Functions of the proteins of the sulfur-oxidizing enzyme system of Paracoccus pantotrophus

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The objective of the project is the determination of the actual reaction of each protein participating in the sulfur-oxidizing (Sox) enzyme system of Paracoccus pantotrophus. This enzyme system is composed of four proteins, SoxYZ, SoxXA, SoxB and SoxCD. SoxYZ can now be activated to yield a uniforn high specific activity which is the basis to examine the catalytic characteristics of the Sox enzyme system. (i) Kinetics of the Sox system will be be unique as no other known enzyme system forms multiple transient complexes to convert one substrate to one product. (ii) Specific reactions of the individual Sox proteins will be analyzed which requires the loading of SoxY with sulfur atoms of different oxidation states. In addition (iii) specific protein-protein interactions among the Sox proteins will be examined by attenuated total reflection analysis and isothermal calorimetry. (iv) The major component of activated SoxYZ protein is the SoxY-Y homodimer linked by an interprotein disulfide. The significance of the interprotein disulfide for the reaction will be examined. (v) SoxZ, the complex partner of SoxY, is essential for activity of SoxY but its role in conformational switch of SoxY, its reactivity or stabilization of the SoxYZ complex is unknown and will be analyzed by kinetics and site-directed mutagenesis.