The E1 conformation binds a phosphate from ATP and hydronium ion on the cytoplasmic side. The enzyme then changes to the E2 conformation, allowing hydronium to be released in the lumen. The E2 conformation binds potassium, and reverts to the E1 conformation to release phosphate and K+ into the cytoplasm where another ATP can be hydrolyzed to repeat the cycle. The β subunit prevents the E2-P conformation from reverting to the E1-P conformation, making proton pumping unidirectional. The number of ions transported per ATP varies from 2H+/2K+ to 1H+/1K+depending on the pH of the stomach.