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Aproton pump is anintegral membrane protein pump that builds up aproton gradient across abiological membrane. Proton pumps catalyze the following reaction:
Mechanisms are based on energy-inducedconformational changes of theprotein structure, or on theQ cycle.
During evolution, proton pumps havearisen independently on multiple occasions. Thus, not only throughout nature, but also within single cells,[clarification needed] different proton pumps that are evolutionarily unrelated can be found. Proton pumps are divided into different major classes of pumps that use different sources of energy, exhibiting different polypeptide compositions and evolutionary origins.
Transport of the positively charged proton is typically electrogenic, i.e.: it generates an electric field across the membrane also called themembrane potential.[citation needed] Proton transport becomes electrogenic if not neutralized electrically by transport of either a corresponding negative charge in the same direction, or a corresponding positive charge in the opposite direction. An example of a proton pump that is not electrogenic, is theproton/potassium pump of thegastric mucosa which catalyzes a balanced exchange of protons and potassium ions.[citation needed]
The combined transmembrane gradient of protons and charges created by proton pumps is called anelectrochemical gradient. An electrochemical gradient represents a store of energy (potential energy) that can be used to drive a multitude of biological processes such asATP synthesis, nutrient uptake and action potential formation.[citation needed]
Incell respiration, the proton pump uses energy to transport protons from the intracellular side to the extracellular side of theplasma membrane.[2] It is an active pump that generates aproton gradient across the membrane. The difference inpH andelectric charge (ignoring differences inbuffer capacity) creates anelectrochemical potential difference that works similar to that of a battery or energy storing unit for the cell.[3][full citation needed] The process could also be seen as analogous to cycling uphill or charging a battery for later use, as it produces potential energy.[tone] The proton pump does not create energy, but forms a gradient that stores energy for later use.[4]
The energy required for the proton pumping reaction may come from light (light energy;bacteriorhodopsins), electron transfer (electrical energy; electron transport complexesI,III andIV) or energy-rich metabolites (chemical energy) such aspyrophosphate (PPi;proton-pumping pyrophosphatase) oradenosine triphosphate (ATP;proton ATPases).[citation needed]
Complex I (EC 1.6.5.3) (also referred to asNADH:ubiquinone oxidoreductase or, especially in the context of the human protein,NADH dehydrogenase) is a proton pump driven by electron transport. It belongs to theH+ or Na+-translocating NADH Dehydrogenase (NDH) Family (TC# 3.D.1), a member of the Na+ transportingMrp superfamily. Itcatalyzes the transfer of electrons from NADH tocoenzyme Q10 (CoQ10) and, ineukaryotes, it is located in theinner mitochondrial membrane. This enzyme helps to establish a transmembrane difference of proton electrochemical potential that theATP synthase then uses tosynthesize ATP.[citation needed]
Complex III (EC 1.10.2.2) (also referred to ascytochrome bc1 or thecoenzyme Q : cytochrome c – oxidoreductase) is a proton pump driven by electron transport. Complex III is a multi-subunittransmembrane protein encoded by both themitochondrial (cytochrome b) and thenuclear genomes (all other subunits). Complex III is present in the inner mitochondrial membrane of all aerobic eukaryotes and the inner membranes of most eubacteria. This enzyme helps to establish a transmembrane difference of proton electrochemical potential that the ATP synthase of mitochondria then uses to synthesize ATP.[citation needed]
Thecytochrome b6f complex (EC 1.10.99.1) (also called plastoquinol—plastocyanin reductase) is an enzyme related to Complex III but found in thethylakoid membrane in chloroplasts of plants,cyanobacteria, and green algae. This proton pump is driven by electron transport and catalyzes the transfer of electrons fromplastoquinol toplastocyanin. The reaction is analogous to the reaction catalyzed by Complex III (cytochrome bc1) of themitochondrial electron transport chain. This enzyme helps to establish a transmembrane difference of proton electrochemical potential that the ATP synthase of chloroplasts then uses to synthesize ATP.[citation needed]
Complex IV (EC 1.9.3.1) (also referred to as cytochrome c oxidase), is a proton pump driven by electron transport. This enzyme is a large transmembrane protein complex found in bacteria and inner mitochondrial membrane of eukaryotes. It receives an electron from each of fourcytochrome c molecules, and transfers them to one oxygen molecule, converting molecular oxygen to two molecules of water. In the process, it binds four protons from the inner aqueous phase to make water and in additiontranslocates four protons across the membrane. This enzyme helps to establish a transmembrane difference of proton electrochemical potential that the ATP synthase of mitochondria then uses to synthesize ATP.[citation needed]
Proton pumps driven by adenosine triphosphate (ATP) (also referred to as proton ATPases orH+
-ATPases) are proton pumps driven by the hydrolysis ofadenosine triphosphate (ATP). Three classes of proton ATPases are found in nature. In a single cell (for example those of fungi and plants), representatives from all three groups of proton ATPases may be present.[citation needed]
Theplasma membraneH+
-ATPase is a single subunit P-type ATPase found in the plasma membrane ofplants,fungi,protists and manyprokaryotes.[citation needed]
Theplasma membraneH+
-ATPase creates theelectrochemical gradients in theplasma membrane ofplants,fungi,protists, and manyprokaryotes. Here, proton gradients are used to drivesecondary transport processes. As such, it is essential for the uptake of mostmetabolites, and also for responses to the environment (e.g., movement of leaves in plants).[citation needed]
Humans (and probably other mammals) have a gastrichydrogen potassium ATPase or H+/K+ ATPase that also belongs to theP-type ATPase family. This enzyme functions as the proton pump of thestomach, primarily responsible for the acidification of the stomach contents (seegastric acid).[citation needed]
TheV-type proton ATPase is a multi-subunit enzyme of theV-type. It is found in various different membranes where it serves to acidifyintracellular organelles or thecell exterior.[citation needed]
TheF-type proton ATPase is a multi-subunit enzyme of theF-type (also referred to asATP synthase or FOF1 ATPase). It is found in the mitochondrial inner membrane where it functions as a proton transport-drivenATP synthase.
Inmitochondria,reducing equivalents provided byelectron transfer orphotosynthesis power this translocation of protons. For example, the translocation of protons bycytochrome c oxidase is powered by reducing equivalents provided by reducedcytochrome c. ATP itself powers this transport in theplasma membrane proton ATPase and in theATPase proton pumps of other cellular membranes.[citation needed]
The FoF1ATP 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. Protons translocate across the inner mitochondrial membrane via proton wire. This series of conformational changes, channeled through the a and b subunits of the FO particle, drives a series of conformational changes in the stalk connecting the FO to the F1 subunit. This process effectively couples the translocation of protons to the mechanical motion between the Loose, Tight, and Open states of F1 necessary to phosphorylate ADP.[citation needed]
Inbacteria and ATP-producing organelles other than mitochondria,reducing equivalents provided byelectron transfer orphotosynthesis power the translocation of protons.[citation needed]
CF1 ATPligase ofchloroplasts correspond to the human FOF1 ATP synthase in plants.[citation needed]
Proton pumping pyrophosphatase (also referred to as HH+
-PPase or vacuolar-type inorganic pyrophosphatases (V-PPase; V is for vacuolar)) is a proton pump driven by the hydrolysis of inorganic pyrophosphate (PPi). In plants, HH+
-PPase is localized to the vacuolar membrane (the tonoplast). This membrane of plants contains two different proton pumps for acidifying the interior of thevacuole, the V-PPase and the V-ATPase.[citation needed]
Bacteriorhodopsin is a light-driven proton pump used byArchaea, most notably inHaloarchaea. Light is absorbed by aretinal pigment covalently linked to the protein, that results in a conformational change of the molecule that is transmitted to the pump protein associated with proton pumping.[5]