Side-chain conformations in 4-{{alpha}}-helical bundles

The distribution of the ₁, ₂ dihedral angles in a dataset consistingof 12 unrelated 4--helical bundle proteins was determined andqualitatively compared with that observed in globular proteins.The analysis suggests that the 4--helical bundle motif couldoccasionally impose steric constraints on side chains: (i) theside-chain conformations are limited to only a subset of theconformations observed in globular proteins and for some aminoacids they are sterically more constrained than those in helicalregions of globular proteins; (ii) aspartic acid and asparagineoccasionally adopt rotamers that have not been previously reportedfor globular or helical proteins; (iii) some rotamers of tyrosineand isoleucine are predominantly or exclusively associated withhydrophobic core positions (a, d); (iv) mutations in the hydrophobiccore occur preferentially between residue types which amongother physicochemical properties also share a predominant rotamer.