Sulfotransferase 1A1 Substrate Selectivity: A Molecular Clamp Mechanism

Area of Science:

• Biochemistry
• Enzymology
• Pharmacology

Background:

• Human cytosolic sulfotransferases (SULTs) catalyze sulfuryl group transfer from PAPS to diverse substrates.
• SULT1A1, abundant in the liver, plays a key role in metabolizing endogenous compounds and detoxifying xenobiotics.
• Understanding SULT1A1 substrate selectivity is crucial for its biological and pharmacological relevance.

Purpose of the Study:

• To investigate the differential effects of nucleotide binding on SULT1A1 substrate affinity and catalytic activity.
• To elucidate the molecular mechanism underlying SULT1A1 substrate selectivity.

Main Methods:

• Enzyme kinetics assays to determine substrate affinity (Km) and catalytic rates (kcat).
• All-atom molecular dynamics simulations to model enzyme-substrate interactions.
• Analysis of point mutants to validate proposed catalytic mechanisms.

Main Results:

• SULT1A1 substrates were classified into positive-synergy and neutral-synergy groups based on nucleotide presence.
• Positive-synergy substrates exhibited significantly enhanced affinity (∼20-fold) and catalytic efficiency (∼3 orders of magnitude).
• Molecular dynamics revealed repositioning of phenylalanine residues (F81, F84) for positive-synergy substrates, facilitating catalysis, while neutral-synergy substrates showed less defined active site positioning.

Conclusions:

• SULT1A1 exhibits distinct substrate classes with differential responses to nucleotide binding, impacting catalytic efficiency.
• A molecular mechanism involving phenylalanine residue repositioning explains the enhanced affinity and catalysis for positive-synergy substrates.
• These findings provide insights into SULT1A1's broad substrate specificity and catalytic regulation.