
2.  Trieber, C. A.; Rothery, R. A.; Weiner, J. H. Journal of Biological Chemistry 1994, 269, 7103-7109.
3.  Rothery, R. A.; Weiner, J. H. Biochemistry 1991, 30, 8296-8305.
4.  Trieber, C. A.; Rothery, R. A.; Weiner, J. H. Journal of Biological Chemistry 1996, 271, 27339-27345.
5.  Trieber, C. A.; Rothery, R. A.; Weiner, J. H. Journal of Biological Chemistry 1996, 271, 4620-4626.
6.  Rothery, R. A.; Weiner, J. H.; Biochemistry 1996, 35, 3247-3257.
7.  Hirst, J.; Sucheta, A.; Ackrell, B. A. C.; Armstrong, F. A. J. Am. Chem. Soc. 1996, 118, 5031-5038.
8.  Heffron, K., Léger, C., Rothery, R. A., Weiner, J. H., and Armstrong, F. A. Biochemistry. 2001, 40, 3117-3126.


back to home page



 
 

15/10/99  AKJ