Proton pump

A proton pump is an integral membrane protein that is capable of moving protons across a cell membrane, mitochondrion, or other organelle. Mechanisms are based on conformational changes of the protein structure or on the Q cycle.

Function
In cell respiration, the pump actively transports protons from the matrix of the mitochondrion into the space between the inner and outer mitochondrial membranes. This action creates a concentration gradient across the inner mitochondrial membrane, as there are more protons outside the matrix than in. This difference in pH and electric charge (ignoring differences in buffer capacity) establishes an electrochemical potential that acts as a kind of battery or reservoir of stored energy for the cell, that functions in a similar way to a dam in a river. The process could also be seen as analogous to cycling uphill or charging a battery for later use, as it produces potential energy. It is important to remember that the proton pump does not create energy, but instead creates a gradient that stores energy that is later used to drive cellular processes.

Mobility
Some of the enzymes involved in the electron transport chain travel along the matrix, like freight boats, navigating up and down the river.

Others such as NADH-Q reductase, act like ferryboats and cross the matrix. Enzymes that cross the matrix may have a secondary role as proton pumps because they can deliver protons to the inner membrane.

In eukaryotes
In mitochondria, reducing equivalents provided by electron transfer or photosynthesis power this translocation of protons. For example, the translocation of protons by cytochrome c oxidase is powered by reducing equivalents provided by reduced cytochrome c. In the plasma membrane proton ATPase and in the ATPase proton pumps of other cellular membranes, ATP itself powers this transport.

The FoF1 ATP synthase of mitochondria, in contrast, usually conduct protons from high to low concentration across the membrane while drawing energy from this flow to synthesize ATP. To allow the passage of protons a proton channel temporarily opens in the inner membrane.

The gastric hydrogen potassium ATPase or H+/K+ ATPase is the proton pump of the stomach, which is responsible primarily for the acidification of the stomach contents (see gastric acid).

In prokaryotes
In bacteria and other ATP-producing organelles than mitochondria, reducing equivalents provided by electron transfer or photosynthesis power the translocation of protons.

CF1 ATP ligase of chloroplasts correspond to the human FoF1 ATP synthase in plants.

Bacteriorhodopsin is a photosynthetic pigment used by archaea, the most notable one being halobacteria.