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Journal of Bacteriology, January 2001, p. 189-199, Vol. 183, No. 1
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.1.189-199.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Two Conserved Glutamates in the Bacterial Nitric Oxide Reductase Are Essential for Activity but Not Assembly of the Enzyme

Gareth Butland,dagger Stephen Spiro, Nicholas J. Watmough, and David J. Richardson*

Centre for Metalloprotein Spectroscopy and Biology, School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom

Received 12 April 2000/Accepted 3 October 2000

The bacterial nitric oxide reductase (NOR) is a divergent member of the family of respiratory heme-copper oxidases. It differs from other family members in that it contains an FeB-heme-Fe dinuclear catalytic center rather than a CuB-heme-Fe center and in that it does not pump protons. Several glutamate residues are conserved in NORs but are absent in other heme-copper oxidases. To facilitate mutagenesis-based studies of these residues in Paracoccus denitrificans NOR, we developed two expression systems that enable inactive or poorly active NOR to be expressed, characterized in vivo, and purified. These are (i) a homologous system utilizing the cycA promoter to drive aerobic expression of NOR in P. denitrificans and (ii) a heterologous system which provides the first example of the expression of an integral-membrane cytochrome bc complex in Escherichia coli. Alanine substitutions for three of the conserved glutamate residues (E125, E198, and E202) were introduced into NOR, and the proteins were expressed in P. denitrificans and E. coli. Characterization in intact cells and membranes has demonstrated that two of the glutamates are essential for normal levels of NOR activity: E125, which is predicted to be on the periplasmic surface close to helix IV, and E198, which is predicted to lie in the middle of transmembrane helix VI. The subsequent purification and spectroscopic characterization of these enzymes established that they are stable and have a wild-type cofactor composition. Possible roles for these glutamates in proton uptake and the chemistry of NO reduction at the active site are discussed.

* Corresponding author. Mailing address: School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom. Phone: 44 1603 593250. Fax: 44 1603 592250. E-mail: d.richardson{at}uea.ac.uk.

dagger Present address: Department of Biochemistry, University of Oxford, Oxford OX1 3QU, United Kingdom.

Journal of Bacteriology, January 2001, p. 189-199, Vol. 183, No. 1
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.1.189-199.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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