Sodium-Coupled Secondary Transporters, Structure and Function
Secondary active transporters are membrane proteins that move a substrate (main substrate) across the cell membrane against its concentration gradient, utilizing the free energy stored in the downhill concentration gradient of one or more coupled substrates, usually ions. Active, in contrast to passive, indicates that the movement of the main substrate is against its concentration gradient, in other words, uphill. Secondary, in contrast to primary, indicates that the energy source of the active transport is not directly from the hydrolysis of ATP (adenosine triphosphate), instead the transporter utilizes a gradient established by a primary transporter, which is often ATP requiring.
Na+-coupled secondary transporters refer to secondary active transporters that utilize the free energy stored in the electrochemical potential difference of sodium ions in and out of the cell membrane. This...