Conformational Remodeling Underlies Activity Loss in Disease-Linked Asparagine Synthetase Variant

Area of Science:

• Biochemistry and structural biology
• Enzymology
• Genetics and molecular biology

Background:

• Asparagine synthetase deficiency (ASNSD) is a severe congenital disorder linked to mutations in the asparagine synthetase (ASNS) gene, leading to neurological impairment and early death.
• The precise molecular mechanisms by which ASNSD-associated missense mutations affect ASNS enzyme function are not well understood, hindering therapeutic development.

Purpose of the Study:

• To elucidate the molecular basis of the recurrent ASNSD-linked R48Q variant in human asparagine synthetase (ASNS).
• To understand how this specific mutation impacts enzyme structure, dynamics, and catalytic activity.

Main Methods:

• Steady-state kinetic assays to assess enzyme catalysis and product stoichiometry.
• Cryogenic electron microscopy (cryo-EM) and 3D variable analysis to determine high-resolution structures.
• Molecular dynamics (MD) simulations to investigate protein dynamics and interdomain communication.

Main Results:

• The R48Q variant significantly reduces L-glutamine-dependent catalysis and disrupts product stoichiometry, indicating impaired interdomain communication.
• Cryo-EM revealed altered loop conformations at the N-terminal active site and subtle changes in the C-terminal domain.
• MD simulations showed that the local mutation propagates conformational changes, decoupling essential domain motions required for catalysis.
• Evolutionary conservation of the affected region highlights its mechanistic importance across related enzymes.

Conclusions:

• The R48Q mutation in ASNS disrupts enzyme function through a combination of local structural changes and propagation of dynamics, leading to ASNSD.
• This study provides the first mechanistic blueprint for an ASNSD-linked variant, deepening the understanding of ASNS function.
• The findings offer a generalized framework for investigating how point mutations affect the dynamics of multidomain enzymes in disease, with implications for precision medicine.