CRYSTALLOGRAPHIC ANALYSIS OF PHE-]LEU SUBSTITUTION IN THE HYDROPHOBIC CORE OF BARNASE

The crystal structure of a barnase mutant, Phe-->Leu7 has been determined to 2.21 Angstrom resolution. No structural rearrangement is observed near the mutated residue. The F7L mutation is highly destabilizing and this is caused by the loss of extensive van der Waals contacts that wild-type Phe7 made with its neighbouring residues, and the exposure of a large hydrophobic pocket on the surface of the protein. The side-chain conformations of the mutated Leu7 residue have torsion angles chi(1) ranging from -138 degrees to -168 degrees and chi(2) ranging from +16 degrees to +70 degrees, for the three molecules in the asymmetric unit. These angles do not agree with the most frequently observed conformations in the protein side-chain rotamer library [Ponder and Richards (1987). J. Mol. Biol. 193, 775-791]. However, when compared to a more recent 'backbone-dependent' rotamer library [Dunbrack and Karplus (1993). J. Mel. Biol. 230, 543-574], the side-chain conformation of Leu7 agrees well with that of the most frequently observed rotamers. The side-chain conformation of Leu7 was found to be dictated by two factors: it has the lowest conformational energy and it buries the most hydrophobic surface area.