Stabilization of halophilic malate dehydrogenase

Area of Science:

• Biochemistry
• Structural Biology
• Extremophile Research

Background:

• Malate dehydrogenase from Halobacterium marismortui is stable only in high salt concentrations.
• Its physiological environment is saturated in KCl, with stability observed in NaCl or KCl solutions.
• The enzyme unfolds below 2.5 M-NaCl or -KCl, indicating a need for high salt concentrations.

Purpose of the Study:

• To investigate the stabilization mechanisms of halophilic malate dehydrogenase in various salt solutions.
• To propose a novel model that explains the enzyme's stability and solvent interactions.
• To reconcile experimental observations with existing theories of protein-salt interactions.

Main Methods:

• Enzymic activity and stability measurements.
• Neutron scattering, ultracentrifugation, and quasi-elastic light-scattering experiments.
• Analysis of protein structure and stabilization in different salt solutions (potassium phosphate, MgCl2, NaCl, KCl).

Main Results:

• Protein structure and stabilization mechanisms differ across various salt solutions.
• In phosphate ions, stabilization resembles non-halophilic proteins, dominated by hydrophobic effects.
• In high concentrations of KCl, NaCl, or MgCl2, solution particles form with protein dimers interacting with numerous salt and water molecules.

Conclusions:

• A novel stabilization model for halophilic malate dehydrogenase is proposed, involving cooperative hydrate bonds with hydrated salt ions.
• This mechanism explains the enzyme's stability in high salt concentrations, particularly MgCl2, NaCl, and KCl.
• Model predictions align with experimental data on solvent interactions, unfolding at non-physiological concentrations, and temperature denaturation.