The four-helix bundle: An attractive fold

A convergence of evidence suggests the core of 4-α-helix bundles is an important structural motif of volatile anesthetic targets. However, structural detail of such sites in natural proteins remains elusive. We screened over 80 soluble proteins for anesthetic binding, and found that ferritin, a 24-mer of 4-α-helix bundles, binds halothane with KA values of ∼ 105 M− 1, higher than any previously reported inhaled anesthetic-protein interaction. The crystal structures of the 1.7 Å halothane/apoferritin complex revealed a binding pocket within an interhelical dimerization interface. The high affinity is explained by favorable entropy and enthalpy, but the acidic proton does not appear to contribute to binding. Halothane produced no detectable alteration of structure or B factors. The remarkably high affinity of the anesthetic/apoferritin complex implies greater selectivity of protein sites than previously thought, and also suggests that direct protein actions may underlie effects at lower than surgical levels of anesthetic, including loss of awareness.