The ultrahigh resolution crystal structure of ribonuclease A containing anisoaspartyl residue: Hydration and sterochemical analysis

Crystals of the deamidated form of bovine pancreatic ribonuclease which con tains an isoaspartyl residue in position 67 diffract to 0.87 Angstrom at 10 0 K. We have refined the crystallographic model using anisotropic displacem ent parameters for all atoms to a conventional crystallographic residual R = 0.101 for all observed reflections in the resolution range 61.0-0.87 Angs trom. The ratio observations /parameters is 7.2 for the final model. This s tructure represents one of the highest resolution protein structures to dat e and interestingly, it is the only example containing more than one molecu le in the asymmetric unit with a resolution better than 1.0 Angstrom. The n on-crystallographic symmetry has been used as a validation check of the geo metrical parameters and it has allowed an estimate for an upper limit of er rors associated with this high resolution model. In the present structure i t was possible to obtain a more accurate picture of the active site whose e lectron density was not clearly interpretable in the previous 1.9 Angstrom resolution structure. In particular, the P1 site is alternatively occupied either by a sulphate anion or by a water molecule network. Most of hydrogen atoms were visible in the electron density maps, including those involved in C-alpha-H-alpha ... O interactions. Analysis of protein-solvent interact ions has revealed the occurrence of an extensive cluster of water molecules , predominantly arranged in pentagonal fused rings and surrounding hydropho bic moiety of side-chains. Finally, in spite of the limited sample of resid ues, we have detected a clear dependence of backbone N-C-alpha-C angle on r esidue conformation. This correlation can be fruitfully used as a valuable tool in protein structure validation. (C) 2000 Academic Press.