943 Risultati per: "AbFrontier"

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Ataxia telangiectasia mutated (ATM) is a serine/threonine-specific protein kinase that is activated by DNA double-strand breaks. It phosphorylates several key proteins that initiate activation of the DNA damage checkpoint, leading to cell cycle arrest, DNA repair or apoptosis. Several of these targets are p53, CHK2, BRCA1, and H2AX.
ATM triggers the G1/S checkpoint; ATR (Ataxia telangiectasia and Rad3-related) prevents G1/S stasis. In this single point in the cell cycle, it would appear that ATM and ATR function in opposition to one another.
Ataxia telangiectasia (AT) is a rare neurodegenerative, autosomal recessive disorder characterized by chromosome instability, radiosensitivity, immunodeficiency and a predisposition for cancer.
The ATR (ATM and Rad3-related) kinase and its downstream effector kinase, Chk1, are central regulators of the replication checkpoint. Loss of these checkpoint proteins causes replication fork collapse and chromosomal rearrangements. ATR are thought to be master controllers of cell cycle checkpoint signaling pathways that are required for cell response to DNA damage and for genome stability.

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α-fetoprotein (AFP) is a glycoprotein of 590 amino acids containing 3.4% carbohydrate content with a molecular weight of 61,000 – 75,000 Da. AFP is normally produced in the developing embryo and fetus by the fetal yolk sac, the fetal gastrointestinal tract, and eventually by the fetal liver. In humans, AFP levels decrease gradually after birth, reaching adult levels by 8 to 12 months. Normal adult AFP levels are low and AFP has no known function in normal adults.
The biologic role of AFP has not been defined yet. Because of its biochemical similarity to albumin, it has been postulated that AFP could be a carrier protein. It may have an immunoregulatory function during pregnancy.
Increased serum levels are found in some tumors, such as hepatocellular
carcinoma (HCC), hepatoblastoma, and germ cell tumors. Although total AFP is a useful serological marker for diagnosis of HCC, the false-negative or positive rate with AFP level is very high. AFP-L3, an isoform of AFP which binds Lens culinaris agglutinin, can be particularly useful in early identification of aggressive tumors associated with HCC. AFP mRNA, the circulating genetic markers, also has been used in monitoring distal metastasis or postoperative recurrence of HCC.

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The Smad family of proteins are functioning in the transmission of extracellular signals in the TGF-β signaling pathway. Binding of a TGF-β superfamily ligands to extracellular receptors triggers phosphorylation of Smad2 at a Serine-Serine-Methionine-Serine (SSMS) motif at its C-terminus. Phosphorylated Smad2 is then able to form a complex with Smad4. These complexes accumulate in the cell nucleus, where they are directly participating in the regulation of gene expression.
In mammals, eight Smad proteins have been identified to date. The Smad family of proteins can be divided into three functional groups: the receptor-activated Smads (R-Smads), common mediator Smads (Co-Smads), and the inhibitory Smads (I-Smads). The R-Smads are directly phosphorylated by the activated type I receptors on their C-terminal Ser-Ser-X-Ser (SSXS) motif and include Smad1, Smad2, Smad3, Smad5, and Smad8. Smad2 and Smad3 are phosphorylated in response to TGF-β and activin, whereas Smad1, Smad5, and Smad8 are phosphorylated in response to BMP (Bone Morphogenetic Protein). This C-terminal phosphorylation allows R-Smad binding to Co-Smad, Smad4, and translocation to the nucleus where they regulate TGF-β target genes. Smad6 and Smad7 belong to the I-Smad which bind to the type I receptor or Smad4 and block their interaction with R-Smads.

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Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a catalytic enzyme commonly known to be involved in glycolysis. The enzyme exists as a tetramer of identical 37 kDa subunits. GAPDH catalyzes the reversible reduction of 1,3-bisphos-phoglycerate to glyceraldehyde 3-phosphophate in the presence of NADPH. Apart from playing a key role in glycolysis, this ubiquitously expressed enzyme also displays other activities unrelated to its glycolytic function. GAPDH is reported to be involved in the processes of DNA replication (1), DNA repair (2), nuclear RNA export (3-4), membrane fusion (5) and microtubule bundling. Other studies also provide evidence of GAPDH playing an essential part of the program of gene expression observed in apoptosis and as part of the cellular phenotype of age-related neurodegenerative diseases (6-7).

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There are ten mitogen-activated protein kinase (MAPK) phosphatases (MKPs) that act as negative regulators of MAPK activity in mammalian cells.
MKP2(DUSP4) is a dual threonine/tyrosine phosphatase that specifically dephosphorylates and inactivates MAPKs(ERK, JNK, p38). It contains one Rhodanese domain and one tyrosine-protein phosphatase domain.
MKP2 is expressed in a variety of tissues, and is localized in the nucleus.
Transcription factor E2F-1 acts as a transcriptional regulator of MKP2. E2F-1/MKP2 pathway mediates apoptosis under oxidative stress and that MKP2 suppresses tumor formation.
MKP2 is a novel transcription target of p53 in mediating apoptosis by 10-bp palindrome motif (CTGGCGCCAG) in the MKP2 promoter.

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Anti-Peroxiredoxin 5 Rabbit Polyclonal Antibody

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Dual specificity phosphatase 13(Also known as BEDP; MDSP; TMDP; SKRP4; DUSP13A; DUSP13B) is an enzyme that in humans is encoded by the DUSP13 gene.[1]
Members of the protein tyrosine phosphatase superfamily cooperate with protein kinases to regulate cell proliferation and differentiation. This superfamily is separated into two families based on the substrate that is dephosphorylated. One family, the dual specificity phosphatases (DSPs) acts on both phosphotyrosine and phosphoserine/threonine residues. This gene encodes different but related DSP proteins through the use of non-overlapping open reading frames, alternate splicing, and presumed different transcription promoters. Expression of the distinct proteins from this gene has been found to be tissue specific and the proteins may be involved in postnatal development of specific tissues. A protein encoded by the upstream ORF was found in skeletal muscle, whereas the encoded protein from the downstream ORF was found only in testis. In mouse, a similar pattern of expression was found. Multiple alternatively spliced transcript variants were described, but the full-length sequence of only some were determined.[1]

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PTEN acts as a phosphatase to dephosphorylate phosphatidylinositol (3,4,5)-trisphosphate (PtdIns (3,4,5)P3). The product of this enzymatic reaction is PtdIns(4,5)P2. This dephosphorylation is important because it results in inhibition of the AKT signaling pathway.
PTEN is a 403 amino acid protein, and a member of the large PTP (protein tyrosine phosphatase) family. PTEN crystal structure revealed that the N-terminal phosphatase domain is followed by a tightly associated C-terminal C2 domain. These two domains together form a minimal catalytic unit. The phosphatase domain contains the active site which carries out the enzymatic function of the protein, whilst the C2 domain allows PTEN to bind to the phospholipid membrane.
PTEN is one of the most commonly lost tumour suppressors in human cancer, and its deregulation is also implicated in several other diseases. Hereditary mutation of PTEN causes tumor-susceptibility diseases such as Cowden disease.

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Anti-PTPRM Mouse Monoclonal Antibody [clone: T5-AF1A8]

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Mammalian target of rapamycin (mTOR), a serine/threonine kinase involved in diverse cellular processes, including protein translation, mRNA turnover, and protein stability, mediates, at least in part, some of the biological actions of Akt. As a Kinase subunit of both mTORC1(complex1) and mTORC2(complex2), mTOR regulates cell growth and survival in response to nutrient and hormonal signals. mTORC1 is activated in response to growth factors or amino-acids. Activated mTORC1 up-regulates protein synthesis by phosphorylating key regulators of mRNA translation and ribosome synthesis. mTORC1 phosphorylates EIF4EBP1 and releases it from inhibiting the elongation initiation factor 4E (eiF4E). mTORC2 seems to function upstream of Rho GTPases to regulate the actin cytoskeleton, probably by activating one or more Rho-type guanine nucleotide exchange factors. mTORC2 promotes the serum-induced formation of stress-fibers or F-actin. mTORC2 plays a critical role in AKT1 'Ser-473' phosphorylation.

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Anti-ACTB Rabbit Polyclonal Antibody

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Signal transducer and activator of transcription (STAT), named after their dual role, generally mediate cytokine, growth factor and hormone receptor signal transduction. In mammals, seven STAT proteins have been identified. STAT5 has been implicated in cellular functions of proliferation, differentiation and apoptosis with relevance to processes of hematopoiesis and immunoregulation, reproduction, and lipid metabolism. Two highly homologous STAT5 isoforms, 96kDa STAT5a and 94kDa STAT5b, are encoded by two tandemly linked genes. Although both STAT5 isoforms are roughly 95% homologous at the level of cDNA, they exhibit both redundant and non-redundant functions in vivo, probably due to differences in their transactivation domain. Aberrant regulation of STAT5 has been observed in solid tumors as well as in patients with either chronic or acute myeloid leukemia. Kinase inhibitors are currently being developed to negatively regulate STAT5 activity for clinical purposes.

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Cyclin-dependent kinase-5(CDK5) is a member of the cyclin-dependent kinase family of serine/threonine kinases. Its mRNA and protein are expressed in the kidneys, testes, and ovaries. And Its activity has been detected almost exclusively in brain extracts.
Similar to other Cdks, monomeric Cdk5 displays no enzymatic activity; however, Cdk5 is not activated by cyclins. Instead, Cdk5 activity requires association with one of two brain-specific regulatory subunits, called p35 and p39. These two activators regulate the spatial and temporal expression of active Cdk5, restricting its activity primarily to post-mitotic neurons.

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The mammalian Phospholipase C(PLC) family has two closely related proteins, PLC1 and PLC2. The PLC isozymes have the core structure domains and a unique array of domains containing an additional PH domain, two SH2 domains and one SH3 domain. In response to extracellular stimuli, such as hormones and growth factors, receptor tyrosine kinases (RTKs) phosphorylate and activate PLC. Activated PLC catalyzes hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) to produce the metabolic second messenger molecules inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG).

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STAT2 (Signal transducer and activator of transcription 2), 113kDa, is a member of the STAT family of cytoplasmic transcription factors. STAT members generally mediate cytokine, growth factor and hormone receptor signal transduction. STAT2 is a transcription factor critical to the signal transduction pathway of type I interferons (e.g. IFNα). STAT2 resides primarily in the cytoplasm and is tyrosine-phosphorylated after IFNα binds to cell surface receptors. In response to tyrosine phosphorylation STAT2 rapidly localizes to the nucleus and acquires the ability to bind specific DNA targets in association with two other proteins, STAT1 and IFN regulatory factor-9 (IRF-9). STAT2 is phosphorylated at Y689 by receptor-associated Janus kinasses (JAKs) leading to STAT2 dimerization and subsequent translocation to the nucleus to activate gene transcription.

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14-3-3, a family of acidic and soluble proteins, highly conserved in amino acid sequences from yeast to mammals, is expressed in all eukaryotic cells. Seven isoforms(β, γ, ε, η, ζ, σ and τ/θ) encoded by seven distinct genes are identified in mammals and forms homo- and hetero- dimeric cup-shaped structures. As 14-3-3 is interacted with more than 100 binding partners, it regulates key proteins involved in various biological processes such as signal trans-duction, cell cycle, transcriptional control, cell proliferation, apoptosis, and ion channel physiology. Most 14-3-3 requires phosphorylation of serine or threonine residues in the target sequence. This protein is abundantly expressed in the brain and has been detected in the cerebrospinal fluid of patients with different neurological disorders.

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Superoxide dismutase (SOD) is an antioxidant enzyme involved in the defense system against reactive oxygen species (ROS). SOD catalyzes the dismutation reaction of superoxide radical anion (O2-) to hydrogen peroxide, which is then catalyzed to innocuous O2 and H2O by glutathione peroxidase and catalase. Several classes of SOD have been identified. These include intracellular copper, zinc SOD (Cu, Zn-SOD/SOD-1), mitochondrial manganese SOD (Mn-SOD/SOD-2) and extracellular Cu, Zn-SOD (EC-SOD/SOD-3) (1). SOD1 is found in all eukaryotic species as a homodimeric 32 kDa enzyme containing one each of Cu and Zn ion per subunit (2). The manganese containing 80 kDa tetrameric enzyme SOD2, is located in the mitochondrial matrix in close proximity to a primary endogenous source of superoxide, the mitochondrial respiratory chain (3). SOD3 is a heparin-binding multimer of disulfide-linked dimers, primarily expressed in human lungs, vessel walls and airways (4). SOD4 is a copper chaperone for superoxide dismutase (CCS), which specifically delivers Cu to copper/zinc superoxide dismutase. CCS may activate copper/zinc superoxide dismutase through direct insertion of the Cu cofactor.

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Peroxiredoxin (Prx) is a growing peroxidase family, whose mammalian members have been known to connect with cell proliferation, differentiation, and apoptosis.
Many isoforms (about 50 proteins), collected in accordance to the amino acid sequence homology, particularly amino-terminal region containing active site cysteine residue, and the thiol-specific antioxidant activity, distribute throughout all the kingdoms. Among them, mammalian Prx consists of 6 different members grouped into typical 2-Cys, atypical 2-Cys Prx, and 1-Cys Prx. Except Prx VI belonging to 1-Cys Prx subgroup, the other five 2-Cys Prx isotypes have the thioredoxin-dependent peroxidase (TPx) activity utilizing thioredoxin, thioredoxin reductase, and NADPH as a reducing system. Mammalian Prxs are 20 – 30 kilodalton in molecular size and vary in subcellular localization: Prx I, II, and VI in cytosol, Prx III in mitochondria, Prx IV in ER and secretion, Prx V showing complicated distribution including peroxisome, mitochondria and cytosol (1).

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Glyceraldehyde-3-phosphate dehydrogenase(GAPDH) is a catalytic enzyme commonly known to be involved in glycolysis. The enzyme exists as a tetramer of identical 37-kDa subunits. GAPDH catalyzes the reversible reduction of 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphophate in the presence of NADPH. Apart from playing a key role in glycolysis, this ubiquitously expressed enzyme also displays other activities unrelated to its glycolytic function. GAPDH is reported to be involved in the processes of DNA replication, DNA repair, nuclear RNA export, membrane fusion and microtubule bundling. Other studies also provide evidence of GAPDH playing an essential part of the program of gene expression observed in apoptosis and as part of the cellular phenotype of age-related neurodegenerative diseases. On recent study, GAPDH has identified of the most oxidant sensitive cell proteins.

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Protein kinase C (PKC) is a family of serine-threonine kinases that regulate a broad spectrum of cellular functions. The family is composed of nine genes that express structurally related phospholipid-dependent kinases with distinct means of regulation and tissue distribution. Based on their structures and sensitivities to Ca2+ and diacylglycerol (DAG), they have been classified into conventional PKCs (alpha, beta, and gamma), novel PKCs (Delta, Epsilon, Eta, and IPA), and atypical PKCs (Zeta and Lambda/Iota). PKC Gamma is a member of the cPKC subfamily which is activated by Ca2+ and diacylglycerol in the presence of phosphatidylserine. Physiologically, PKC Gamma is activated by a mechanism coupled with receptor-mediated breakdown of inositol phospholipid as other cPKC isotypes. PKC Gamma is expressed solely in the brain and spinal cord and its localization is restricted to neurons. Within the brain, PKC Gamma is the most abundant in the cerebellum, hippocampus and cerebral cortex, where the existence of neuronal plasticity has been demonstrated.

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Catalase is a homotetrameric heme-containing enzyme present within the matrix of all peroxisomes. It carries out a dismutation reaction in which hydrogen peroxide is converted to water and oxygen. Human catalase has the last four amino acids (-KANL) at the extreme C-terminus for peroxisome targeting. The monomer of human catalase is 61.3 kD in molecular size. Catalase has been implicated as an important factor in inflammation, mutagenesis, prevention of apoptosis, and stimulation of a wide spectrum of tumors. Loss of catalase leads to the human genetic disease, acatalasemia, or Takahara’s disease (1).

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Platelet-derived growth factors (PDGFs) have been implicated in the control of cell proliferation, survival and migration. The PDGF family of growth factors consists of five different disulphide-linked dimers built up of four different polypeptide chains encoded by four different genes. Theses isoforms, PDGF-AA, PDGF-AB, PDGF-BB, PDGF-CC and PDGF-DD, act via two receptor tyrosine kinases, PDGF receptors α and β. Thus far, gene-targeting experiments have been attempted to create knockout mice deficient for PDGFR-α or PDGFR-β. Those mice, however, died either at the embryonic stage or several days after birth. Platelet-derived growth factor receptors, PDGFR-α and PDGFR-β, have five extracellular immunoglobulin-like domains and an intracellular tyrosine kinase domain. Upon binding a PDGF, the receptors form homo- and heterodimers. Dimerization of the receptors juxtaposes the intracellular part of the receptors, which allow phosphorylation in trans between the two receptors in the complex. These autophosphorylation provide docking sites for downstream signal transduction molecules. More than 10 different SH2–domain-containing molecules have been shown to bind to different autophosphorylation sites in the PDGF α- and β-receptors. There are signal transduction molecules with enzymatic activity, such as PI3-kinase, PLC-γ, Src, SHP-2, GAP, as well as adaptor molecules such as Grb2, Shc, Nck, Grb7 and Crk, and Stats. Each of the different partners recruited by the activated receptor initiates different signaling pathways, making possible a great variety of cellular response.

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The epidermal growth factor receptor (EGFR) is a transmembrane receptor tyrosine kinase of the ErbB (also known as HER) family in which four members have been identified: EGFR (ErbB1), HER2/Neu (ErbB2), HER3 (ErbB3), and HER4 (ErbB4). All four erbB receptors are composed of an extracellular ligand-binding region consisting of glycosylated domains, a transmembrane domain containing a single hydrophobic anchor sequence, an intracellular region containing the catalytic tyrosine kinase domain, and a carboxyl-terminal region containing several tyrosine residues that become phosphorylated after receptor activation.
The epidermal growth factor receptor (EGFR) signaling pathway is one of the most important pathways that regulate growth, survival, proliferation, and differentiation in mammalian cells. EGFR and other members of the erbB family form either homodimers or heterodimers upon ligand binding, resulting in conformational changes that allow activation of protein kinases and transphosphorylation of key tyrosine residues within the carboxyl-terminal domain. After the induction of tyrosine phosphorylation, some signaling pathways appear to start with the recognition of the C-terminal phosphotyrosines by appropriate adaptor or signaling molecules. The aberrant activation of the EGFR leads to enhanced proliferation and other tumor-promoting activities. Several mechanisms lead to aberrant receptor activation, including receptor overexpression, gene amplification, activating mutations, overexpression of receptor ligands, and/or loss of their negative regulatory mechanisms.
The epidermal growth factor receptor (EGFR) has been extensively investigated as a target for anti-neoplastic therapy. Anti-EGFR antibodies that interfere with ligand-dependent receptor activation have shown clinical activity in a variety of solid tumors.

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Superoxide dismutase (SOD) is an antioxidant enzyme involved in the defense system against reactive oxygen species (ROS). SOD catalyzes the dismutation reaction of superoxide radical anion (O2-) to hydrogen peroxide, which is then catalyzed to innocuous O2 and H2O by glutathione peroxidase and catalase. Several classes of SOD have been identified. These include intracellular copper, zinc SOD (Cu, Zn-SOD/SOD-1), mitochondrial manganese SOD (Mn-SOD/SOD-2) and extracellular Cu, Zn-SOD (EC-SOD/SOD-3) (1). SOD-1 is found in all eukaryotic species as a homodimeric 32-kDa enzyme containing one each of Cu and Zn ion per subunit (2). The manganese containing 80-kDa tetrameric enzyme SOD2, is located in the mitochondrial matrix in close proximity to a primary endogenous source of superoxide, the mitochondrial respiratory chain (3). SOD-3 is a heparin-binding multimer of disulfide-linked dimers, primarily expressed in human lungs, vessel walls and airways (4). SOD-4 is a copper chaperone for superoxide dismutase (CCS), which specifically delivers Cu to copper/zinc superoxide dismutase. CCS may activate copper/zinc superoxide dismutase through direct insertion of the Cu cofactor.

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The human Catecholamine-Sulfating Phenol Sulfotransferase (CSPS) is the only sulfotransferase that catalyses the sulfation of catecholamins, in particular the neurotransmitter dopamine, with high activity. CSPS is required for stimulation by Mn2+ of the sulfating activity and expressed in the human intestine, brain, platelet and other tissues. In the brain it may play a role in regulating the levels of dopamine. It also serves as a detoxifying function in the intestine, where it may detoxify potentially lethal dietary monoamines.

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α-1-antichymotrypsin (ACT) is a glycoprotein with a molecular weight of 55~68 kDa and 25% by weight of various sugars, whose heterogeneity results in variation of molecular weight. It inhibits chymotrypsin-like proteinases in vivo and has cytotoxic killer-cell activity in vitro. The protein also has a role as an acute-phase protein and is active in the control of immunologic and inflammatory processes, and as a tumor marker. Plasma ACT levels might increase rapidly more than 5-fold during an acute phase reaction, ACT has been shown to promote Aβ polymerization in vitro and in vivo, and levels of ACT protein in plasma and cerebrospinal fluid from Alzheimer's patients have been found to correlate with progression of dementia.

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The mammalian thioredoxin reductases (TrxRs) are a family of selenocysteine-containing pyridine nucleotide-disulfide oxido-reductases. All the mammalian TrxRs are homologous to glutathione reductase with respect to primary structure including the conserved redox catalytic site (-Cys-Val-Asn-Val-Gly-Cys-) but distinctively with a C-terminal extension containing a catalytically active penultimate selenocysteine (SeCys) residue in the conserved sequence(-Gly-Cys-SeCys-Gly). TrxR is homodimeric protein in which each monomer includes an FAD prosthetic group, a NADPH binding site and a redox catalytic site. Electrons are transferred from NADPH via FAD and the active-site disulfide to C-terminal SeCys-containing redox center, which then reduces the substrate like thioredoxin. The members of TrxR family are 55 – 58 kilodalton in molecular size and composed of three isoforms including cytosolic TrxR1, mitochondrial TrxR2, and TrxR3, known as Trx and GSSG reductase (TGR). TrxR plays a key role in protection of cells against oxidative stress and redox-regulatory mechanism of transcription factors and various biological phenomena (1).

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The complement system is a part of the larger immune system and three biochemical pathways are present: the classical complement pathway, the alternative pathway, and the mannose-binding lectin pathway.
Human complement factor C6 is one of five components (C5b, C6, C7, C8, and C9) that interact to form the cytolytic membrane attack complex (MAC) which is the cytolytic end product of the complement cascade. MAC is typically formed on the surface of intruding pathogenic bacterial as a result of the activation of the complement system, and it is one of the ultimate weapons of the immune system.
The sixth component of complement, C6, is a 913 amino acid single polypeptide chain serum glycoprotein. Homology study suggests that C6 could contain two domains, an amino-terminal region that is related to complement C8 and C9, and a carboxyl-terminal region that has partial homology to the complement regulatory proteins factor H and factor I.
Genetic deficiencies of terminal complement components lead to markedly increased susceptibility to only one particular Gram-negative genus, the Neisseria. The susceptibility is attributable to the major role played by complement-mediated killing in host defense against the pathogen.

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The complement system is a part of the larger immune system and three biochemical pathways are present: the classical complement pathway, the alternative pathway, and the mannose-binding lectin pathway.
Complement component C4 is an essential component of humoral immune response. In its activated form, C4b becomes a subunit of the C3 convertase, which is an enzymatic complex that activates C3 of the classical and lectin complement activation pathways. The classical pathway is initiated by the activation of the C1-complex (C1q, C1r and C1s) by C1q's binding to antibody-antigen. The C1-complex now binds to and splits C2 and C4 producing C2a and C4b. C4b and C2a bind to form C3-convertase. Production of C3-convertase leads to cleavage of C3 into C3a and C3b and C3b joins with the C3 convertase to make C5 convertase.
Human C4 is the most polymorphic protein of the complement system. Complement C4 exists as two isotypes, C4A (acidic) and C4B (basic). Although the sequence identity is very high, they have different hemolytic activities, covalent affinities to antigens and immune complexes, and serological reactivities. Each C4 contains β chain, α chain, C4a anaphyltoxin, C4b, and γ chain.
C4-deficient mice shows incomplete clearance of microbial attack and C4-deficiency in human shows increased autoimmune diseases.

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Cyclin-dependent kinase-5 (CDK5) is a member of the cyclin-dependent kinase family of serine/threonine kinases. Its mRNA and protein are expressed in kidney, testes, and ovary. And Its activity is detected almost exclusively in brain extracts.
Similar to other Cdks, monomeric Cdk5 displays no enzymatic activity, but Cdk5 is not activated by cyclins. Instead, Cdk5 activity requires association with one of two brain-specific regulatory subunits called p35 and p39. The two activators regulate the spatial and temporal expression of active Cdk5 to restrict its activity primarily to post-mitotic neurons.