Rewiring V-type and K-type enzyme allostery through subunit interface mutations

Technical Summary: Rewiring V-type and K-type Enzyme Allostery through Subunit Interface Mutations

Problem Statement
The heterodimeric enzyme IGPS relies on long-range allosteric communication to regulate its function, transmitting signals from the effector-binding HisF subunit to the catalytic HisH subunit. This signaling is mediated by a densely packed interdomain interface characterized by conserved noncovalent contacts. While the existence of this interface is established, the specific role individual interfacial contacts play in tuning the structural and dynamic features that govern allosteric control remains to be fully elucidated. The central challenge addressed is determining how specific mutations at this interface alter the enzyme's conformational ensemble and, consequently, its allosteric output.

Methodology
The study employs a targeted mutagenesis approach focused on the interdomain interface of IGPS to dissect the contributions of specific residues. Two primary variants were generated:

1. hK181A: Disrupts a critical salt bridge between HisF residue K181 and fD98.
2. hR18A: Disrupts a secondary salt bridge between HisF residue R18 and fE71.

These mutants were characterized using a multi-faceted experimental and computational framework:

• Enzymatic Assays: Measurement of basal glutaminase activity and substrate affinity to assess functional changes.
• NMR Spectroscopy: Chemical shift perturbation (CSP) experiments to map structural changes, and CPMG relaxation dispersion experiments to probe millisecond-timescale dynamics.
• Molecular Dynamics (MD) Simulations: Used to correlate experimental observations with conformational sampling and to identify catalytically competent states.
• Network and Energetic Analysis: Network traffic analysis was utilized to map correlated communication pathways, while energetic analysis quantified the redistribution of enthalpic and entropic contributions.

Key Results
The investigation yielded distinct functional and dynamic outcomes for the two interface mutations, revealing functional asymmetry among interfacial interactions:

• Constitutive Activation (hK181A): Disruption of the K181–fD98 salt bridge converted IGPS into a constitutively more active enzyme. This variant exhibited increased basal glutaminase activity and enhanced substrate affinity. NMR and MD data indicate that this mutation remodels millisecond-timescale dynamics throughout the HisF subunit, promoting enhanced sampling of catalytically competent conformations.
• Attenuated Activation (hR18A): In contrast, disrupting the R18–fE71 salt bridge weakened effector-induced activation, demonstrating that not all interfacial contacts contribute equally to the allosteric mechanism.
• Rewired Signaling: Network traffic analysis combined with energetic profiling revealed that the hK181A mutation redistributes enthalpic and entropic contributions, effectively rewiring the long-range allosteric signaling pathways.

Key Contributions
The paper provides a mechanistic dissection of how specific interfacial residues act as molecular gates that shape the conformational ensemble accessible to IGPS. By correlating specific salt bridge disruptions with changes in dynamics and energetics, the study demonstrates that the interdomain interface is not a static scaffold but a tunable regulatory element. The work establishes a direct link between specific noncovalent contacts, the redistribution of thermodynamic contributions, and the resulting allosteric phenotype.

Significance
The authors conclude that these results identify specific interfacial residues as critical determinants of allosteric control. The primary significance of the work lies in demonstrating that interface reengineering can be used to rationally reprogram allosteric output. By identifying these "molecular gates," the study offers a framework for understanding how long-range communication is tuned in heterodimeric enzymes and suggests a pathway for the rational design of allosteric regulation through targeted interface mutations.