Fig. 1.

Topology and reaction schemes of flippase and Na⁺,K⁺-ATPase. (A) Diagram of P4-ATPase topology indicating P-type ATPase domains and residues studied here by mutagenesis, as well as K873 of M5 studied previously (12). (B) Proposed flippase reaction cycle. (C) Reaction cycle of Na⁺,K⁺-ATPase [Post-Albers scheme (10, 11)]. E₁, E₁P, E₂P, and E₂ are the major enzyme forms, the P indicating phosphorylation.