Crystallographic temperature B factors and intramolecular mobility of RNase A

The temperature B factors for the atoms of RNase A in its various crystal s tructures and the causes of possible strong differences in their values wer e analyzed. It was shown that the large and, in different structures, varyi ng contribution of the rigid body disordering of the crystal lattice to the experimental values of B factors can be partially set off by using Delta B , the difference of mean B factors for the side chain and backbone atoms. F or six different crystal structures of RNase A and two structures of pancre atic trypsin inhibitor (PTI), the Delta B values were close to zero for the internal amino acid residues, which implies a similarity of amplitudes of thermal vibrations of the side chain and backbone atoms. Positive Delta B v alues are characteristic of the surface residues, well accessible for the s olvent, including some residues of the active center, but in the crystallin e RNase-inhibitor complex their Delta B values are close to zero as for the internal residues. Similarly to other globular proteins, the small-scale d ynamics of thermal vibrations of such internal residues in the crystal stru cture can be modeled by calculations of low-frequency harmonic modes and, b ased on the H-1 NMR data, remains close to harmonic also in the RNase and P TI solution. The dual role of the small-scale collective harmonic vibration s in the protein function is discussed.