Transport Across Cell Membranes

Structure and Function of H+ Pumps

Proton (H+) pumps generate electrochemical proton gradients, which can be used to control pH levels or synthesize ATP.

Plants, fungi, and bacteria all utilize electrochemical gradients for secondary active transport of substrates. Unlike animals, which most commonly use Na+ gradients for this purpose, plants, fungi, and bacteria use H+ gradients. The pump that generates these gradients is the plasma membrane proton-pumping (H+) ATPase. An H+ pump is a membrane-bound ATPase that generates a proton gradient. Some may be run in reverse to synthesize ATP. It transports one proton from the cell interior to the cell exterior for each ATP molecule hydrolyzed and is phosphorylated as part of its pumping cycle. The proton pump controls intracellular pH, so its function is carefully regulated.

Plants also use a H+ gradient for the motion of leaves in response to environmental changes, as well as functions related to growth. Bacteria use an H+ gradient to power the flagellar motor that allows them to swim.

The cells lining the stomach of animals also contain a proton pump, the H+/ K+ ATPase. The task of this pump is to acidify the stomach. The action of this pump is electrically neutral, because it exchanges one K+ ion for each H+ ion with each ATP hydrolyzed. Like the other ATPases, this pump undergoes a conformational change associated with phosphorylation and dephosphorylation, which facilitates ion transport.

Proton gradients are also generated by mitochondria and chloroplasts, through the reactions involved in cellular respiration and photosynthesis. A different type of H+ ATPase, found in the inner mitochondrial membrane and the thylakoid membrane of chloroplasts, utilizes this proton gradient to operate in the reverse direction, known as an ATP synthase. Protons are pumped from the intermembrane space into the mitochondrial matrix (or the stroma of the chloroplast), causing a rotation of the transmembrane portion of the pump. This conformational change facilitates the production of ATP from ADP and phosphate, which are bound in an active site on the interior side of the pump.