Environmental contaminants perturb fragile protein assemblies and inhibit normal protein function

Area of Science:

• Biochemistry
• Toxicology
• Enzyme kinetics

Background:

• The molecular mechanisms of environmental contaminant toxicity are poorly understood.
• Human porphobilinogen synthase (HsPBGS) is crucial for heme biosynthesis and its activity is regulated by oligomeric state.
• Small molecules can inhibit HsPBGS by stabilizing its inactive hexameric form.

Purpose of the Study:

• To investigate if environmental contaminants from the National Toxicology Program library can stabilize the HsPBGS hexamer and inhibit its activity.
• To identify specific contaminants that perturb HsPBGS oligomeric distribution and function.

Main Methods:

• Screening of 1,408 compounds from the National Toxicology Program library using native polyacrylamide gel electrophoresis.
• Confirmation of preliminary hits and dose-response analysis of compounds affecting HsPBGS oligomeric distribution.
• In vitro assays to determine the inhibitory effects of identified compounds on HsPBGS catalytic activity.

Main Results:

• Seventeen compounds were identified that shift HsPBGS oligomeric distribution towards the hexamer and inhibit its activity.
• The most potent inhibitor, Mutagen X, was characterized with an IC50 of 1.4 μM.
• These findings demonstrate that environmental contaminants can act as allosteric inhibitors of HsPBGS.

Conclusions:

• Environmental contaminants can inhibit HsPBGS by stabilizing its inactive hexameric form, representing a novel toxicity mechanism.
• The identified inhibitors may exacerbate conditions associated with PBGS dysfunction, such as ALAD porphyria and lead poisoning.
• Perturbation of protein oligomeric equilibria by small molecules is a viable mechanism for environmental toxicity.