Tartrate resistant acid phosphatase
Encyclopedia
Tartrate-resistant acid phosphatase or acid phosphatase 5, tartrate resistant (ACP5 or TRAP) is a glycosylated monomer
ic metalloenzyme
expressed in mammals. It has a molecular weight of approximately 35kDa, a basic isoelectric point
(7.6-9.5) , and optimal activity in acidic conditions. TRAP is synthesized as latent proenzyme and activated by proteolytic cleavage and reduction. It is differentiated from other mammalian acid phosphatases by its resistance to inhibition by tartrate, molecular weight and characteristic purple colour.
The mechanism of phosphate ester hydrolysis by TRAP is through a nucleophilic attack mechanism, whereby, catalysis occurs with the binding of a phosphate-substrate to the Fe2+ in the active site of TRAP. This is then followed by a nucleophilic attack by a hydroxide ligand on the bound phosphorus atom, resulting in cleavage of the phosphate ester bond and production of an alcohol. The exact identity and mechanism of the hydroxide ligand is unclear, but it is thought to be either a hydroxide that bridges the metal ions within the active site or a terminal hydroxide bound to Fe3+, with conflicting reports for both mechanisms.
s, activated macrophages, neurons, and by the porcine endometrium during pregnancy. In newborn rats, TRAP is also detectable in the spleen, thymus, liver, kidneys, skin, lung, and heart at low levels. TRAP expression is increased in certain pathological conditions. These include leukaemic reticuloendotheliosis (hairy cell leukaemia), Gaucher’s disease, HIV-induced encephalopathy, osteoclastoma and osteoporosis
, and metabolic bone diseases.
In osteoclasts, TRAP is localized within the ruffled border area, the lysosomes, the Golgi cisternae and vesicles.
, highly conserved throughout the class mammalia. The TRAP gene has been cloned and sequenced in porcine, rat, human, and murine species.
Human, murine, and porcine TRAP genes all contain 5 exons, and have the ATG codon at the beginning of exon 2, with exon 1 being non-coding. Within the exon 1 promoter, there are three distinct “tissue-specific” promoters: 1A, 1B, and 1C. This would allow TRAP expression to be tightly controlled.
Transcribed from this gene is a 1.5kb mRNA with an open reading frame (ORF) of 969-975 bp encoding a 323-325 amino acid protein. In the rat, the ORF is 981 bp in length and encodes for a 327-amino acid protein. TRAP is translated as a single polypeptide.
TRAP gene transcription is regulated by the Microphthalmia-associated transcription factor
.
In knockout studies, TRAP-/- mice exhibit mild osteopetrosis
, associated with reduced osteoclast activity. These result in thickening and shortening of the cortices, formation of club-like deformities in the distal femur
, and widened epiphyseal growth plates with delayed mineralization of cartilage, all of which increase with age. In TRAP overexpressing transgenic mice, mild osteoporosis occurs along with increased osteoblast
activity and bone synthesis.
Proposed functions of TRAP include osteopontin
/bone sialoprotein
dephosphorylation
, the generation of reactive oxygen species
(ROS), iron transport, and as a cell growth and differentiation factor.
, which bind to osteoclasts when phosphorylated. Upon partial dephosphorylation, both osteopontin and bone sialoprotein are incapable of binding to osteoclasts. From this effect, it has been hypothesized that TRAP is secreted from the ruffled border, dephosphorylates osteopontin and allows osteoclast migration, and further resorption to occur.
(O2-.), which forms from the action of NADPH-oxidase on oxygen (O2). They play an essential role in the function of phagocytic cells.
TRAP, containing a redox active iron, catalyzes the generation of ROS through Fenton chemistry:
producing hydroxyl radicals, hydrogen peroxide
, and singlet oxygen.
In osteoclasts, ROS are generated at the ruffled border and seem to be required for resorption and degradation to occur.
to bone resorption sites, and, once there, TRAP is believed to initiate osteoblast differentiation, activation, and proliferation. This hypothesis was formed from the examination of the bone structure of TRAP-null mice. It was noted that, in addition to osteopetrosis
, bone formation occurred in a haphazard manner, where the microarchitecture was highly irregular.
In TRAP overexpressing mice, it has been found that the affected mice are grossly obese. This has led to the hypothesis that TRAP has involvement in hyperplastic obesity.
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
ic metalloenzyme
Metalloprotein
Metalloprotein is a generic term for a protein that contains a metal ion cofactor. Metalloproteins have many different functions in cells, such as enzymes, transport and storage proteins, and signal transduction proteins. Indeed, about one quarter to one third of all proteins require metals to...
expressed in mammals. It has a molecular weight of approximately 35kDa, a basic isoelectric point
Isoelectric point
The isoelectric point , sometimes abbreviated to IEP, is the pH at which a particular molecule or surface carries no net electrical charge....
(7.6-9.5) , and optimal activity in acidic conditions. TRAP is synthesized as latent proenzyme and activated by proteolytic cleavage and reduction. It is differentiated from other mammalian acid phosphatases by its resistance to inhibition by tartrate, molecular weight and characteristic purple colour.
The mechanism of phosphate ester hydrolysis by TRAP is through a nucleophilic attack mechanism, whereby, catalysis occurs with the binding of a phosphate-substrate to the Fe2+ in the active site of TRAP. This is then followed by a nucleophilic attack by a hydroxide ligand on the bound phosphorus atom, resulting in cleavage of the phosphate ester bond and production of an alcohol. The exact identity and mechanism of the hydroxide ligand is unclear, but it is thought to be either a hydroxide that bridges the metal ions within the active site or a terminal hydroxide bound to Fe3+, with conflicting reports for both mechanisms.
TRAP expression and cell localization
Under normal circumstances, TRAP is highly expressed by osteoclastOsteoclast
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...
s, activated macrophages, neurons, and by the porcine endometrium during pregnancy. In newborn rats, TRAP is also detectable in the spleen, thymus, liver, kidneys, skin, lung, and heart at low levels. TRAP expression is increased in certain pathological conditions. These include leukaemic reticuloendotheliosis (hairy cell leukaemia), Gaucher’s disease, HIV-induced encephalopathy, osteoclastoma and osteoporosis
Osteoporosis
Osteoporosis is a disease of bones that leads to an increased risk of fracture. In osteoporosis the bone mineral density is reduced, bone microarchitecture is deteriorating, and the amount and variety of proteins in bone is altered...
, and metabolic bone diseases.
In osteoclasts, TRAP is localized within the ruffled border area, the lysosomes, the Golgi cisternae and vesicles.
TRAP gene, promoter organisation and transcription
Mammalian TRAP is encoded by one gene, which is localized on chromosome 19 (19p13.2-13.3) in humans, and on chromosome 9 in mice. TRAP DNA is, as expected from protein sequencingProtein sequencing
Protein sequencing is a technique to determine the amino acid sequence of a protein, as well as which conformation the protein adopts and the extent to which it is complexed with any non-peptide molecules...
, highly conserved throughout the class mammalia. The TRAP gene has been cloned and sequenced in porcine, rat, human, and murine species.
Human, murine, and porcine TRAP genes all contain 5 exons, and have the ATG codon at the beginning of exon 2, with exon 1 being non-coding. Within the exon 1 promoter, there are three distinct “tissue-specific” promoters: 1A, 1B, and 1C. This would allow TRAP expression to be tightly controlled.
Transcribed from this gene is a 1.5kb mRNA with an open reading frame (ORF) of 969-975 bp encoding a 323-325 amino acid protein. In the rat, the ORF is 981 bp in length and encodes for a 327-amino acid protein. TRAP is translated as a single polypeptide.
TRAP gene transcription is regulated by the Microphthalmia-associated transcription factor
Microphthalmia-associated transcription factor
Microphthalmia-associated transcription factor is a basic helix-loop-helix leucine zipper transcription factor involved in melanocyte and osteoclast development.-Clinical significance:...
.
Physiology
The exact physiologic role(s) of TRAP is unknown, but many functions have been attributed to this protein.In knockout studies, TRAP-/- mice exhibit mild 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,...
, associated with reduced osteoclast activity. These result in thickening and shortening of the cortices, formation of club-like deformities in the distal femur
Femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...
, and widened epiphyseal growth plates with delayed mineralization of cartilage, all of which increase with age. In TRAP overexpressing transgenic mice, mild osteoporosis occurs along with increased osteoblast
Osteoblast
Osteoblasts are mononucleate cells that are responsible for bone formation; in essence, osteoblasts are specialized fibroblasts that in addition to fibroblastic products, express bone sialoprotein and osteocalcin.Osteoblasts produce a matrix of osteoid, which is composed mainly of Type I collagen...
activity and bone synthesis.
Proposed functions of TRAP include osteopontin
Osteopontin
Osteopontin , also known as bone sialoprotein I , early T-lymphocyte activation , secreted phosphoprotein 1 , 2ar and Rickettsia resistance , is a human gene product, which is also conserved in other species...
/bone sialoprotein
Bone sialoprotein
Bone sialoprotein is a component of mineralized tissues such as bone, dentin, cementum and calcified cartilage. BSP is a significant component of the bone extracellular matrix and has been suggested to constitute approximately 8% of all non-collagenous proteins found in bone and cementum...
dephosphorylation
Dephosphorylation
Dephosphorylation is the essential process of removing phosphate groups from an organic compound by hydrolysis. Its opposite is phosphorylation...
, the generation of reactive oxygen species
Reactive oxygen species
Reactive oxygen species are chemically reactive molecules containing oxygen. Examples include oxygen ions and peroxides. Reactive oxygen species are highly reactive due to the presence of unpaired valence shell electrons....
(ROS), iron transport, and as a cell growth and differentiation factor.
Protein dephosphorylation and osteoclast migration
It has been shown that osteopontin and bone sialoprotein, bone matrix phosphoproteins, are highly efficient in vitro TRAP substratesSubstrate (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...
, which bind to osteoclasts when phosphorylated. Upon partial dephosphorylation, both osteopontin and bone sialoprotein are incapable of binding to osteoclasts. From this effect, it has been hypothesized that TRAP is secreted from the ruffled border, dephosphorylates osteopontin and allows osteoclast migration, and further resorption to occur.
ROS generation
Reactive oxygen species (ROS) are generated in macrophages and osteoclasts from superoxideSuperoxide
A superoxide, also known by the obsolete name hyperoxide, is a compound that possesses the superoxide anion with the chemical formula O2−. The systematic name of the anion is dioxide. It is important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature...
(O2-.), which forms from the action of NADPH-oxidase on oxygen (O2). They play an essential role in the function of phagocytic cells.
TRAP, containing a redox active iron, catalyzes the generation of ROS through Fenton chemistry:
- O2 → (NADPH-oxidase) O2- ∙ → (superoxide dismutase) H2O2 → (catalase) H2O + O2
- TRAP-Fe3+ (purple) + O2- ∙→ TRAP-Fe2+ (pink) + O2
- H2O2 + TRAP-Fe2+ (pink) → HO∙ + HO− + TRAP-Fe3+
producing hydroxyl radicals, hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
, and singlet oxygen.
In osteoclasts, ROS are generated at the ruffled border and seem to be required for resorption and degradation to occur.
Iron transport
In the pregnant sow, uteroferrin is highly expressed in the uterine fluids. Due to the unique anatomy of the porcine uterus, and the specific, progesterone-induced expression of TRAP; it is hypothesized that uteroferrin acts as an iron transport protein.Cell growth and differentiation factor
TRAP is associated with osteoblast migrationCell migration
Cell migration is a central process in the development and maintenance of multicellular organisms. Tissue formation during embryonic development, wound healing and immune responses all require the orchestrated movement of cells in particular directions to specific locations...
to bone resorption sites, and, once there, TRAP is believed to initiate osteoblast differentiation, activation, and proliferation. This hypothesis was formed from the examination of the bone structure of TRAP-null mice. It was noted that, in addition to 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,...
, bone formation occurred in a haphazard manner, where the microarchitecture was highly irregular.
In TRAP overexpressing mice, it has been found that the affected mice are grossly obese. This has led to the hypothesis that TRAP has involvement in hyperplastic obesity.