SULT6B1

Model summary

A soluble cytosolic enzyme that transfers sulfate from the activated donor PAPS to a wide spectrum of endogenous and xenobiotic substrates. It preferentially modifies bile acids and steroid hormones and can also decorate flavonoids and anthracycline drugs, thereby tuning hormone signaling, promoting xenobiotic clearance, and routing diverse small molecules into sulfate-conjugated pools. Its nucleotide-cofactor–binding scaffold positions the sulfonyl group for efficient transfer, and its hydrophobic substrate channel accommodates steroidal and polyphenolic ligands. Operating in the cytoplasm, it integrates with pathways that maintain the cellular sulfate donor pool and coordinate broader metabolic and detoxification circuits.

Predicted Gene Ontology terms

1. 01

   I begin with the domain architecture.

   The N-terminal to central region (residues 25–301) is encompassed by IPR027417, a P-loop containing nucleoside triphosphate hydrolase homologous superfamily. This fold creates a nucleotide-binding cradle that stabilizes phosphate-rich ligands and positions them for chemistry rather than serving primarily as an ATPase. Nested within this scaffold, residues 56–289 form IPR000863, the sulfotransferase domain. This catalytic core binds the sulfate donor 3′-phosphoadenosine-5′-phosphosulfate (PAPS) and catalyzes transfer of the sulfonyl group to nucleophilic acceptors. The ordered arrangement—an NTP-binding P-loop-like superfamily that enforces nucleotide geometry, overlaid by a canonical sulfotransferase catalytic module—causally establishes a PAPS-dependent sulfonyl transfer mechanism.

2. 02

   From this architecture, the molecular function resolves as follows.

   The sulfotransferase domain dictates GO:0008146 sulfotransferase activity, while the P-loop-like scaffold rationalizes high-affinity interactions with nucleotide cofactors and diverse small-molecule substrates. The broad ligand-binding surface and soluble nature of the fold explain GO:0005515 protein binding through transient assemblies with metabolic partners and chaperones. The hydrophobic substrate channel typical of cytosolic sulfotransferases accommodates planar hydrophobes, supporting GO:1990239 steroid hormone binding and GO:0015485 cholesterol binding; these interactions arise because the active-site cavity and surrounding hydrophobic patches favor steroidal and polyphenolic ligands. The soluble nucleotide-binding architecture can also mediate GO:0003676 nucleic acid binding through electropositive grooves that engage RNA/DNA transiently, a property observed for several cytosolic enzymes that moonlight in RNA-associated assemblies.

3. 03

   The catalytic chemistry links directly to biological processes.

   By transferring sulfate from PAPS to small molecules, the enzyme executes GO:0051923 sulfation. Because PAPS is the immediate sulfate donor, the enzyme’s activity is embedded in GO:0050427 3′-phosphoadenosine 5′-phosphosulfate metabolic process. Its substrate spectrum—steroids, bile acids, thyroid hormones, flavonoids, and xenobiotics—drives GO:0009812 flavonoid metabolic process and GO:0006805 xenobiotic metabolic process. The ability to sulfate anthracyclines and related planar dyes situates it in GO:0044598 doxorubicin metabolic process and GO:1901616 organic hydroxy compound catabolic process. Broader alcohol and polyketide turnover terms (GO:0006067 ethanol metabolic process and GO:0030638 polyketide metabolic process) follow from the enzyme’s role in O-sulfation pathways that funnel diverse alcohol and polyketide-derived metabolites into excretable sulfates.

4. 04

   Cellular localization follows from the soluble, signal-peptide–free architecture and the absence of transmembrane segments: the enzyme operates in the cytoplasm, specifically the GO:0005829 cytosol.

   This soluble environment provides access to both endogenous substrates and xenobiotics arriving from cytosolic detoxification routes.

5. 05

   Mechanistically, the enzyme binds PAPS within the sulfotransferase core, aligning the sulfonyl group for in-line transfer to hydroxyl- or amine-bearing acceptors.

   The P-loop-like superfamily fold stabilizes the nucleotide and positions catalytic residues that activate the acceptor and stabilize the transition state. Substrate recognition explains its preference for bile acids and steroid hormones and its capacity to sulfate flavonoids and anthracyclines. I hypothesize transient assemblies with cytosolic metabolic hubs: enzymes that generate or consume PAPS (e.g., PAP-processing enzymes), phase II/III detoxification partners, and lipid/steroid carriers. Likely partners include PAPS-synthesis and turnover systems (to maintain sulfate donor flux), cytosolic detoxification modules that handle xenobiotics and flavonoids, and cytosolic carriers that deliver steroidal and cholesterol-derived substrates. These assemblies would channel substrates efficiently and coordinate with nucleic acid–associated complexes that transiently recruit soluble enzymes through nucleic acid binding surfaces.