Proton ATPase
Encyclopedia
- This article is about the H+-ATPase found in plants and fungi. For the gastric H+/K+ ATPase (involved in the acidification of the stomach in mammals), see Hydrogen potassium ATPaseHydrogen potassium ATPaseGastric hydrogen potassium ATPase is also known as H+/K+ ATPase- Function and location :The gastric hydrogen potassium ATPase or H+/K+ ATPase is the proton pump of the stomach and, as such, is the enzyme primarily responsible for the acidification of the stomach contents...
.
In the field of enzymology, the proton-ATPase is an enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
that catalyzes
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
the following chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
:
- ATP + H2O + H+in
ADP + phosphate + H+out
The 3 substrates
Substrate (biochemistry)
In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...
of this enzyme are ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
, H2O
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, and H+
Hydrogen ion
Hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes.Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions....
, whereas its 3 products
Product (chemistry)
Product are formed during chemical reactions as reagents are consumed. Products have lower energy than the reagents and are produced during the reaction according to the second law of thermodynamics. The released energy comes from changes in chemical bonds between atoms in reagent molecules and...
are ADP
Adenosine diphosphate
Adenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....
, phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
, and H+
Hydrogen ion
Hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes.Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions....
.
This enzyme belongs to the family of hydrolase
Hydrolase
In biochemistry, a hydrolase is an enzyme that catalyzes the hydrolysis of a chemical bond. For example, an enzyme that catalyzed the following reaction is a hydrolase:-Nomenclature:...
s, specifically those acting on acid anhydrides to catalyse transmembrane movement of substances. To be specific, the protein is a part of the P-type ATPase
P-type ATPase
The P-type ATPases, also known as E1-E2 ATPases, are a large group of evolutionarily related ion pumps that are found in bacteria, archaea and eukaryotes. They are α-helical bundle primary transporters referred to as P-type ATPases because they catalyze auto- phosphorylation of a key conserved...
family. The systematic name of this enzyme class is ATP phosphohydrolase (H+-exporting).
H+-exporting ATPase is also known as proton ATPase or more simply proton pump. Other names in common use include proton-translocating ATPase, yeast plasma membrane H+-ATPase, yeast plasma membrane ATPase, and ATP phosphohydrolase.
Function and location
The H+-ATPaseATPase
ATPases are a class of enzymes that catalyze the decomposition of adenosine triphosphate into adenosine diphosphate and a free phosphate ion. This dephosphorylation reaction releases energy, which the enzyme harnesses to drive other chemical reactions that would not otherwise occur...
or proton pump creates the electrochemical gradients in the plasma membrane of plants, fungi, protists, and many prokaryotes. Here, proton gradients are used to drive secondary transport
Secondary active transport
In secondary active transport or co-transport, uses energy to transport molecules across a membrane; however, in contrast to primary active transport, there is no direct coupling of ATP; instead, the electrochemical potential difference created by pumping ions out of the cell is used...
processes. As such, it is essential for the uptake of most metabolites, and also for plant responses to the environment (e.g., movement of leaves).
It is interesting to note that H+-ATPases are specific for plants, fungi, and protists; and Na+/K+-ATPases are specific for animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
cells. These two groups of P-type ATPase
P-type ATPase
The P-type ATPases, also known as E1-E2 ATPases, are a large group of evolutionarily related ion pumps that are found in bacteria, archaea and eukaryotes. They are α-helical bundle primary transporters referred to as P-type ATPases because they catalyze auto- phosphorylation of a key conserved...
s, although not from the same subfamily, seem to perform a complementary function in plants/fungi/protists and animal cells, namely the creation of an electrochemical gradient
Electrochemical gradient
An electrochemical gradient is a spatial variation of both electrical potential and chemical concentration across a membrane; that is, a combination of the membrane potential and the pH gradient...
used as an energy source for secondary transport
Secondary active transport
In secondary active transport or co-transport, uses energy to transport molecules across a membrane; however, in contrast to primary active transport, there is no direct coupling of ATP; instead, the electrochemical potential difference created by pumping ions out of the cell is used...
.
Structural studies
Structural information on P-type proton ATPases are scarce compared to that obtained for SERCA1a. A low resolution structure from 2D crystals of the plasma membrane (PM) H+-ATPase from Neurospora crassaNeurospora crassa
Neurospora crassa is a type of red bread mold of the phylum Ascomycota. The genus name, meaning "nerve spore" refers to the characteristic striations on the spores. The first published account of this fungus was from an infestation of French bakeries in 1843. N...
is, as of medio 2011, the only structural information on the fungal H+-ATPase . For the plant counterpart, a crystal structure of the AHA2 PM H+-ATPase from Arabidopsis thaliana
Arabidopsis thaliana
Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...
has been obtained from 3D crystals with a resolution of 3.6 Å . The structure of AHA2 clearly identifies three cytosolic domains corresponding to the N (nucleotide binding), P (phosphorylation), and A (actuator) domains, similar to those observed in the SR Ca2+-ATPase
SERCA
SERCA, or sarco/endoplasmic reticulum Ca2+-ATPase, or SR Ca2+-ATPase, is a calcium ATPase-type P-ATPase.-Function:SERCA resides in the sarcoplasmic reticulum within muscle cells...
and also verifies the presence of ten transmembrane helices. The 3D crystal structure shows the AHA2 PM H+-ATPase in a so-called quasi-occluded E1 state with the non-hydrolysable ATP analogue AMPPCP bound, and the overall fold of the catalytic unit reveals a high degree of structural similarity to the SR Ca2+-ATPase
SERCA
SERCA, or sarco/endoplasmic reticulum Ca2+-ATPase, or SR Ca2+-ATPase, is a calcium ATPase-type P-ATPase.-Function:SERCA resides in the sarcoplasmic reticulum within muscle cells...
and the Na+,K+-ATPase. The overall arrangement of the domains is similar to that observed for the occluded E1 conformation of the SR Ca2+-ATPase
SERCA
SERCA, or sarco/endoplasmic reticulum Ca2+-ATPase, or SR Ca2+-ATPase, is a calcium ATPase-type P-ATPase.-Function:SERCA resides in the sarcoplasmic reticulum within muscle cells...
, and based on comparison with structural data for the other conformations of the SR Ca2+-ATPase
SERCA
SERCA, or sarco/endoplasmic reticulum Ca2+-ATPase, or SR Ca2+-ATPase, is a calcium ATPase-type P-ATPase.-Function:SERCA resides in the sarcoplasmic reticulum within muscle cells...
, it was suggested that the structure of the AHA2 PM H+-ATPase represents a novel E1 intermediate. A distinct feature of the PM H+-ATPase not observed in other P-type ATPases is the presence of a large cavity in the transmembrane domain formed by M4, M5 and M6.
Regulation
Precise regulation of PM H+-ATPase activity is crucial to the plant. Over-expression of the PM H+-ATPase is compensated by a down-regulation of activity, whereas deletion of an isoform is compensated by redundancy as well as augmented activity of other isoforms by increased level of post-translational modifications.The PM H+-ATPase is subject to autoinhibition, which negatively regulates the activity of the pump and keeps the enzyme in a low activity state where ATP hydrolytic activity is partly uncoupled from ATP hydrolysis,. Release from the autoinhibitory restraints requires posttranslational modifications such as phosphorylation and interacting proteins. Autoinhibition is achieved by the N- and C-termini of the protein - communication between the two termini facilitates the necessary precise control of pump activity. The PM H+-ATPase is the first P-type ATPase for which both termini have been demonstrated to take part in the regulation of protein activity .
Physiological roles in plants
Plasma membrane H+-ATPases are found throughout the plant in all cell types investigated, but some cell types have much higher concentrations of H+-ATPase than others. In general, these cell types are specialised for intensive active transportActive transport
Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...
and accumulate solutes from their surroundings. Most studies of these roles come from genetic studies on Arabidopsis thaliana. H+-ATPases in plants are expressed from a multigene subfamily, and Arabidopsis thaliana for instance, have 12 different H+-ATPase genes.
Some important physiological processes the plant H+-ATPase is involved in are:
- Phloem loading. The phloemPhloemIn vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
is a tissue specialised for long-distance transport of organic compounds, and is well-known for its involvement in the transport of sugar from leaves or other source areas. Here the H+-ATPase powers the sucrose/H+ cotransporters and is found to be essential for the loading of sucroseSucroseSucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...
into the phloemPhloemIn vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
. - Solute uptake in roots. H+-ATPases energize the uptake of nutrients from the soil into the rootRootIn vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s, and is also involved in the further loading of these solutes into the xylemXylemXylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...
, a tissue specialised for long-distance transport of water and micronutrients. - Tip-growing systems. Pollen tubes and root hairs are examples of plant tip-growing systemsTip growthTip growth is an extreme form of polarised growth of living cells that results in an elongated cylindrical cell morphology with a rounded tip at which the growth activity takes place. Tip growth occurs in algae , fungi and plants Tip growth is an extreme form of polarised growth of living cells...
, where a single cell expands in one direction only. The direction of growth is controlled by an asymmetrical proton gradient, where protons enter at the extreme tip and are pumped out just below the tip. - Size of stomatal aperture. The somatal pore controls the diffusion of CO2 into the leaves to be utilized for photosynthesisPhotosynthesisPhotosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
. The pore is formed by two guard cells, which control the size of the pore by swelling in response to the activity of the H+-ATPase. Opening and closure of the pore is partly controlled by regulation of the H+-ATPase. - Plant movements. Like the somatal pore, other movements of plant organs are controlled by motor cells' changing cell turgor. These cells control phenomena such as solar tracking by the plant to optimize orientation of photosynthetic leaves, and the swift and spectacular reactions to touch found in some plant species (e.g., carnivorous plants). All of these swelling and shrinking processes take place by massive water and ion fluxes through channels. Here, activation of the H+-ATPase leads to plasma membrane hyperpolarizationHyperpolarizationHyperpolarization has several meanings:* Hyperpolarization occurs when the strength of the electric field across the width of a cell membrane increases...
and the opening of voltage sensitive potassium channels. The K+ influx leads to water uptake and turgor increase in the cell. - Salt and osmotolerance. Salinity imposes two stresses on the cell: one is the loss of turgor due to the hypertonicity of the extracellular medium, and the other is a direct effect of toxic ions on metabolismMetabolismMetabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
. Therefore plants have developed several defence mechanisms. The Na/H+ antiporterAntiporterAn antiporter is an integral membrane protein involved in secondary active transport of two or more different molecules or ions across a phospholipid membrane such as the plasma membrane in opposite directions.In secondary active transport, one species of solute moves along its electrochemical...
is heavily involved and is powered by the action of the H+-ATPase, which is highly expressed in leaves and roots during salt stress. - Intracellular pH regulation. Intracellular pHIntracellular pHAll cells contain an intracellular fluid whose pH value is known as the intracellular pH . The pHi plays a critical role in the function of the cell, and close regulation is required for cells to survive...
remains constant during cell growth, it is presumed, to ensure optimal activity of the cytoplasmic enzymes. This is controlled by the proton pump. - Acid growth. Acidification of the external medium caused by activation of the plasma membrane H+-ATPase initiates cellular expansion. It is believed that the plant hormonePlant hormonePlant hormones are chemicals that regulate plant growth, which, in the UK, are termed 'plant growth substances'. Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and, when moved...
auxinAuxinAuxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...
activates the proton pump. The apoplastApoplastWithin a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant....
ic acidification leads to loosening of the cell wallCell wallThe cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
and hyperpolarizationHyperpolarizationHyperpolarization has several meanings:* Hyperpolarization occurs when the strength of the electric field across the width of a cell membrane increases...
of the plasma membrane inducing K+ uptake and swelling.