Sodium V-ATPase rotor ring, molecular model. V-ATPases (vacuolar-type ATPase) are evolutionarily ancient enzymes that helps transport ions across cell membranes, usually protons and in this case sodium ions. The underlying reaction providing the energy is the hydrolysis of adenosine triphosphate (ATP), which is catalysed by this enzyme. This example of a V-ATPase is from the Enterococcus hirae bacterium. This rotor ring is part of the structure through which the ions pass as they cross the cell membrane. The ring contains 10 subunits (protein chains), indicated by the colours.