Ca2+ binding induced sequential allosteric activation of sortase A: An example for ion-triggered conformational...

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Staphylococcus aureus sortase A (SrtA) is an intrinsically disordered enzyme.
• Allosteric activation of SrtA is initiated by calcium ion (Ca2+) binding.
• The precise mechanism of Ca2+ induced allosteric activation and its effect on enzyme dynamics remain unclear.

Purpose of the Study:

• To elucidate the molecular mechanism of allosteric activation of SrtA by Ca2+.
• To investigate the role of intrinsically disordered regions in SrtA's dynamic regulation.
• To understand how Ca2+ binding influences the enzyme's catalytic activity and substrate binding.

Main Methods:

• Long-term molecular dynamics (MD) simulations of SrtA.
• Simulations were performed with and without Ca2+ bound.
• Analysis of protein structural changes, loop ordering, hinge motions, and binding groove conformations.

Main Results:

• Ca2+ binding induces ordering of the disordered β6/β7 loop.
• Ca2+ binding modulates hinge motions in the dynamic β7/β8 loop, crucial for catalysis.
• Signal transmission from Ca2+ site to β7/β8 loop occurs via repetitive folding/unfolding of the β6/β7 loop.
• Correlated rearrangements lead to distinct binding groove conformations with significantly enhanced binding energies for the sorting signal motif (up to 20 kcal/mol more favorable).

Conclusions:

• SrtA activation by Ca2+ involves a highly correlated, conformational selection-based mechanism.
• The dynamics of intrinsically disordered regions are essential for allosteric regulation in SrtA.
• Ca2+ binding optimizes the enzyme's binding groove for efficient substrate recognition and catalysis.