Activity Regulation of a Glutamine Amidotransferase Bienzyme Complex by Substrate-Induced Subunit Interface Expansion

Area of Science:

• Biochemistry
• Enzymology
• Structural Biology

Background:

• Glutamine amidotransferases are multienzyme complexes that utilize ammonia generated by a glutaminase domain.
• Ammonia is transferred via a protein tunnel to a synthase domain for metabolite synthesis.
• Synchronized catalysis is crucial to prevent wasteful consumption of metabolites, but the underlying mechanism remains unclear.

Purpose of the Study:

• To investigate the mechanism controlling glutaminase catalysis in glutamine amidotransferases.
• To elucidate how synchronized catalysis is achieved in these multienzyme systems.
• To utilize aminodeoxychorismate synthase, involved in folate biosynthesis, as a model system.

Main Methods:

• Analysis of the aminodeoxychorismate synthase system in different catalytic states.
• Structural analysis to observe changes in subunit interfaces and active site accessibility.

Main Results:

• Incubation with glutamine induced a significant expansion of the subunit interface in aminodeoxychorismate synthase.
• This expansion resulted in the complete enclosure of the glutaminase active site.
• The enclosed active site facilitates sequestered ammonia generation and transport to the synthase active site.

Conclusions:

• The observed subunit interface expansion provides a mechanism for synchronized catalysis in glutamine amidotransferases.
• This structural rearrangement ensures efficient and controlled ammonia transfer, preventing wasteful reactions.
• The findings suggest a general mechanism applicable to the entire glutamine amidotransferase enzyme family.