V-ATPase
Encyclopedia
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...
with remarkably diverse functions in eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
organisms. V-ATPases acidify a wide array of intracellular organelles and pump protons
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:...
across the plasma membranes
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
of numerous cell types. V-ATPases couple the energy of ATP hydrolysis
ATP hydrolysis
ATP hydrolysis is the reaction by which chemical energy that has been stored and transported in the high-energy phosphoanhydridic bonds in ATP is released, for example in the muscles, to produce work. The product is ADP and an inorganic phosphate, orthophosphate...
to proton transport
Transport protein
A membrane transport protein is a membrane protein involved in the movement of ions, small molecules, or macromolecules, such as another protein across a biological membrane. Transport proteins are integral membrane proteins; that is they exist within and span the membrane across which they...
across intracellular and plasma membranes of eukaryotic cells.
Roles played by V-ATPases
V-ATPases are found within the membranes of many organelles, such as endosomes, lysosomes, and secretory vesicles, where they play a variety of roles crucial for the function of these organelles. For example, the proton gradient across the yeast vacuolar membrane generated by V-ATPases drives calcium uptake into the vacuole through an H+/Ca2+ antiporter system (Ohya, 1991). V-ATPases also play an important role in synaptic transmission in neuronal cells. NorepinephrineNorepinephrine
Norepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter...
enters vesicles in exc by V-ATPase.
V-ATPases are also found in the plasma membranes of a wide variety of cells such as intercalated cells of the kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
, osteoclasts (bone resorbing cells), macrophages, neutrophils, sperm
Spermatozoon
A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...
, midgut
Midgut
The midgut is the portion of the embryo from which most of the intestines develop. After it bends around the superior mesenteric artery, it is called the "midgut loop"...
cells of insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s, and certain tumor
Tumor
A tumor or tumour is commonly used as a synonym for a neoplasm that appears enlarged in size. Tumor is not synonymous with cancer...
cells. Plasma membrane V-ATPases are involved in processes such as pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
homeostasis
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...
, coupled transport
Co-transport
Co-transport, also known as coupled transport or secondary active transport, refers to the simultaneous or sequential passive transfer of molecules or ions across biological membranes in a fixed ratio...
, and tumor metastasis
Metastasis
Metastasis, or metastatic disease , is the spread of a disease from one organ or part to another non-adjacent organ or part. It was previously thought that only malignant tumor cells and infections have the capacity to metastasize; however, this is being reconsidered due to new research...
. V-ATPases in the acrosomal membrane of sperm acidify the acrosome
Acrosome
The acrosome is an organelle that develops over the anterior half of the head in the spermatozoa of many animals. It is a cap-like structure derived from the Golgi apparatus. Acrosome formation is completed during testicular maturation. In Eutherian mammals the acrosome contains digestive enzymes...
. This acidification activates proteases required to drill through the plasma membrane of the egg
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
. V-ATPases in the osteoclast plasma membrane pump protons onto the bone surface, which is necessary for bone resorption. In the intercalated cells of the kidney, V-ATPases pump protons into the urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
, allowing for bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
reabsorption into the blood.
V-ATPase structure
The yeastYeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...
V-ATPase is the best characterized. There are at least 13 subunits identified to form a functional V-ATPase complex, which consists of two domains. The subunits belong to either the Vo domain (membrane associated subunits, lowercase letters on the figure) or the V1 domain (peripherally associated subunits, uppercase letters on the figure).
The V1 includes 8 subunits, A-H, with three copies of the catalytic A and B subunits, three copies of the stator subunits E and G, and one copy of the regulatory C and H subunits. In addition, the V1 domain also contains the subunits D and F, which form a central rotor axle (Kitagawa et al., 2008). The V1 domain contains tissue-specific subunit isoforms including B, C, E, and G. Mutations to the B1 isoform result in the human disease distal renal tubular acidosis
Acidosis
Acidosis is an increased acidity in the blood and other body tissue . If not further qualified, it usually refers to acidity of the blood plasma....
and sensorineural deafness.
The Vo domain contains 6 different subunits, a, d, c, c', c", and e, with the stoichiometry of the c ring still a matter of debate with a decamer being postulated for the tobacco Hornworm M. Sexta V-ATPase. The mammalian Vo domain contains tissue-specific isoforms for subunits a and d, while yeast V-ATPase contains two organelle-specific subunit isoforms of a, Vph1p, and Stv1p. Mutations to the a3 isoform result in the human disease infantile malignant osteopetrosis
Osteopetrosis
Osteopetrosis, literally "stone bone", also known as marble bone disease and Albers-Schonberg disease is an extremely rare inherited disorder whereby the bones harden, becoming denser, in contrast to more prevalent conditions like osteoporosis, in which the bones become less dense and more brittle,...
, and mutations to the a4 isoform result in distal renal tubular acidosis, in some cases with sensorineural deafness.
The V1 domain is responsible for ATP hydrolysis, whereas the Vo domain is responsible for proton translocation. ATP hydrolysis at the catalytic nucleotide binding sites on subunit A drives rotation of a central stalk composed of subunits D and F, which in turn drives rotation of a barrel of c subunits relative to the a subunit. The complex structure of the V-ATPase has been revealed through the structure of the M. Sexta and Yeast complexes that were solved by singe-particle cryo-EM and negative staining, respectively (Muench 2009, Diepholz 2008, Zhang 2008). These structures have revealed that the V-ATPase has a 3-stator network, linked by a collar of density formed by the C, H, and a subunits, which, while dividing the V1 and V0 domains, make no interactions with the central rotor axle formed by the F, D, and d subunits. Rotation of this central rotor axle caused by the hydrolysis of ATP within the catalytic AB domains results in the movement of the barrel of c subunits past the a subunit, which drives proton transport across the membrane. A stoichiometry
Stoichiometry
Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...
of two protons translocated for each ATP hydrolyzed has been proposed by (Johnson, 1982).
In addition to the structural subunits of yeast V-ATPase, associated proteins that are necessary for assembly have been identified. These associated proteins are essential for Vo domain assembly and are termed Vma12p, Vma21p, and Vma22p (Hirata, 1993; Ho, 1993; Hill, 1994; Jackson, 1997). Two of the three proteins, Vma12p and Vma22p, form a complex that binds transiently to Vph1p (subunit a) to aid its assembly and maturation (Hill, 1994; Hill, 1995; Graham, 1998; Graham, 2003). Vma21p coordinates assembly of the Vo subunits as well as escorting the Vo domain into vesicles for transport to the Golgi
Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
(Malkus, 2004).
V-ATPase assembly
Yeast V-ATPases fail to assemble when any of the genes that encode subunits are deleted except for subunits H and c" (Whyteside, 2005; Forgac, 1999; Stevens, 1997). Without subunit H, the assembled V-ATPase is not active (Ho, 1993; Parra, 2000) and the loss of the c" subunit results in uncoupling of enzymatic activity (Whyteside, 2005).The precise mechanisms by which V-ATPases assembly are still controversial, with evidence suggesting two different possibilities. Mutational analysis and in vitro assays have shown that preassembled Vo and V1 domains can combine to form one complex in a process called independent assembly. Support for independent assembly includes the findings that the assembled Vo domain can be found at the vacuole in the absence of the V1 domain, whereas free V1 domains can be found in the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
and not at the vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
(Kane, 1995; Sumner, 1995). In contrast, in vivo pulse-chase experiments have revealed early interactions between Vo and V1 subunits, to be specific, the a and B subunits, suggesting that subunits are added in a step-wise fashion to form a single complex in a concerted assembly process (Kane, 1999).
Regulation of V-ATPase activity
In vivo regulation of V-ATPase activity is accomplished by reversible dissociation of the V1 domain from the Vo domain. After initial assembly, both the insect Manduca sexta and yeast V-ATPases can reversibly disassemble into free Vo and V1 domains after a 2- to 5-minute deprivation of glucose (Kane, 1995). Reversible disassembly may be a general mechanism of regulating V-ATPase activity, since it exists in yeast and insects. Reassembly is proposed to be aided by a complex termed RAVERAVE
RAVE can refer to:*Radial Velocity Experiment, spectroscopic astronomical survey*RAVE , a manga series*Reducing Americans' Vulnerability to Ecstasy Act...
(regulator of H+-ATPase of vacuolar and endosomal membranes) (Kane and Smardon, 2003). Dissasembly and reassembly of V-ATPases does not require new protein synthesis but does need an intact microtubular network (Holliday, 2000).
Osteopetrosis
OsteopetrosisOsteopetrosis
Osteopetrosis, literally "stone bone", also known as marble bone disease and Albers-Schonberg disease is an extremely rare inherited disorder whereby the bones harden, becoming denser, in contrast to more prevalent conditions like osteoporosis, in which the bones become less dense and more brittle,...
is generic name that represents a group of heritable conditions in which there is a defect in osteoclastic bone resorption
Osteoclast
An osteoclast is a type of bone cell that removes bone tissue by removing its mineralized matrix and breaking up the organic bone . This process is known as bone resorption. Osteoclasts were discovered by Kolliker in 1873...
. Both dominant and recessive osteopetrosis occur in humans {Michigami, 2002; Frattini, 2000}. Autosomal dominant osteopetrosis shows mild symptoms in adults experiencing frequent bone fractures due to brittle bones {Michigami, 2002}. A more severe form of osteopetrosis is termed autosomal recessive infantile malignant osteopetrosis {Frattini, 2000; Sobacchi, 2001; Fasth, 1999}. Three genes that are responsible for recessive osteopetrosis in humans have been identified. They are all directly involved in the proton generation and secretion pathways that are essential for bone resorption. One gene is carbonic anhydrase
Carbonic anhydrase
The carbonic anhydrases form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons , a reversible reaction that occurs rather slowly in the absence of a catalyst...
II (CAII), which, when mutated, causes osteopetrosis with renal tubular acidosis
Renal tubular acidosis
Renal tubular acidosis is a medical condition that involves an accumulation of acid in the body due to a failure of the kidneys to appropriately acidify the urine. When blood is filtered by the kidney, the filtrate passes through the tubules of the nephron, allowing for exchange of salts, acid...
(type 3) {Sly, 1983}. Mutations to the chloride channel ClC7 gene also lead to both dominant and recessive osteopetrosis {Michigami, 2002}. Approximately 50% of patients with recessive infantile malignant osteopetrosis have mutations to the a3 subunit isoform of V-ATPase {Sobacchi, 2001; Kornak, 2000; Frattini, 2003}. In humans, 26 mutations have been identified in V-ATPase subunit isoform a3, found in osteoclasts, that result in the bone disease autosomal recessive osteopetrosis {Frattini, 2000; Kornak, 2000; Sobacchi, 2001; Susani, 2004}.
Distal renal tubular acidosis (dRTA)
The importance of V-ATPase activity in renal proton secretion is highlighted by the inherited disease distal renal tubular acidosisRenal tubular acidosis
Renal tubular acidosis is a medical condition that involves an accumulation of acid in the body due to a failure of the kidneys to appropriately acidify the urine. When blood is filtered by the kidney, the filtrate passes through the tubules of the nephron, allowing for exchange of salts, acid...
. In all cases, renal tubular acidosis results from a failure of the normal renal mechanisms that regulate systemic pH. There are four types of renal tubular acidosis. Type 1 is distal renal tubular acidosis and results from a failure of the cortical collecting duct to acidify the urine below pH 5. {Alper, 2002}. Some patients with autosomal recessive dRTA also have sensorineural hearing loss
Sensorineural hearing loss
Sensorineural hearing loss is a type of hearing loss in which the root cause lies in the vestibulocochlear nerve , the inner ear, or central processing centers of the brain....
{Karet, 1999}. Inheritance of this type of RTA results from either mutations to V-ATPase subunit isoform B1 or isoform a4 or mutations of band 3
Band 3
Anion Exchanger 1 or Band 3 is a phylogenetically preserved transport protein responsible for mediating the exchange of chloride for bicarbonate across a plasma membrane. Functionally similar members of the AE clade are AE2 and AE3.It is ubiquitous throughout the vertebrates...
(also called AE1), a Cl-/HCO3- exchanger {Stehberger, 2003; Karet, 1999; Karet, 1998}. Twelve different mutations to V-ATPase isoform B1 (Stover, 2002) and twenty-four different mutations in a4 lead to dRTA {Smith, 2000; Karet, 1999; Stover, 2005}. Reverse transcription polymerase chain reaction
Polymerase chain reaction
The polymerase chain reaction is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence....
studies have shown expression of the a4 subunit in the intercalated cell of the kidney and in the cochlea
Cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....
{Stover, 2002}. dRTA caused by mutations in the a4 subunit gene in some cases can be associated with deafness due to a failure to normally acidify the endolymph
Endolymph
Endolymph is the fluid contained in the membranous labyrinth of the inner ear. It is also called Scarpa's fluid, after Antonio Scarpa.-Composition:...
of the inner ear
Inner ear
The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...
{Stehberger, 2003}.
Nomenclature
The term Vo has a lowercase letter "o" (not the number "zero") in subscript. The "o" stands for oligomycinOligomycin
Oligomycins are macrolides created by Streptomyces that can be poisonous to other organisms.-Function:They have use as antibiotics.In addition, oligomycin inhibits ATP synthase by blocking its proton channel , which is necessary for oxidative phosphorylation of ADP to ATP . The inhibition of ATP...
.