Binding of monovalent cations induces large changes in the secondary structure of Na⁺,K⁺‐ATPase as probed by Raman spectroscopy

Raman spectra of active Na⁺,K⁺-ATPase from pig kidney in media containing Na⁺ (E₁), K⁺ (E₂) or without exogenous ions (E₁ conformation) were recorded in order to calculate the changes in the enzyme's secondary structure induced by binding of monovalent cations. It is demonstrated that: (i) K⁺ binding to the E₁ form of the enzyme leads to conversion of 100 peptide groups from the β-structure to α-helical conformation; (ii) the transition is reversible and fully reproducible in the E₁→E₂→E₁ and E₂→E₁→E₂ experimental schemes. Predictional calculations revealed polypeptide chain segments involved in the α ↔ β transformations. These segments reside mainly in the two highly conserved regions of the α-subunit in the cytoplasmic domain of Na⁺,K⁺-ATPase. A possible role for the β-subunit is discussed.