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 2.2 Å 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 χ₁ ranging from −138° to −168° and χ₂ ranging from +16° to +70°, 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 & Richards (1987). J. Mol. Biol. 193, 775–791] . However, when compared to a more recent `backbone-dependent' rotamer library [Dunbrack & Karplus (1993). J. Mol. 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.