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Effect of exchange of the cysteine molybdenum ligand with selenocysteine on the structure and function of the active site in human sulfite oxidase

  • Sulfite oxidase (SO) is an essential molybdoenzyme for humans, catalyzing the final step in the degradation of sulfur-containing amino acids and lipids, which is the oxidation of sulfite to sulfate. The catalytic site of SO consists of a molybdenum ion bound to the dithiolene sulfurs of one molybdopterin (MPT) molecule, carrying two oxygen ligands, and is further coordinated by the thiol sulfur of a conserved cysteine residue. We have exchanged four non-active site cysteines in the molybdenum cofactor (Moco) binding domain of human SO (SOMD) with serine using site-directed mutagenesis. This facilitated the specific replacement of the active site Cys207 with selenocysteine during protein expression in Escherichia coli. The sulfite oxidizing activity (k(cat)/K-M) of SeSOMD4Ser was increased at least 1.5-fold, and the pH optimum was shifted to a more acidic value compared to those of SOMD4Ser and SOMD4Cys(wt) X-ray absorption spectroscopy revealed a Mow Se bond length of 2.51 A, likely caused by the specific binding of Sec207 to theSulfite oxidase (SO) is an essential molybdoenzyme for humans, catalyzing the final step in the degradation of sulfur-containing amino acids and lipids, which is the oxidation of sulfite to sulfate. The catalytic site of SO consists of a molybdenum ion bound to the dithiolene sulfurs of one molybdopterin (MPT) molecule, carrying two oxygen ligands, and is further coordinated by the thiol sulfur of a conserved cysteine residue. We have exchanged four non-active site cysteines in the molybdenum cofactor (Moco) binding domain of human SO (SOMD) with serine using site-directed mutagenesis. This facilitated the specific replacement of the active site Cys207 with selenocysteine during protein expression in Escherichia coli. The sulfite oxidizing activity (k(cat)/K-M) of SeSOMD4Ser was increased at least 1.5-fold, and the pH optimum was shifted to a more acidic value compared to those of SOMD4Ser and SOMD4Cys(wt) X-ray absorption spectroscopy revealed a Mow Se bond length of 2.51 A, likely caused by the specific binding of Sec207 to the molybdenum, and otherwise rather similar square-pyramidal S/Se(Cys)(O2MoS2)-S-VI(MPT) site structures in the three constructs. The low-pH form of the Mo(V) electron paramagnetic resonance (EPR) signal of SeSOM4Ser was altered compared to those of SOMD4Ser and SOMD4cy,(,), with g, in particular shifted to a lower magnetic field, due to the Se ligation at the molybdenum. In contrast, the Mo(V) EPR signal of the high-pH form was unchanged. The substantially stronger effect of substituting selenocysteine for cysteine at low pH as compared to high pH is most likely due to the decreased covalency of the Mo Se bond.show moreshow less

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Metadaten
Author:Stefan Reschke, Dimitri Niks, Heather Wilson, Kajsa G. V. Sigfridsson, Michael Haumann, K. V. Rajagopalan, Russ Hine, Silke Leimkühler
DOI:https://doi.org/10.1021/bi4008512
ISSN:0006-2960 (print)
Parent Title (English):Biochemistry
Publisher:American Chemical Society
Place of publication:Washington
Document Type:Article
Language:English
Year of first Publication:2013
Year of Completion:2013
Release Date:2017/03/26
Volume:52
Issue:46
Pagenumber:9
First Page:8295
Last Page:8303
Funder:Deutsche Forschungsgemeinschaft (DFG) Cluster of Excellence "Unifying Concepts in Catalysis" [EXC 314]; DFG [LEI171/6-1]; Heisenberg Fellowship [Ha3265/3-1, Ha3265/6-1]; DFG; U.S. Department of Energy
Organizational units:Mathematisch-Naturwissenschaftliche Fakultät / Institut für Biochemie und Biologie
Peer Review:Referiert