Sulfur oxygenase reductase SOR
We have been using X-rays diffraction to determine the 3D structure of a minute biological reactor, aiming to understand its working mechanisms. This is a project in collaboration with Prof. Arnulf Kletzin, TU Darmstadt, Germany.
Numerous microorganisms oxidize sulfur for energy conservation and contribute to the global biogeochemical sulfur cycle. Acidianus ambivalens, a thermoacidophilic archaeon found in volcanic vents, transforms inorganic sulphur with assistance of oxygen into sulphur metabolites. The reaction occurs within nano-reactors built up by 24 sulphur oxygenase reductase molecules. Their surface contains 6 hydrophobic chimney-like protrusions that work as elemental sulphur entry portals. Reactors include 24 catalytic centres composed of iron sites and cysteine persulfides, at positively charged inner compartments accessible only from the inside.
In order to map sulfur internal pathways and to highlight catalytic residues several point mutations and inhibition procedures were constructed, and corresponding 3D structures determined. Surface “chimneys” are not essential for the reactor activity, they presumably control the access of hydrophobic sulphur to the inner hollow. Products exit might occur via hydrophilic channels, with 8 outlets at 3-fold symmetry axes. Enlargement of both openings increased enzyme activity by several-fold. In contrast, the inner passage to the catalytic centre cannot be opened without decreasing the specific enzyme activity.
The SOR particle provides a compartment where inorganic sulfur reacts into metabolites, thus providing an example of prototypes of proto-organelles in archaea.
A. ambivalens habitat:hot vents of volcanic origin, by 80 °C
and pH 2.5.
SOR nano reactor is composed of 24 monomers (Mr 870 kDa,
7632 aa) forming a hollow sphere (15 nm diameter) with
positively charged interior.
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Veith & Kletzin et al. Front. in Microbiol. 2, 37 (2011)
