Redox and chemical activities of the hemes in the sulfur oxidation pathway enzyme SoxAX.
BACKGROUND
SoxAX enzymes initiate microbial oxidation of reduced inorganic sulfur compounds. Their catalytic mechanism is unknown.


RESULTS
Cyanide displaces the CysS(-) ligand to the active site heme following reduction by S(2)O(4)(2-) but not Eu(II).


CONCLUSIONS
An active site heme ligand becomes labile on exposure to substrate analogs.


CONCLUSIONS
Elucidation of SoxAX mechanism is necessary to understand a widespread pathway for sulfur compound oxidation. SoxAX enzymes couple disulfide bond formation to the reduction of cytochrome c in the first step of the phylogenetically widespread Sox microbial sulfur oxidation pathway. Rhodovulum sulfidophilum SoxAX contains three hemes. An electrochemical cell compatible with magnetic circular dichroism at near infrared wavelengths has been developed to resolve redox and chemical properties of the SoxAX hemes. In combination with potentiometric titrations monitored by electronic absorbance and EPR, this method defines midpoint potentials (E(m)) at pH 7.0 of approximately +210, -340, and -400 mV for the His/Met, His/Cys(-), and active site His/CysS(-)-ligated heme, respectively. Exposing SoxAX to S(2)O(4)(2-), a substrate analog with E(m) ~-450 mV, but not Eu(II) complexed with diethylene triamine pentaacetic acid (E(m) ~-1140 mV), allows cyanide to displace the cysteine persulfide (CysS(-)) ligand to the active site heme. This provides the first evidence for the dissociation of CysS(-) that has been proposed as a key event in SoxAX catalysis.
Centre for Molecular and Structural Biochemistry, School of Chemistry and School of Biological Sciences, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, UK.
Justin M
JM
Bradley
Sophie J
SJ
Marritt
Margaret A
MA
Kihlken
Kate
K
Haynes
Andrew M
AM
Hemmings
Ben C
BC
Berks
Myles R
MR
Cheesman
Julea N
JN
Butt
D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D002384:Catalysis; D020134:Catalytic Domain; D003574:Cytochrome c Group / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D006418:Heme / Q000378:metabolism*; D007700:Kinetics; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D041903:Rhodovulum / Q000737:chemistry / Q000201:enzymology* / Q000235:genetics; D013455:Sulfur / Q000378:metabolism*
D001426:Bacterial Proteins; D003574:Cytochrome c Group; D006418:Heme; D013455:Sulfur; D010088:Oxidoreductases; C090099:diheme cytochrome c