Figure 10.

Role of rotamer averaging in conformational changes associated with hinge motions. (A) Location of helix αF in the X-ray structure of the hE:FOL:NADP⁺ complex (PDB entry 4M6K).¹⁵ Helix αF is stabilized in its hinge-closed conformation by hydrogen bonds, shown by the black dashed lines. (B) X-ray structure of the hE:NADPH complex (PDB entry 4M6J), showing movement of helix αF toward the active site and breaking of the restraining hydrogen bonds upon opening of the hinges.¹⁵ Leu97 undergoes increased rotamer averaging in the hE:NADPH complex. Helix αF is colored red in panels A and B, and side chains are shown as sticks. NADP/H is colored cyan and atom colors and folate yellow and atom colors. (C) Superposition of the structures of the hE:NADPH (green) and hE:FOL:NADP⁺ (pink) complexes, showing conformational changes associated with sliding of helix αF and at the C-terminus of helix αE. Methyl-containing side chains (in PDB entry 4M6J) that undergo rotamer averaging in the hE:NADPH complex are shown as spheres: red for side chains on or in contact with helix αF, blue for side chains that pack against the C-terminal region of helix αE, pink for side chains in hinge 1, and gray for all other methyl-containing side chains with rotamer averaging. Leu133 also exhibits rotamer averaging in the crystal structure of the hE:NADPH complex. The NADPH in PDB entry 4M6J is colored cyan and atom colors; the folate and NADP⁺ ligands have been omitted from the structure of the hE:FOL:NADP⁺ complex for the sake of clarity.