Allosteric Mechanisms Triggering Substrate and Cofactor Binding in the SULT1A1 Dimer as Revealed by Molecular Dynamics Simulations.

Sulfotransferases (SULTs) are phase II drug-metabolizing enzymes metabolizing a wide range of endogenous compounds and xenobiotics including drugs. SULTs form dimers in vivo, and most isoforms share a conserved dimerization motif. Since it has been shown that the monomers of the SULT1A1 isoform maintain their activity in vitro, the biological significance of dimerization remains unclear. To elucidate the mechanism and the effects of dimerization on the SULT1A1 structure and function, we performed molecular dynamics (MD) simulations on both the monomer and dimer form of the enzyme and investigated the effect of cofactor and substrate binding into the dimer structure and dynamics. Our results show a clear dynamical effect on the dimerization of the apoenzyme, resulting in an increase of the ligand binding gate opening and greater fluctuation of the functional loops of one monomeric subunit. Furthermore, in the dimer, we uncovered intra- and intersubunit allosteric effects as a direct consequence of cofactor and the substrate binding, and we present the corresponding allosteric pathways. Our analyses suggest that the asymmetric behavior of the dimer in the presence of one PAPS molecule may reflect a half-site reactivity mechanism, previously suggested for SULT dimer function, which may be particularly important for large substrates. Thus, our study shed new light in our understanding of SULT1A1 structural dynamics and dimerization as related to enzyme function.