title: Thermodynamic analysis of the unfolding and stability of the dimeric DNA-binding protein HU from the hyperthermophilic eubacterium Thermotoga maritima and its E34D mutant.
abstract: We have studied the stability of the histone-like, DNA-binding protein HU from the hyperthermophilic eubacterium Thermotoga maritima and its E34D mutant by differential scanning microcalorimetry and CD under acidic conditions at various concentrations within the range of 2-225 micro m of monomer. The thermal unfolding of both proteins is highly reversible and clearly follows a two-state dissociation/unfolding model from the folded, dimeric state to the unfolded, monomeric one. The unfolding enthalpy is very low even when taking into account that the two disordered DNA-binding arms probably do not contribute to the cooperative unfolding, whereas the quite small value for the unfolding heat capacity change (3.7 kJ.K(-1).mol(-1)) stabilizes the protein within a broad temperature range, as shown by the stability curves (Gibbs energy functions vs. temperature), even though the Gibbs energy of unfolding is not very high either. The protein is stable at pH 4.00 and 3.75, but becomes considerably less so at pH 3.50 and below, to the point that a simple decrease in concentration will lead to unfolding of both the wild-type and the mutant protein at pH 3.50 and low temperatures. This indicates that various acid residues lose their charges leaving uncompensated positively charged clusters. The wild-type protein is more stable than its E34D mutant, particularly at pH 4.00 and 3.75 although less so at 3.50 (1.8, 1.6 and 0.6 kJ.mol(-1) at 25 degrees C for DeltaDeltaG at pH 4.00, 3.75 and 3.50, respectively), which seems to be related to the effect of a salt bridge between E34 and K13.
authors.affiliation: Department of Physical Chemistry, Faculty of Sciences, University of Granada, 18071 Granada, Spain.
authors.forename: Javier
authors.initials: J
authors.lastname: Ruiz-Sanz
authors.forename: Vladimir V
authors.initials: VV
authors.lastname: Filimonov
authors.forename: Evangelos
authors.initials: E
authors.lastname: Christodoulou
authors.forename: Constantinos E
authors.initials: CE
authors.lastname: Vorgias
authors.forename: Pedro L
authors.initials: PL
authors.lastname: Mateo
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D001426:Bacterial Proteins / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics* / Q000302:isolation & purification; D002152:Calorimetry, Differential Scanning / Q000379:methods; D002942:Circular Dichroism; D004268:DNA-Binding Proteins / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics* / Q000302:isolation & purification; D019281:Dimerization; D006863:Hydrogen-Ion Concentration; D008956:Models, Chemical; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D011489:Protein Denaturation; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification; D016415:Sequence Alignment; D013696:Temperature; D013816:Thermodynamics; D020124:Thermotoga maritima / Q000737:chemistry* / Q000235:genetics
chemical_list: D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D011994:Recombinant Proteins; C040330:histone-like protein HU, bacteria