Glutathione Transferases as Efficient Ketosteroid Isomerases

Area of Science:

• Biochemistry
• Enzymology
• Steroid Metabolism

Background:

• Glutathione transferases (GSTs) are primarily known for detoxification roles.
• Emerging evidence highlights their involvement in diverse biological functions beyond detoxification.
• Ketosteroid isomerase activity of GSTs is implicated in steroid hormone biosynthesis.

Purpose of the Study:

• To investigate the ketosteroid isomerase activity of GST A3-3.
• To understand the enzyme's role in steroid hormone biosynthesis.
• To explore the evolutionary and mechanistic aspects of this enzymatic function.

Main Methods:

• Enzyme kinetic analysis (kcat/Km) of GST A3-3 in humans and horses.
• Assessing the impact of GST A3-3 inhibition on progesterone production in human cells.
• Comparative analysis of active-site residues and their role in catalysis.
• Investigating homologous enzymes in other species, including Drosophila melanogaster.

Main Results:

• Human and equine GST A3-3 exhibit exceptionally high catalytic efficiency (kcat/Km ~10^7 M^-1s^-1) for ketosteroid isomerization.
• GST A3-3 expression in steroidogenic tissues supports 3β-hydroxysteroid dehydrogenase activity.
• Inhibition of GST A3-3 in human cells significantly reduces progesterone formation.
• Specific active-site residues (Tyr9, Arg15) and H-site residues are critical for catalysis.

Conclusions:

• GST A3-3 plays a significant, previously underappreciated role in mammalian steroid hormone biosynthesis.
• The enzyme's high catalytic efficiency suggests an evolved specialization for this function.
• Species-specific variations in GST A3-3 activity warrant further investigation.
• Conserved active-site residues are essential, but substrate-binding pocket architecture dictates efficiency.