 The enzyme transfers CO2 from glutaconyl-CoA to a biotin carrier protein (the γ-subunit) that is subsequently decarboxylated by the carboxybiotin decarboxylation site within the Na+ pumping beta subunit (Gcdβ).A proposed structure of the holoenzyme positions the water-filled central channel of the Gcdα dimer coaxial with the ion channel in Gcdβ.The central channel is blocked by arginines, which could allow Na+ passage by conformational movement or by entry through two side channels.