STRUCTURAL AND KINETIC IMPLICATIONS OF SUBSTRATE INHIBITION IN THE HUMAN SULFOTRANSFERASE 1A1 ENZYME

Niranjali U. Gamage,^a,c Ronald G.Duggleby,^b Amanda C. Barnett,^a Michael Tresillian,^a Catherine F. Latham,^a,c Nancy E. Liyou,^a Michael E. McManus^a and Jennifer L. Martin^c

^aDepartment of Physiology and Pharmacology, School of Biomedical Sciences, University of Queensland, Brisbane, Queensland, 4072, Australia; ^bDepartment of Biochemistry and Molecular Biology, School of Molecular and Microbial Sciences, University of Queensland, Brisbane, Queensland, 4072, Australia; ^cInstitute for Molecular Bioscience, University of Queensland, Brisbane, Queensland, 4072, Australia (n.gamage1@mailbox.uq.edu.au)

Sulfonation catalyzed by sulfotransferases is an important pathway in detoxification of a broad range of endobiotics and xenobotics, but evidence has emerged in recent years that sulfonation can lead to bioactivation of carcinogens. A major human sulfotransferase, SULT1A1, has been shown to play a significant role in metabolizing many endogenous compounds and is implicated in a range of cancers due to its ability to bioactivate carcinogenic and mutagenic xenobiotic compounds. The crystal structure that we report here is that of SULT1A1 complexed with PAP and the xenobiotic substrate p-nitrophenol (pNP) ^[1]. This is the first sulfotransferase structure complexed with a xenobiotic substrate and unexpectedly it reveals two molecules of pNP in the active site. The kinetics of SULT1A1with pNP shows slight non-hyperbolic behaviour at low concentrations and substrate inhibition at higher pNP concentrations. These kinetic features are fully consistent with the SULT1A1 structure. The extended active site of SULT1A1 revealed by the crystal structure is also in agreement with binding of the endogenous ligand di-iodothyronine but it cannot accommodate its other substrate b-estradiol as easily. This therefore suggests that the binding site is flexible to accept diverse hydrophobic molecules of varying sizes and shapes. Therefore, the SULT1A1 structure provides the structural basis for substrate inhibition and reveals the first clues as to how this enzyme sulfonates various liphophlic compounds.

References

1 Gamage NU, Duggleby RG, Barnett AC, Tresillian M, Latham CF, Liyou NE, McManus ME, Martin J (2003). Structure of a human carcinogen converting enzyme, SULT1A1: structural and kinetic implications of substrate inhibition. J Biol Chem.278, 7655-7662.