Solvent Interactions of Halophilic Malate Dehydrogenase

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Abstract
Malate dehydrogenase from the extreme halophilic Haloarcula marismortui (Hm MalDH) is an acidic protein that is unstable below molar salt concentrations. The solvated folded protein was studied by small-angle neutron scattering in solvents containing salt: NaCl, NaCH3CO2, KF, NH4Cl, NH4CH3CO2, (NH4)2SO4, MgCl2, and MgSO4. It was found that the global solvent interactions depend mainly on the nature of the cation. Complementary mass density measurements in MgCl2, NaCl, NaCH3CO2, and (NH4)2SO4 allowed determining the partial molal volumes of the protein, which were found to increase slightly with the salt, and the preferential salt binding parameters for each solvent condition. These are strongly dependent on the cation type and salt concentration. Hm MalDH can be modeled as an invariant particle binding 4100 water molecules in MgCl2 and 2000 ± 200 in NaCl, NaCH3CO2, or (NH4)2SO4. The number of salt molecules associated to the particle decreases from about 85 to 0 in the order MgCl2 > NaCl = NaCH3CO2 > (NH4)2SO4. Alternatively, we considered exchangeable sites for water and salt with the effects of solvent nonideality. It does not change the description of the solvent interactions. Solvent anions act on Hm MalDH stability through a limited number of strong binding sites, as those seen at the interfaces of Hm MalDH by crystallography. Cations would act through some strong and numerous weak binding sites defined on the folded protein, in possible addition to nonspecific hydration effects.