Amino acid transport in thermophiles: characterization of an arginine-binding protein from Thermotoga maritima. 3. Conformational dynamics and stability.

Arginine-binding protein from Thermotoga maritima (TmArgBP) is a 27.7 kDa protein possessing the typical two domain structure of the periplasmic binding protein family. The protein is characterized by high specificity and affinity for binding a single molecule of l-arginine. In this work, the effect of temperature and/or guanidine hydrochloride on structure and stability of the protein in the absence and in the presence of l-arginine has been investigated by differential scanning calorimetry, far-UV circular dichroism and intrinsic tryptophan phosphorescence and fluorescence. The results revealed that TmArgBP undergoes an irreversible one-step thermal unfolding process in a cooperative mode. The TmArgBP melting temperature was recorded at 115 °C. The presence of l-arginine did not change the protein secondary structure content as well as the intrinsic phosphorescence and fluorescence protein properties, even if it increases the structural stability of the protein. The obtained results are discussed in combination with a detailed inspection of the three-dimensional structure of the protein.