Structural and Chemical Profiling of the Human Cytosolic Sulfotransferases

Abstract

The human cytosolic sulfotransfases (hSULTs) comprise a family of 12 phase II enzymes involved in the metabolism of drugs and hormones, the bioactivation of carcinogens, and the detoxification of xenobiotics. Knowledge of the structural and mechanistic basis of substrate specificity and activity is crucial for understanding steroid and hormone metabolism, drug sensitivity, pharmacogenomics, and response to environmental toxins. We have determined the crystal structures of five hSULTs for which structural information was lacking, and screened nine of the 12 hSULTs for binding and activity toward a panel of potential substrates and inhibitors, revealing unique “chemical fingerprints” for each protein. The family-wide analysis of the screening and structural data provides a comprehensive, high-level view of the determinants of substrate binding, the mechanisms of inhibition by substrates and environmental toxins, and the functions of the orphan family members SULT1C3 and SULT4A1. Evidence is provided for structural “priming” of the enzyme active site by cofactor binding, which influences the spectrum of small molecules that can bind to each enzyme. The data help explain substrate promiscuity in this family and, at the same time, reveal new similarities between hSULT family members that were previously unrecognized by sequence or structure comparison alone. We metabolize many hormones, drugs, and bioactive chemicals and toxins from the environment. One family of enzymes that participate in the metabolic process consists of the cytosolic sulfotransferases, or SULTs. SULTs have a variety of mechanisms of action—sometimes they inactivate the biological activity of the chemical (e.g., in the case of estrogen). At other times, the enzymes make the chemical more toxic (e.g., for certain carcinogens). Humans have 12 distinct SULT enzymes. Determining how each of these human enzymes recognizes and distinguishes between the thousands of chemicals we confront each day is essential for understanding hormone regulation, assessing environmental risk, and eventually developing better, more-effective drugs. We have studied the human SULT family of enzymes to profile which small molecules are recognized by each enzyme. We also visualized and compared the detailed structural features that determine which enzyme interacts with which molecule. By studying the entire family, we discovered new ways in which chemicals interact with each enzyme. Furthermore, we identified new inhibitors and inhibitory mechanisms. Finally, we discovered functions for many of the human enzymes that were previously uncharacterized.