The cytosolic sulfotransferases (SULTs) in vertebrates catalyze the sulfonation of endogenous thyroid/steroid hormones and catecholamine neurotransmitters, as well as a variety of xenobiotics, using 3'-phosphoadenosine 5'-phosphosulfate (PAPS) as the sulfonate donor. In this study, we determined the structures of SULT1A2 and an allozyme of SULT1A1, SULT1A1*3, bound with 3'-phosphoadenosine 5'-phosphate (PAP), at 2.4 and 2.3 angstrom resolution, respectively. The conformational differences between the two structures revealed a plastic substrate-binding pocket with two channels and a switch-like substrate selectivity residue Phe247, providing clearly a structural basis for the substrate inhibition. In SULT1A2, Tyr149 extends approximately 2.1 angstrom further to the inside of the substrate-binding pocket, compared with the corresponding His149 residue in SULTIA1*3. Site-directed mutagenesis study showed that, compared with the wild-type SULT1A2, mutant Tyr149Phe SULT1A2 exhibited a 40 times higher K-m, and two times lower V-max with p-nitrophenol as substrate. These latter data imply a significant role of Tyr149 in the catalytic mechanism of SULT1A2. (C) 2010 Elsevier Inc. All rights reserved.