Proton pump

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A proton pump is an integral membrane protein pump that builds up a proton gradient across a biological membrane. Proton pumps catalyze the following reaction:

Integral membrane protein type of membrane protein that is permanently attached to the biological membrane

An integral membrane protein (IMP) is a type of membrane protein that is permanently attached to the biological membrane. All transmembrane proteins are IMPs, but not all IMPs are transmembrane proteins. IMPs comprise a significant fraction of the proteins encoded in an organism's genome. Proteins that cross the membrane are surrounded by annular lipids, which are defined as lipids that are in direct contact with a membrane protein. Such proteins can only be separated from the membranes by using detergents, nonpolar solvents, or sometimes denaturing agents.

Biological membrane enclosing or separating membrane that acts as a selectively permeable barrier within living thing

A biological membrane or biomembrane is an enclosing or separating membrane that acts as a selectively permeable barrier within living things. Biological membranes, in the form of eukaryotic cell membranes, consist of a phospholipid bilayer with embedded, integral and peripheral proteins used in communication and transportation of chemicals and ions. The bulk of lipid in a cell membrane provides a fluid matrix for proteins to rotate and laterally diffuse for physiological functioning. Proteins are adapted to high membrane fluidity environment of lipid bilayer with the presence of an annular lipid shell, consisting of lipid molecules bound tightly to surface of integral membrane proteins. The cell membranes are different from the isolating tissues formed by layers of cells, such as mucous membranes, basement membranes, and serous membranes.

Contents

H+
[on one side of a biological membrane] + energy H+
[on the other side of the membrane]

Mechanisms are based on energy-induced conformational changes of the protein structure or on the Q cycle.

Q cycle series of reactions in cellular respiration

The Q cycle describes a series of reactions that describe how the sequential oxidation and reduction of the lipophilic electron carrier, Coenzyme Q10 (CoQ10), between the ubiquinol and ubiquinone forms, can result in the net movement of protons across a lipid bilayer.

During evolution, proton pumps have arisen independently on multiple occasions. Thus, not only throughout nature but also within single cells, different proton pumps that are evolutionarily unrelated can be found. Proton pumps are divided into different major classes of pumps that utilize different sources of energy, have different polypeptide compositions and evolutionary origins.

Function

Transport of the positively charged proton is typically electrogenic, i.e. it generates an electrical field across the membrane also called the membrane potential. 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 the proton/potassium pump of the gastric mucosa which catalyzes a balanced exchange of protons and potassium ions.

Membrane potential physical quantity

Membrane potential is the difference in electric potential between the interior and the exterior of a biological cell. With respect to the exterior of the cell, typical values of membrane potential, normally given in units of millivolts and denoted as mV, ranges from –40 mV to –80 mV.

Gastric hydrogen potassium ATPase, also known as H+/K+ ATPase, is an enzyme which functions to acidify the stomach.

Gastric mucosa

The gastric mucosa is the mucous membrane layer of the stomach, which contains the glands and the gastric pits. In humans, it is about 1 mm thick, and its surface is smooth, soft, and velvety. It consists of simple columnar epithelium, lamina propria, and the muscularis mucosae.

The combined transmembrane gradient of protons and charges created by proton pumps is called an electrochemical gradient. An electrochemical gradient represents a store of energy (potential energy) that can be used to drive a multitude of biological processes such as ATP synthesis, nutrient uptake and action potential formation.

Electrochemical gradient gradient of electrochemical potential, usually for an ion that can move across a membrane

An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and the electrical gradient, or difference in charge across a membrane. When there are unequal concentrations of an ion across a permeable membrane, the ion will move across the membrane from the area of higher concentration to the area of lower concentration through simple diffusion. Ions also carry an electric charge that forms an electric potential across a membrane. If there is an unequal distribution of charges across the membrane, then the difference in electric potential generates a force that drives ion diffusion until the charges are balanced on both sides of the membrane.

Potential energy Potential energy

In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors.

In cell respiration, the proton pump uses energy to transport protons from the matrix of the mitochondrion to the inter-membrane space. [1] It is an active pump that generates a proton concentration gradient across the inner mitochondrial membrane because there are more protons outside the matrix than inside. The difference in pH and electric charge (ignoring differences in buffer capacity) creates an electrochemical potential difference that works similar to that of a battery or energy storing unit for the cell. [2] The process could also be seen as analogous to cycling uphill or charging a battery for later use, as it produces potential energy. The proton pump does not create energy, but forms a gradient that stores energy for later use. [3]

In biology, matrix is the material in animal or plant l structure of connective tissues is an extracellular matrix. Finger nails and toenails grow from matrices. It is found in various connective tissue. It is generally used as a jelly like structure instead of cytoplasm in connective tissue.

Mitochondrion Semi-autonomous, self-replicating organelle that occurs in varying numbers, shapes, and sizes in the cytoplasm of virtually all eukaryotic cells; the site of tissue respiration

The mitochondrion is a double-membrane-bound organelle found in most eukaryotic organisms. Some cells in some multicellular organisms may, however, lack them. A number of unicellular organisms, such as microsporidia, parabasalids, and diplomonads, have also reduced or transformed their mitochondria into other structures. To date, only one eukaryote, Monocercomonoides, is known to have completely lost its mitochondria. The word mitochondrion comes from the Greek μίτος, mitos, "thread", and χονδρίον, chondrion, "granule" or "grain-like". Mitochondria generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. A mitochondrion is thus termed the powerhouse of the cell.

Gradient Multi-variable generalization of the derivative of a function

In vector calculus, the gradient is a multi-variable generalization of the derivative. Whereas the ordinary derivative of a function of a single variable is a scalar-valued function, the gradient of a function of several variables is a vector-valued function. Specifically, the gradient of a differentiable function of several variables, at a point , is the vector whose components are the partial derivatives of at .

Diversity

The energy required for the proton pumping reaction may come from light (light energy; bacteriorhodopsins), electron transfer (electrical energy; electron transport complexes I, III and IV) or energy-rich metabolites (chemical energy) such as pyrophosphate (PPi; proton-pumping pyrophosphatase) or adenosine triphosphate (ATP; proton ATPases).

Bacteriorhodopsin is a protein used by Archaea, most notably by halobacteria, a class of the Euryarchaeota. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. The resulting proton gradient is subsequently converted into chemical energy.

Coenzyme Q – cytochrome c reductase class of enzymes

The coenzyme Q : cytochrome c – oxidoreductase, sometimes called the cytochrome bc1 complex, and at other times complex III, is the third complex in the electron transport chain, playing a critical role in biochemical generation of ATP. Complex III is a multisubunit transmembrane protein encoded by both the mitochondrial and the nuclear genomes. Complex III is present in the mitochondria of all animals and all aerobic eukaryotes and the inner membranes of most eubacteria. Mutations in Complex III cause exercise intolerance as well as multisystem disorders. The bc1 complex contains 11 subunits, 3 respiratory subunits, 2 core proteins and 6 low-molecular weight proteins.

Cytochrome c oxidase class of enzyme complexes

The enzyme cytochrome c oxidase or Complex IV, EC 1.9.3.1 is a large transmembrane protein complex found in bacteria, archaea, and in eukaryotes in their mitochondria.

Electron transport driven proton pumps

Electron Transport Complex I

Complex I (EC 1.6.5.3) (also referred to as NADH:ubiquinone oxidoreductase or, especially in the context of the human protein, NADH dehydrogenase) is a proton pump driven by electron transport. It belongs to the H+ or Na+-translocating NADH Dehydrogenase (NDH) Family (TC# 3.D.1), a member of the Na+ transporting Mrp superfamily. It catalyzes the transfer of electrons from NADH to coenzyme Q10 (CoQ10) and, in eukaryotes, it is located in the inner mitochondrial membrane. This enzyme helps to establish a transmembrane difference of proton electrochemical potential that the ATP synthase then uses to synthesize ATP.

Electron Transport Complex III

Complex III ((EC 1.10.2.2)) (also referred to as cytochrome bc1 or the coenzyme Q : cytochrome c – oxidoreductase) is a proton pump driven by electron transport. Complex III is a multisubunit transmembrane protein encoded by both the mitochondrial (cytochrome b) and the nuclear 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.

The cytochrome b6f complex

The cytochrome b6f complex (EC 1.10.99.1) (also called plastoquinol—plastocyanin reductase) is an enzyme related to Complex III but found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae. This proton pump is driven by electron transport and catalyzes the transfer of electrons from plastoquinol to plastocyanin. The reaction is analogous to the reaction catalyzed by Complex III (cytochrome bc1) of the mitochondrial 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.

Electron Transport Complex IV

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 four cytochrome 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 addition translocates 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.

ATP driven proton pumps

Adenosine triphosphate (ATP) driven proton pumps (also referred to as proton ATPases or H+
-ATPases) are proton pumps driven by the hydrolysis of adenosine 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.

P-type proton ATPase

The plasma membrane H+
-ATPase
is a single subunit P-type ATPase found in the plasma membrane of plants, fungi, protists and many prokaryotes.

The plasma membrane H+
-ATPase
creates the electrochemical gradients in the plasma membrane of plants, fungi, protists, and many prokaryotes. Here, proton gradients are used to drive secondary transport processes. As such, it is essential for the uptake of most metabolites, and also for responses to the environment (e.g., movement of leaves in plants).

Humans (and probably other mammals) have a gastric hydrogen potassium ATPase or H+/K+ ATPase that also belongs to the P-type ATPase family. This enzyme functions as the proton pump of the stomach, primarily responsible for the acidification of the stomach contents (see gastric acid).

V-type proton ATPase

The V-type proton ATPase is a multisubunit enzyme of the V-type. It is found in various different membranes where it serves to acidify intracellular organelles or the cell exterior.

F-type proton ATPase

The F-type proton ATPase is a multisubunit enzyme of the F-type (also referred to as ATP synthase or FOF1 ATPase). It is found in the mitochondrial inner membrane where is functions as a proton transport-driven ATP synthase.

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. ATP itself powers this transport in the plasma membrane proton ATPase and in the ATPase proton pumps of other cellular membranes.

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. 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.

In bacteria and ATP-producing organelles other 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.

Pyrophosphate driven proton pumps

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 the vacuole, the V-PPase and the V-ATPase.

Light driven proton pumps

Bacteriorhodopsin is a light-driven proton pump and is used by Archaea, most notably in Halobacteria. Light is absorbed by a retinal pigment covalently linked to the protein, that result in a conformational change of the molecule that is transmitted to the pump protein associated with proton pumping.

See also

Related Research Articles

Adenosine triphosphate chemical compound

Adenosine triphosphate (ATP) is a complex organic chemical that provides energy to drive many processes in living cells, e.g. muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all forms of life, ATP is often referred to as the "molecular unit of currency" of intracellular energy transfer. When consumed in metabolic processes, it converts either to adenosine diphosphate (ADP) or to adenosine monophosphate (AMP). Other processes regenerate ATP so that the human body recycles its own body weight equivalent in ATP each day. It is also a precursor to DNA and RNA, and is used as a coenzyme.

Oxidative phosphorylation the phosphorylation of ADP to ATP that accompanies the oxidation of a metabolite through the operation of the respiratory chain. Oxidation of compounds establishes a proton gradient across the membrane, providing the energy for ATP synthesis.

Oxidative phosphorylation is the metabolic pathway in which cells use enzymes to oxidize nutrients, thereby releasing energy which is used to produce adenosine triphosphate (ATP). In most eukaryotes, this takes place inside mitochondria. Almost all aerobic organisms carry out oxidative phosphorylation. This pathway is so pervasive because it is a highly efficient way of releasing energy, compared to alternative fermentation processes such as anaerobic glycolysis.

Electron transport chain A process in which a series of electron carriers operate together to transfer electrons from donors to any of several different terminal electron acceptors to generate a transmembrane electrochemical gradient.

An electron transport chain (ETC) is a series of complexes that transfer electrons from electron donors to electron acceptors via redox (both reduction and oxidation occurring simultaneously) reactions, and couples this electron transfer with the transfer of protons (H+ ions) across a membrane. This creates an electrochemical proton gradient that drives the synthesis of adenosine triphosphate (ATP), a molecule that stores energy chemically in the form of highly strained bonds. The molecules of the chain include peptides, enzymes (which are proteins or protein complexes), and others. The final acceptor of electrons in the electron transport chain during aerobic respiration is molecular oxygen although a variety of acceptors other than oxygen such as sulfate exist in anaerobic respiration.

Respiratory complex I class of enzymes

Respiratory complex I, EC 1.6.5.3 is the first large protein complex of the respiratory chains of myriad organisms from bacteria to humans. It catalyzes the transfer of electrons from NADH to coenzyme Q10 (CoQ10) and translocates protons across the inner mitochondrial membrane in eukaryotes or the plasma membrane of bacteria.

Cellular respiration Cellular enzymatic release of energy from compounds

Cellular respiration is a set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy in the process, as weak so-called "high-energy" bonds are replaced by stronger bonds in the products. Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. Cellular respiration is considered an exothermic redox reaction which releases heat. The overall reaction occurs in a series of biochemical steps, most of which are redox reactions themselves. Although cellular respiration is technically a combustion reaction, it clearly does not resemble one when it occurs in a living cell because of the slow release of energy from the series of reactions.

ATPase dephosphorylation enzyme

ATPases (EC 3.6.1.3, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, SV40 T-antigen, adenosine 5'-triphosphatase, ATP hydrolase, complex V (mitochondrial electron transport), (Ca2+ + Mg2+)-ATPase, HCO3-ATPase, adenosine triphosphatase) are a class of enzymes that catalyze the decomposition of ATP into ADP and a free phosphate ion or the inverse reaction. This dephosphorylation reaction releases energy, which the enzyme (in most cases) harnesses to drive other chemical reactions that would not otherwise occur. This process is widely used in all known forms of life.

ATP synthase class of enzymes

ATP synthase is an enzyme that creates the energy storage molecule adenosine triphosphate (ATP). ATP is the most commonly used "energy currency" of cells for all organisms. It is formed from adenosine diphosphate (ADP) and inorganic phosphate (Pi). The overall reaction catalyzed by ATP synthase is:

Crista Any of the inward folds of the mitochondrial inner membrane. Their number, extent, and shape differ in mitochondria from different tissues and organisms. They appear to be devices for increasing the surface area of the mitochondrial inner membrane, w

A crista is a fold in the inner membrane of a mitochondrion. The name is from the Latin for crest or plume, and it gives the inner membrane its characteristic wrinkled shape, providing a large amount of surface area for chemical reactions to occur on. This aids aerobic cellular respiration, because the mitochondrion requires oxygen. Cristae are studded with proteins, including ATP synthase and a variety of cytochromes.

Thylakoid part of a plant

A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as grana. Grana are connected by intergranal or stroma thylakoids, which join granum stacks together as a single functional compartment.

A membrane transport protein is a membrane protein involved in the movement of ions, small molecules, and macromolecules, such as another protein, across a biological membrane. Transport proteins are integral transmembrane protein; that is they exist permanently within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion or active transport. The two main types of proteins involved in such transport are broadly categorized as either channels or carriers. The solute carriers and atypical SLCs are secondary active or facilitative transporters in humans.

Chemiosmosis Movement of ions across a semi permeable membrane, down their electrochemical gradient

Chemiosmosis is the movement of ions across a semipermeable membrane, down their electrochemical gradient. An example of this would be the generation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a membrane during cellular respiration or photosynthesis.

Mitochondrial matrix The gel-like material, with considerable fine structure, that lies in the matrix space, or lumen, of a mitochondrion. It contains the enzymes of the tricarboxylic acid cycle and, in some organisms, the enzymes concerned with fatty acid oxidation.

In the mitochondrion, the matrix is the space within the inner membrane. The word "matrix" stems from the fact that this space is viscous, compared to the relatively aqueous cytoplasm. The mitochondrial matrix contains the mitochondria's DNA, ribosomes, soluble enzymes, small organic molecules, nucleotide cofactors, and inorganic ions.[1] The enzymes in the matrix facilitate reactions responsible for the production of ATP, such as the citric acid cycle, oxidative phosphorylation, oxidation of pyruvate and the beta oxidation of fatty acids.

Photophosphorylation Attachment of a phosphoryl group to ADP to form ATP in presence of light

In the process of photosynthesis, the phosphorylation of ADP to form ATP using the energy of sunlight is called photophosphorylation. Only two sources of energy are available to living organisms: sunlight and reduction-oxidation (redox) reactions. All organisms produce ATP, which is the universal energy currency of life. In photosynthesis this commonly involves photolysis, or photodissociation, of water and a continuous unidirectional flow of electrons from water to photosystem II.

Intermembrane space part of a cell

The intermembrane space (IMS) is the space occurring between or involving two or more membranes. In cell biology, it's most commonly described as the region between the inner membrane and the outer membrane of a mitochondrion or a chloroplast. It also refers to the space between the inner and outer nuclear membranes of the nuclear envelope, but is often called the perinuclear space. The IMS of mitochondria plays a crucial role in coordinating a variety of cellular activities, such as regulation of respiration and metabolic functions. Unlike the IMS of the mitochondria, the IMS of the chloroplast does not seem to have any obvious function.

Inner mitochondrial membrane

The inner mitochondrial membrane (IMM) is the mitochondrial membrane which separates the mitochondrial matrix from the intermembrane space.

In biology, an ion transporter is a transmembrane protein that moves ions across a biological membrane against their concentration gradient through active transport. These primary transporters are enzymes that convert energy from various sources—including adenosine triphosphate (ATP), sunlight, and other redox reactions—to potential energy stored in an electrochemical gradient. This potential energy is then used by secondary transporters, including ion carriers and ion channels, to drive vital cellular processes, such as ATP synthesis.

MT-ATP8 A mitochondrial protein-coding gene whose product is involved in ATP synthesis

MT-ATP8 is a mitochondrial gene with the full name 'mitochondrially encoded ATP synthase membrane subunit 8' that encodes a subunit of mitochondrial ATP synthase, ATP synthase Fo subunit 8. This subunit belongs to the Fo complex of the large, transmembrane F-type ATP synthase. This enzyme, which is also known as complex V, is responsible for the final step of oxidative phosphorylation in the electron transport chain. Specifically, one segment of ATP synthase allows positively charged ions, called protons, to flow across a specialized membrane inside mitochondria. Another segment of the enzyme uses the energy created by this proton flow to convert a molecule called adenosine diphosphate (ADP) to ATP. Subunit 8 differs in sequence between Metazoa, plants and Fungi.

ATP5D protein-coding gene in the species Homo sapiens

ATP synthase subunit delta, mitochondrial, also known as ATP synthase F1 subunit delta or F-ATPase delta subunit is an enzyme that in humans is encoded by the ATP5F1D gene. This gene encodes a subunit of mitochondrial ATP synthase. Mitochondrial ATP synthase catalyzes ATP synthesis, utilizing an electrochemical gradient of protons across the inner membrane during oxidative phosphorylation.

Light-dependent reactions photosynthetic reactions

In photosynthesis, the light-dependent reactions take place on the thylakoid membranes. The inside of the thylakoid membrane is called the lumen, and outside the thylakoid membrane is the stroma, where the light-independent reactions take place. The thylakoid membrane contains some integral membrane protein complexes that catalyze the light reactions. There are four major protein complexes in the thylakoid membrane: Photosystem II (PSII), Cytochrome b6f complex, Photosystem I (PSI), and ATP synthase. These four complexes work together to ultimately create the products ATP and NADPH.

References

  1. Yoshikawa, Shinya; Shimada, Atsuhiro; Shinzawa-Itoh, Kyoko (2015). "Chapter 4, Section 4 Proton Pump Mechanism". In Peter M.H. Kroneck and Martha E. Sosa Torres (ed.). Sustaining Life on Planet Earth: Metalloenzymes Mastering Dioxygen and Other Chewy Gases. Metal Ions in Life Sciences. 15. Springer. pp. 108–111. doi:10.1007/978-3-319-12415-5_4.
  2. Campbell, N.A., 2008. Resource Acquisition and Transport in Vascular Plants. 8th ed., Biology. San Francisco: Pearson Benjamin Cummings.
  3. Nature, Structural biology: Piston drives a proton pump. by Tomoko Ohnishi, 26 May 2010