Structure of a Human Carcinogen-converting Enzyme, SULT1A1

Abstract

Sulfonation catalyzed by sulfotransferase enzymes plays an important role in chemical defense mechanisms against various xenobiotics but also bioactivates carcinogens. A major human sulfotransferase, SULT1A1, metabolizes and/or bioactivates many endogenous compounds and is implicated in a range of cancers because of its ability to modify diverse promutagen and procarcinogen xenobiotics. The crystal structure of human SULT1A1 reported here is the first sulfotransferase structure complexed with a xenobiotic substrate. An unexpected finding is that the enzyme accommodates not one but two molecules of the xenobiotic model substrate p-nitrophenol in the active site. This result is supported by kinetic data for SULT1A1 that show substrate inhibition for this small xenobiotic. The extended active site of SULT1A1 is consistent with binding of diiodothyronine but cannot easily accommodate ␤-estradiol, although both are known substrates. This observation, together with evidence for a disorder-order transition in SULT1A1, suggests that the active site is flexible and can adapt its architecture to accept diverse hydrophobic substrates with varying sizes, shapes and flexibility. Thus the crystal structure of SULT1A1 provides the molecular basis for substrate inhibition and reveals the first clues as to how the enzyme sulfonates a wide variety of lipophilic compounds.Sulfonation is a widely distributed biological reaction catalyzed by members of the supergene family of enzymes called sulfotransferases (SULTs).1 SULTs utilize the sulfonate donor 3Ј-phosphoadenosine 5Ј-phosphosulfate (PAPS) to catalyze sulfonation, yielding PAP as the desulfonated product of the reaction. These enzymes modify the biological activities of a diverse range of compounds including neurotransmitters, hormones, and drugs. The reaction aids excretion of foreign chemicals but, in some cases, causes bioactivation of carcinogens and mutagens (1-3). SULTs are the focus of intense research in the fields of cancer, drug metabolism, and pharmacogenetics because genetic variation, particularly in isoenzyme SULT1A1, may predispose to lung cancers (4), protect against colorectal cancers (5), and affect the age of onset in breast cancer (6).To date, five distinct mammalian gene families for cytosolic SULTs (SULTs 1-5) have been identified (7), although SULT3 and SULT5 have not been found in humans (8). The most extensive group of the human cytosolic SULTs is the SULT1 family, which includes SULTs 1A1, 1A2, 1A3, 1B1, 1C1, 1C2, and 1E1. Each SULT has distinct substrate preferences but may also exhibit broad and overlapping substrate specificity to span the diversity of chemicals requiring sulfonation. For example, SULT1A3 catalyzes dopamine sulfonation but also accepts p-nitrophenol (pNP) with lower affinity (9, 10). SULT1A1, which shares 93% identity with SULT1A3, also sulfonates dopamine and pNP but with the reverse preference, and in addition it can sulfonate a wide range of hydrophobic molecules (Fig. 1) including xenobiotics and endogenous substrates 3,3Ј-diio...

The Structure of Human SULT1A1 Crystallized with Estradiol

“…1). The binding mode of the product, PAP, is similar to that described for the SULT1A1⅐PAP⅐pNP structure (11).…”

“…In that study we showed that the active site of SULT1A1 is capable of binding two molecules of pNP simultaneously (11). Further, we have demonstrated, using both molecular modeling and site-directed mutagenesis, that SULT1A3 can bind two molecules of dopamine in its active site (12).…”

“…The crystal structure of SULT1A1 and modeling studies have shown how small planar molecules such as pNP and L-shaped aromatics (iodothyronines) can bind to the active site (11) but have not explained how extended multi-ring systems such as E2 interact with the enzyme. We proposed that the SULT1A1 active site is plastic and adopts various shapes to accept different types of molecules (11). Previously, we reported an L-shaped binding site in the structure when SULT1A1 was crystallized with pNP (11).…”

para-Nitrophenyl Sulfate Activation of Human Sulfotransferase 1A1 Is Consistent with Intercepting the E·PAP Complex and Reformation of E·PAPS

“…A SULT1A1 crystal structure was solved that showed two molecules of p-nitrophenol (pNP) in the same active site. However, computer modeling of this structure indicated that the active site could not easily accommodate even one molecule of a larger substrate such as ␤-estradiol (17). Other SULT crystal struc-tures solved with the bound substrate indicated that only one substrate is possible in the crystal structure (18 -23).…”

Interindividual Variability in 2-Hydroxylation, 3-Sulfation, and 3-Glucuronidation of Ethynylestradiol in Human Liver

“…A similar substrate inhibition was commonly observed for phenol sulfotransferases. 18,19) Clinically relevant plasma concentrations of EE 2 are very low (Ͻ2 nM). 7) These values indicate that the possibility of substrate inhibition of EE 2 3-sulfation in human liver can be completely ruled out.…”