Lack of substrate inhibition in a monomeric form of human cytosolic SULT2A1

Department of Pharmacology and Toxicology, University of Alabama at Birmingham, Birmingham, AL, USA.
Hormone molecular biology and clinical investigation 12/2010; 3(1):357-366. DOI: 10.1515/HMBCI.2010.041
Source: PubMed


Mammalian cytosolic sulfotransferases (SULTs) frequently show substrate inhibition during the sulfation of increasing concentrations of substrates. SULT2A1, a major human liver isoform responsible for the conjugation of hydroxysteroids, bile acids and aliphatic hydroxyl groups in drugs and xenobiotics, is a homodimer and displays substrate inhibition during the conjugation of dehydroepiandrosterone (DHEA). Maltose binding protein (MBP)-SULT2A1 fusion protein, produced as an intermediate step in the purification of the SULT2A1 homodimer, elutes during size exclusion chromatography as a monomer. The initial-rate parameters (K(m) and V(max)) of the monomer (MBP-SULT2A1) and native SULT2A1 dimer for DHEA sulfation are extremely similar; however, the monomer is not inhibited by DHEA. Intrinsic fluorescence studies show that two DHEA molecules bind each SULT2A1 subunit, one in the catalytic site and one in an apparent allosteric site. Lack of dimerization in the MBP-SULT2A1 fusion protein decreased the K(d) for binding of DHEA at the allosteric site. These results suggest that formation of the homodimer is associated with structural rearrangements leading to increased DHEA binding at an allosteric site that is associated with substrate inhibition.

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    • "This model of the involvement of SULT dimerization in substrate inhibition remains to be tested. However , it is worth noting that SULT isoforms have been reported to lose susceptibility to substrate inhibition upon monomerization (Cook et al. 2010a). "
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    • "The physiological significance of sulfotransferase dimerization has been examined for a number of different enzymes. For SULT2A1, dimerization affects enzyme kinetics, substrate binding to the allosteric site and substrate inhibition [18]. By contrast, the homomeric form of SULT1A1 is structurally unstable and is more readily denatured by temperature and urea compared to the wild-type protein [19]. "
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