proteasome & lisosom

Apa proteasome itu? Protein apa saja yang terdegradasi?

Proteasomes are very large protein complexes inside all eukaryotes and archaea , and in some bacteria . In eukaryotes, they are located in the nucleus and the cytoplasm .  The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis , a chemical reaction that breakspeptide bonds . Enzymes that carry out such reactions are called proteases . Proteasomes are part of a major mechanism by which cells regulate theconcentration of particular proteins and degrade misfolded proteins . The degradation process yields peptides of about seven to eight amino acids long, which can then be further degraded into amino acids and used in synthesizing new proteins.  Proteins are tagged for degradation with a small protein calledubiquitin . The tagging reaction is catalyzed by enzymes called ubiquitin ligases . Once a protein is tagged with a single ubiquitin molecule, this is a signal to other ligases to attach additional ubiquitin molecules. The result is a polyubiquitin chain that is bound by the proteasome, allowing it to degrade the tagged protein. 

In structure , the proteasome is a cylindrical complex containing a "core" of four stacked rings around a central pore. Each ring is composed of seven individual proteins. The inner two rings are made of seven β subunits that contain the six protease active sites . These sites are located on the interior surface of the rings, so that the target protein must enter the central pore before it is degraded. The outer two rings each contain seven α subunits whose function is to maintain a "gate" through which proteins enter the barrel. These α subunits are controlled by binding to "cap" structures or regulatory particlesthat recognize polyubiquitin tags attached to protein substrates and initiate the degradation process. The overall system of ubiquitination and proteasomal degradation is known as the ubiquitin-proteasome system .

The proteasomal degradation pathway is essential for many cellular processes, including the cell cycle , the regulation of gene expression , and responses to oxidative stress . The importance of proteolytic degradation inside cells and the role of ubiquitin in proteolytic pathways was acknowledged in the award of the 2004 Nobel Prize in Chemistry to Aaron Ciechanover , Avram Hershko and Irwin Rose .

The proteasome subcomponents are often referred to by their Svedberg sedimentation coefficient (denoted S ). The most common form of the proteasome is known as the 26S proteasome, which is about 2000 kilodaltons (kDa) in molecular mass and contains one 20S core particle structure and two 19S regulatory caps. The core is hollow and provides an enclosed cavity in which proteins are degraded; openings at the two ends of the core allow the target protein to enter.Each end of the core particle associates with a 19S regulatory subunit that contains multiple ATPase active sites and ubiquitin binding sites; it is this structure that recognizes polyubiquitinated proteins and transfers them to the catalytic core. An alternative form of regulatory subunit called the 11S particle can associate with the core in essentially the same manner as the 19S particle; the 11S may play a role in degradation of foreign peptides such as those produced after infection by a virus .

20S core particle

The number and diversity of subunits contained in the 20S core particle depends on the organism; the number of distinct and specialized subunits is larger in multicellular than unicellular organisms and larger in eukaryotes than in prokaryotes. All 20S particles consist of four stacked heptameric ring structures that are themselves composed of two different types of subunits; α subunits are structural in nature, whereas β subunits are predominantly catalytic . The outer two rings in the stack consist of seven α subunits each, which serve as docking domains for the regulatory particles and the alpha subunits N-termini form a gate that blocks unregulated access of substrates to the interior cavity.  The inner two rings each consist of seven β subunits and contain the protease active sites that perform the proteolysis reactions. The size of the proteasome is relatively conserved and is about 150 angstroms (Å) by 115 Å. The interior chamber is at most 53 Å wide, though the entrance can be as narrow as 13 Å, suggesting that substrate proteins must be at least partially unfolded to enter. 

In archaea such as Thermoplasma acidophilum , all the α and all the β subunits are identical, while eukaryotic proteasomes such as those in yeast contain seven distinct types of each subunit. In mammals , the β1, β2, and β5 subunits are catalytic; although they share a common mechanism, they have three distinct substrate specificities considered chymotrypsin -like, trypsin -like, and peptidyl-glutamyl peptide-hydrolyzing (PHGH).  Alternative β forms denoted β1i, β2i, and β5i can be expressed in hematopoietic cells in response to exposure to pro- inflammatory signals such as cytokines , in particular, interferon gamma . The proteasome assembled with these alternative subunits is known as the immunoproteasome , whose substrate specificity is altered relative to the normal proteasome. 

19S regulatory particle

The 19S particle in eukaryotes consists of 19 individual proteins and is divisible into two subassemblies, a 10-protein base that binds directly to the α ring of the 20S core particle, and a 9-protein lid where polyubiquitin is bound. Six of the ten base proteins are ATPase subunits from the AAA Family, and an evolutionary homolog of these ATPases exists in archaea, called PAN (Proteasome Activating Nucleotidase).  The association of the 19S and 20S particles requires the binding of ATP to the 19S ATPase subunits, and ATP hydrolysis is required for the assembled complex to degrade folded and ubiquitinated proteins.Interestingly, only the step of substrate unfolding requires energy from ATP hydrolysis, while ATP-binding alone can support all the other steps required for protein degradation (eg complex assembly, gate opening, translocation and proteolysis).  In fact, ATP binding to the ATPases by itself supports the rapid degradation of unfolded proteins. However, while ATP hydrolysis is required for unfolding only it is not yet clear whether this energy may be used in the coupling of some of these steps.  As of 2011, the atomic structure of the 26S proteasome has not been solved, despite massive efforts to do so.Nevertheless, it is understood generally how the 19S associates with and regulates the 20S core particle.  In fact, the 19S and 11S particles bind to the same sites in the α rings of the 20S core particle although, they each induce gate opening by different mechanism. 

Regulation of the 20S by the 19S

The 19S regulatory particle is responsible for stimulating the 20S to degrade proteins. A primary function of the 19S regulatory ATPases is to open the gate in the 20S that blocks the entry of substrates into the degradation chamber.  The mechanism by which the proteasomal ATPase open this gate has been recently elucidated.  20S gate opening, and thus substrate degradation, requires the C-termini of the proteasomal ATPases, which contains a specific motif (ie HbYX motif). The ATPases C-termini bind into pockets in the top of the 20S, and tether the ATPase complex to the 20S proteolytic complex thus joining the substrate unfolding equipment with the 20S degradation machinery. Binding of these C-termini into these 20S pockets by themselves stimulates opening of the gate in the 20S much like a "key-in-a-lock" opens a door.  The precise mechanism by which this "key-in-a-lock" mechanism functions has been structurally elucidated. 

11S regulatory particle

20S proteasomes can also associate with a second type of regulatory particle, the 11S regulatory particle, a heptameric structure that does not contain any ATPases and can promote the degradation of short peptides , but not of complete proteins. It is presumed that this is because the complex cannot unfold larger substrates. This structure is also known as PA28 or REG. The mechanisms by which it binds to the core particle through the C-terminal tails of its subunits and induces α-ring conformational changes to open the 20S gate suggest a similar mechanism for the 19S particle. The expression of the 11S particle is induced by interferon gamma and is responsible, in conjunction with the immunoproteasome β subunits, for the generation of peptides that bind to the major histocompatibility complex .

Ubiquitination and targeting

Proteins are targeted for degradation by the proteasome by covalent modification of a lysine residue that requires the coordinated reactions of threeenzymes . In the first step, a ubiquitin-activating enzyme (known as E1) hydrolyzes ATP and adenylates a ubiquitin molecule. This is then transferred to E1's active-site cysteine residue in concert with the adenylation of a second ubiquitin. ^[ 28 ] This adenylated ubiquitin is then transferred to a cysteine of a second enzyme, ubiquitin-conjugating enzyme (E2). In the last step, a member of a highly diverse class of enzymes known as ubiquitin ligases (E3) recognizes the specific protein to be ubiquitinated and catalyzes the transfer of ubiquitin from E2 to this target protein. A target protein must be labeled with at least four ubiquitin monomers (in the form of a polyubiquitin chain) before it is recognized by the proteasome lid.  It is therefore the E3 that confers substrate specificity to this system.  The number of E1, E2, and E3 proteins expressed depends on the organism and cell type, but there are many different E3 enzymes present in humans, indicating that there is a huge number of targets for the ubiquitin proteasome system.

The mechanism by which a polyubiquitinated protein is targeted to the proteasome is not fully understood. Ubiquitin-receptor proteins have an N-terminal ubiquitin-like (UBL) domain and one or more ubiquitin-associated (UBA) domains. The UBL domains are recognized by the 19S proteasome caps and the UBA domains bind ubiquitin via three-helix bundles . These receptor proteins may escort polyubiquitinated proteins to the proteasome, though the specifics of this interaction and its regulation are unclear. 

The ubiquitin protein itself is 76 amino acids long and was named due to its ubiquitous nature, as it has a highly conserved sequence and is found in all known eukaryotic organisms. The genes encoding ubiquitin in eukaryotes are arranged in tandem repeats , possibly due to the heavy transcription demands on these genes to produce enough ubiquitin for the cell. It has been proposed that ubiquitin is the slowest- evolving protein identified to date. 

Unfolding and translocation

After a protein has been ubiquitinated, it is recognized by the 19S regulatory particle in an ATP-dependent binding step.  The substrate protein must then enter the interior of the 20S particle to come in contact with the proteolytic active sites. Because the 20S particle's central channel is narrow and gated by the N-terminal tails of the α ring subunits, the substrates must be at least partially unfolded before they enter the core. The passage of the unfolded substrate into the core is called translocation and necessarily occurs after deubiquitination.  However, the order in which substrates are deubiquitinated and unfolded is not yet clear.  Which of these processes is the rate-limiting step in the overall proteolysis reaction depends on the specific substrate; for some proteins, the unfolding process is rate-limiting , while deubiquitination is the slowest step for other proteins.  The extent to which substrates must be unfolded before translocation is not known, but substantial tertiary structure , and in particular nonlocal interactions such as disulfide bonds , are sufficient to inhibit degradation. 

The gate formed by the α subunits prevents peptides longer than about four residues from entering the interior of the 20S particle. The ATP molecules bound before the initial recognition step are hydrolyzed before translocation. While energy is needed for substrate unfolding it is not required for translocation.  The assembled 26S proteasome can degrade unfolded proteins in the presence of a non-hydrolyzable ATP analog , but cannot degrade folded proteins, indicating that energy from ATP hydrolysis is used for substrate unfolding.  Passage of the unfolded substrate through the opened gate occurs via facilitated diffusion if the 19S cap is in the ATP-bound state. 

The mechanism for unfolding of globular proteins is necessarily general, but somewhat dependent on the amino acid sequence . Long sequences of alternating glycine and alanine have been shown to inhibit substrate unfolding decreasing the efficiency of proteasomal degradation; this results in the release of partially degraded byproducts, possibly due to the decoupling of the ATP hydrolysis and unfolding steps.  Such glycine-alanine repeats are also found in nature, for example in silk fibroin ; in particular, certain Epstein-Barr virus gene products bearing this sequence can stall the proteasome, helping the virus propagate by preventing antigen presentation on the major histocompatibility complex . 

A cutaway view of the proteasome 20S core particle illustrating the locations of the active sites . The α subunits are represented as green spheres and the β subunits as protein backbones colored by individualpolypeptide chain . The small pink spheres represent the location of the active-site threonine residue in each subunit. Light blue chemical structures are the inhibitor bortezomib bound to the active sites.

Proteolysis

The mechanism of proteolysis by the β subunits of the 20S core particle is through a threonine -dependent nucleophilic attack . This echanism may depend on an associated water molecule for deprotonation of the reactive threonine hydroxyl . Degradation occurs within the central chamber formed by the association of the two β rings and normally does not release partially degraded products, instead reducing the substrate to short polypeptides typically 7–9 residues long, though they can range from 4 to 25 residues depending on the organism and substrate. The biochemical mechanism that determines product length is not fully characterized.  Although the three catalytic β subunits have a common mechanism, they have slightly different substrate specificities, which are consideredchymotrypsin -like, trypsin -like, and peptidyl-glutamyl peptide-hydrolyzing (PHGH)-like. These variations in specificity are the result of interatomic contacts with local residues near the active sites of each subunit. Each catalytic β subunit also possesses a conserved lysine residue required for proteolysis. 

Although the proteasome normally produces very short peptide fragments, in some cases these products are themselves biologically active and functional molecules. Certain transcription factors regulating the expression of specific genes, including one component of the mammalian complex NF-κB , are synthesized as inactive precursors whose ubiquitination and subsequent proteasomal degradation converts them to an active form. Such activity requires the proteasome to cleave the substrate protein internally: rather than processively degrading it from one terminus. It has been suggested that long loops on these proteins' surfaces serve as the proteasomal substrates and enter the central cavity, while the majority of the protein remains outside. Similar effects have been observed in yeast proteins; this mechanism of selective degradation is known as regulated ubiquitin/proteasome dependent processing (RUP). 

Ubiquitin-independent degradation

Although most proteasomal substrates must be ubiquitinated before being degraded, there are some exceptions to this general rule, especially when the proteasome plays a normal role in the post- translational processing of the protein. The proteasomal activation of NF-κB by processing p105 into p50 via internal proteolysis is one major example. Some proteins that are hypothesized to be unstable due to intrinsically unstructured regions,  are degraded in a ubiquitin-independent manner. The most well-known example of a ubiquitin-independent proteasome substrate is the enzyme ornithine decarboxylase . Ubiquitin-independent mechanisms targeting key cell cycle regulators such as p53 have also been reported, although p53 is also subject to ubiquitin-dependent degradation.  Finally, structurally abnormal, misfolded, or highly oxidized proteins are also subject to ubiquitin-independent and 19S-independent degradation under conditions of cellular stress. 

The proteasome plays a straightforward but critical role in the function of the adaptive immune system . Peptide antigens are displayed by the major histocompatibility complex class I (MHC) proteins on the surface of antigen-presenting cells . These peptides are products of proteasomal degradation of proteins originated by the invading pathogen . Although constitutively expressed proteasomes can participate in this process, a specialized complex composed of proteins whose expression is induced by interferon gamma produces peptides of the optimal size and composition for MHC binding.These proteins whose expression increases during the immune response include the 11S regulatory particle, whose main known biological role is regulating the production of MHC ligands, and specialized β subunits called β1i, β2i, and β5i with altered substrate specificity. The complex formed with the specialized β subunits is known as the immunoproteasome .  Another β5i variant subunit, β5t, is expressed in the thymus, leading to a thymus-specific "thymoproteasome" whose function is as yet unclear. 

The strength of MHC class I ligand binding is dependent on the composition of the ligand C-terminus , as peptides bind by hydrogen bonding and by close contacts with a region called the "B pocket" on the MHC surface. Many MHC class I alleles prefer hydrophobic C-terminal residues, and the immunoproteasome complex is more likely to generate hydrophobic C-termini. 

Due to its role in generating the activated form of NF-κB , an anti- apoptotic and pro- inflammatory regulator of cytokine expression, proteasomal activity has been linked to inflammatory andautoimmune diseases . Increased levels of proteasome activity correlate with disease activity and have been implicated in autoimmune diseases including systemic lupus erythematosus andrheumatoid arthritis . 

The proteasome is also involved in Intracellular antibody-mediated proteolysis of antibody bound virions. In this neutralisation pathway, TRIM21 (a protein of the tripartite motif family) binds withimmunoglobulin G to direct the virion to the proteasome where it is degraded. 

Sumber : http://en.wikipedia.org/wiki/Proteasome

The Proteasomes

General information

Proteasomes are protein degradative machines that are found in the nucleus and the cytoplasm. They are not only found in all eukaryotic organisms, but have also been found in archaebacteria. Proteasomes play many roles in the cell's life:

1. They remove abnormal and misfolded proteins from the cell.

2. They are involved in the cell's stress response, where they degrade Ub-conjugated regulatory proteins.

3. As part of the Ub system, they are involved in regulating the cell cycle.

4. They are involved in cellular differentiation (where they degrade transcription factors and metabolic enzymes).

5. They play an important role in the immune system by generating antigenic peptides that are presented by the major histocompatibility complex (MHC) class I molecules (such molecules are studied in Immunology).

In short, proteasome activity is involved in most of the processes that also involve ubiquitin. We know they are essential because the removal of proteasome genes in eukaryotes is lethal.

Proteasomes are cylindrical structures very similar to hsp60 chaperonins (we'll discuss this later). Like all cellular machines, proteasomes use ATP to drive conformational changes in their subunits. ATP hydrolysis is not needed to actually cleave the peptide bonds of a protein, but instead is thought to involve recognition of target proteins, their unfolding, the translocation of the substrate protein into the proteasome's chamber, and/or the opening and closing of proteasome gates.

20S Proteasome Chamber in Archaebacteria

Let's consider the structure of the proteasome core. We will focus on archaebacterial proteasomes because they are simpler. Eucaryotice proteasomes are very similar, yet they employ many more different subunits to construct the chamber.

As noted above, the proteasome core is actually a cylinder like hsp60. It is about the same size as hsp60, namely, 15nm in length and 11 nm in diameter. It is composed of four rings stacked on top of each other (like tires).Consider the following figure below.

Each of the four rings is composed of seven individual protein subunits. The two outer rings are made up of alpha subunits and are proteolytically inactive. Note that they also define the "pore" which allows substrate proteins inside. This pore is roughly 0.13 nm in diameter. The two inner rings are composed of beta subunits and are proteolytically active. Together, these four rings define three chambers within the proteasome, the largest one being in the center and defined by the beta subunits. It is in this chamber that proteins are cleaved into small peptides.

Click here to see the protein structure of the alpha and beta subunits. Note that they are very similar in shape. The biggest difference is that the alpha subunit has an extra alpha helix across the top of the molecule. This helix is part of the "pore" and may help guide substrate proteins inside the chamber. The beta subunit shown is mature. At an earlier point in its life, a prosequence was attached to the N-terminal end. This prosequence masks the proteolytic activity of this subunit when it exists as an individual subunit. During assembly of the proteasome, it is cut away to expose the beta-sheet cleft (which defines the active site). In fact, the N-terminal threonine that is created by such cleaving of the prosequence is the amino acid that is directly responsible for proteolysis.

20S Chamber Function

The proteasome represents a unique type of protease - a threonine protease. Most proteases use other amino acids (like serine) as part of the proteolytic active site. When a substrate protein is unfolded and guided into the middle chamber, peptide bonds are cleaved every 8-9 amino acids. Thus, the proteasome takes a single polypeptide chain containing hundreds of amino acids and converts it into numerous short peptides 8-9 amino acids in length. That this regular size is seen suggests a molecular ruler is involved. More specifically, the polypeptide chain is apparently stretched across the chamber so that it interacts with two proteolytic sites concurrently.The distance between these two sites would be thus bridged by 8-9 amino acids of the substrate protein. However, once cut, it is still not clear how the individual short peptides are removed from the chamber, although some have speculated the proteasome may have side windows that serve this purpose.

Proteasome Assembly

Click here to view one model for proteasome assembly. According to this model, individual alpha subunits first bind to individual beta subunits. These dimers are not active. Then, these protein dimers interact with each other to form a ring complex. This activity may be guided by chaperones. Once the rings are formed, two such rings can come together with the concurent removal of the beta subunit prosequences. The cleavage of these subunits not only drives the assembly of the 20S chamber, but also activates the subunits inside the chamber. This assembly process thus protects the cytoplasm from indiscriminate proteolysis.

26S Proteasome

The 20S chamber is simply the core degradative machinery. In eukaryotic cells, the complex is typically associated with what are called 19S caps. The 26S proteasome is thus a complex of the 20S core chamber attached to two 19S caps on each end. Click here to see the 26S proteasome.

It is the 19S caps that tie the proteasome to the Ub system. These caps are composed of about 20 different proteins. Some of these proteins apparently interact with Ub (recall how Ub functions to decrease the rate of dissociation between substrate proteins and the proteasome), although it has yet to be determined which proteins actually interact with Ub. Without the 19S caps, Ub-conjugated proteins are no more likely to be degraded by the proteasome than any other protein. It is also worth noting that the 19S caps appear to be flexibly attached, raising the possibility that movement is involved in the capturing of Ub-conjugated proteins.

The best defined component of the 19S caps involve six proteins with ATPase activity. These six proteins form a ring that sits adjacent to the pore of the proteasome defined by the alpha subunits. ATP hydrolysis is clearly involved with the entry of proteins into the chamber, but the exact mechanism involved remains to be determined.

Put simply, the 20S chamber is the heart of the proteasome that does the degradation. The 19S caps serve to capture and guide proteins into this chamber. 19S caps are not seen in archaebacteria. This is consistent with the fact that neither is Ub found in these bacteria.

Immunoproteasomes as "Hot Rod" Complexes

A very interesting feature of proteasomes is their role in the immune system. Proteasomes can syntheisize short peptide fragments that are then used as antigens in lymphocytes. These antigens are presented on the surface of these cells (through the MHC complex) and play an important role in the cells ability to mount a specific immune response. This role for the proteasome is a demanding one and calls for a "souped-up"

proteasome. In higher vertebrates (not yeast) another complex is used to replace the 19S cap. This complex is know as PA28 (or the 11S cap). This cap is smaller than the 19S cap, but serves to make the proteasome more efficient at generated peptides. It is thought to do so by stretching open the mouth of the proteasome and creating a strain on it so that it is easier for peptides to escape from the chamber. PA28 is a six member ring made up of two different subunits.

Furthermore, in immune cells, some of the beta subunits are replaced by what are called gamma-interferon inducible homologs. These subunits are like the beta subunits, but allow for alterations on the degradation process that better fit the immune system's need. Interferon is a cell hormone that is excreted locally when an infection occurs. Thus, infections cause immune cells to replace certain proteasome parts with more effective ones. The immunoproteasome is one mean proteasome. Click here to see it.

Comparing the proteasome and chaperonin

As the figure below shows, the 20S proteasome is very similar to the hsp60 chaperonin. Both are cylindrical structures of the same size. Both are composed of stacked rings each made up of seven subunits.

Yet in spite of these remarkable similarities, there are important differences. First, the two structures are not evolutionarily related. The amino acid sequences, along with the tertiary structure, of proteasome and chaperone subunits are quite different.

Secondly, the functions are different. Where proteasomes degrade proteins, chaperones provide a protective environment that facilitates proper folding. Of course, it is not surprising that the same conformational solution (the "isolation chamber") is employed for these different functions. In the case of proteasomes, the proteins in the cytoplasm must be protected from the proteolytic processes. Otherwise, indiscriminate degradation would occur. In the case of the chaperones, the unfolded proteins must be protected from the proteins in the cytoplasm. Otherwise, indisciminate aggregation would occur.

When these functional differences are realized, some of the slight differences in architecture are explained. Notice the mouth sizes of both complexes. The diameter of the chaperone's mouth is over three times larger than that of the proteasome. Where a chaperone would not want to greatly restrict access to its chamber, the proteasome would. Thus, the smaller mouth of the proteasome makes sense. Furthermore, remember that the proteasome mouth is complexed to the 19S cap, which addes a further layer of restriction. Hsp60 chaperones do not employ 19S-like caps.

Sumber: http://www2.bw.edu/~mbumbuli/cell/protlec/index.html

The proteasome

degradasi Protein adalah sebagai penting untuk sel sebagai sintesis protein. Misalnya,

• untuk memasok asam amino untuk sintesis protein segar
• untuk menghapus kelebihan enzim
• untuk menghapus faktor transkripsi yang tidak lagi diperlukan.

Ada dua perangkat intraselular utama di mana protein yang rusak atau tidak dibutuhkan dipecah:

• lisosom dan
• proteasomes

Lisosom berurusan terutama dengan

• ekstraselular protein, misalnya, protein plasma, yang diambil ke dalam sel, misalnya, oleh endositosis
• permukaan sel-protein membran yang digunakan dalam endositosis yang dimediasi reseptor .
• protein (dan makromolekul lainnya) ditelan oleh autophagosomes .
  Proteasomes berurusan terutama dengan protein endogen, yaitu, protein yang disintesis di dalam sel seperti:

• faktor transkripsi
• siklin (yang harus dihancurkan untuk mempersiapkan langkah berikutnya dalam siklus sel )
• mereka dikodekan oleh gen yang rusak
• protein yang disandikan oleh virus dan patogen intraselular lainnya
• protein yang terlipat salah karena
• dari terjemahan kesalahan
• mereka telah rusak oleh molekul lain di sitosol.

Struktur proteasome

The Particle Core (CP)

• Partikel inti ini terbuat dari 2 salinan dari masing-masing dari 14 protein yang berbeda.
• Ini adalah dirakit di kelompok 7 membentuk cincin.
• 4 cincin ditumpuk satu sama lain (seperti 4 donat).

The Particle Regulatory (RP)

• Ada dua RP identik, satu di setiap akhir dari partikel inti.
• Setiap terbuat dari 18 protein yang berbeda (tidak satupun dari mereka sama dengan yang dalam CP).
• 6 dari ini ATPase .
• Beberapa subunit memiliki situs yang mengakui ubiquitin protein.

Ubiquitin

• protein kecil (76 asam amino);
• dilestarikan di seluruh kerajaan kehidupan, yaitu, hampir identik dalam urutan baik pada bakteri, ragi, atau mamalia;
• digunakan oleh semua makhluk untuk menargetkan protein untuk kehancuran.

Proses

Protein ditakdirkan untuk kehancuran

• adalah konjugasi molekul ubiquitin yang mengikat ke grup amino terminal dari lisin residu.
• molekul Tambahan mengikat ubiquitin yang pertama membentuk rantai.
• Mengikat kompleks untuk ubiquitin-mengenali situs (s) pada partikel regulasi.
• Protein ini membuka oleh ATPase menggunakan energi ATP.
• Dilipat translokasi protein ke dalam rongga sentral dari partikel inti.
• Beberapa situs aktif pada permukaan bagian dalam dari dua tengah "donat" mematahkan berbagai ikatan peptida spesifik dari rantai.
• Ini menghasilkan satu set peptida rata-rata sekitar 8 asam amino panjang.
• Ini meninggalkan partikel inti oleh rute yang tidak dikenal dimana
• mereka mungkin lebih lanjut dipecah menjadi asam amino individual oleh peptidases dalam sitosol atau
• pada mamalia, mereka mungkin dimasukkan di kelas, saya molekul histocompatability akan dipresentasikan kepada sistem kekebalan tubuh sebagai antigen potensial [ lihat di bawah ].
• Partikel peraturan melepaskan ubiquitins untuk digunakan kembali.

Antigen Pengolahan dengan Proteasomes

Pada mamalia, aktivasi sistem kekebalan tubuh

• menyebabkan pelepasan sitokin gamma-interferon .
• Ini menyebabkan tiga subunit dalam partikel inti untuk digantikan oleh subunit pengganti.
• Peptide yang dihasilkan dalam proteasome berubah dipetik oleh TAP (= ransporter t ssociated dengan p rocessing antigen) protein dan diangkut dari sitosol ke dalam retikulum endoplasma mana
• setiap memasuki alur pada permukaan histocompatability saya molekul kelas .
• Kompleks ini kemudian bergerak melalui aparatus Golgi dan dimasukkan ke dalam membran plasma di tempat yang dapat "diakui" oleh sel T CD8 ^+.

Sumber: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Proteasome.html

Apa yang disebut Lisosom? Protein apa saja yang didegradasi di dalamnya?

Lisosom adalah seluler organel yang mengandung asam hidrolase enzim untuk memecah bahan limbah dan puing-puing selular.Mereka ditemukan di sel-sel hewan, sedangkan dalam ragi dan tanaman peran yang sama dilakukan oleh litik vakuola .  Lisosom mencerna kelebihan atau lusuh organel , partikel makanan, dan menelan virus atau bakteri . The membran sekitar lisosom yang memungkinkan enzim pencernaan untuk bekerja di 4,5 pH yang mereka butuhkan. sekering Lisosom dengan vakuola dan mengeluarkan enzim mereka ke dalam vakuola , mencerna isinya. Mereka diciptakan oleh penambahan enzim hidrolitik untuk endosomes awal dari aparatus Golgi . Nama ini lisosom berasal dari kata Yunani lisis , untuk memisahkan, dan soma, tubuh.Mereka sering dijuluki "bunuh diri-kantong" atau "bunuh diri-kantung" oleh ahli biologi sel karena peran mereka dalam otolisis .Lisosom ditemukan oleh cytologist Belgia Christian de Duve pada tahun 1950.

Ukuran lisosom bervariasi 0,1-1,2 pM .  Pada pH 4,8, bagian dalam lisosom adalah asam dibandingkan dengan sedikit basa sitosol(pH 7.2). lisosom mempertahankan pH ini yang berbeda dengan memompa proton (H ⁺ ion) dari sitosol melintasi membran melaluipompa proton dan klorida saluran ion . Membran lisosomal melindungi sitosol, dan karena itu sisa dari sel , dari degradatif enzimdalam lisosom. Sel adalah tambahan dilindungi dari asam lisosomal hidrolisis yang bocor ke sitosol, karena enzim adalah pH-sensitif dan tidak berfungsi juga dalam lingkungan alkalin sitosol.

Beberapa enzim penting yang ditemukan dalam lisosom meliputi:

§  Lipase , yang mencerna lipid

§  Amilase , yang mencerna amilosa , pati , dan maltodekstrin

§  Protease , yang mencerna protein

§  Nucleases , yang mencerna asam nukleat

§  Asam fosfat monoesters.

lisosomal enzim disintesis dalam sitosol dan retikulum endoplasma , di mana mereka menerima mannose-6-fosfat tag yang menargetkan mereka untuk lisosom^. Menyimpang lisosomal menargetkan menyebabkan inklusi-sel penyakit , dimana enzim tidak benar mencapai lisosom, mengakibatkan akumulasi limbah dalam organel ini. 

Lisosom adalah sistem pembuangan limbah sel dan dapat putus apa-apa. Mereka mencerna hampir semuanya. Satu pengecualian adalah asbes. Mereka digunakan untuk pencernaan makromolekul dari fagositosis (menelan sel mati lain atau bahan ekstraselular yang lebih besar, seperti mikroba menyerang asing), endositosis (dimana protein reseptor yang didaur ulang dari permukaan sel), danautophagy (dimana dalam atau tidak dibutuhkan organel tua atau protein, atau mikroba yang telah menginvasi sitoplasma dikirim ke lisosom). Autophagy juga dapat menyebabkan kematian sel autophagic , suatu bentuk penghancuran diri diprogram , atau otolisis , dari sel, yang berarti bahwa sel mencerna sendiri.

Fungsi lain termasuk mencerna bakteri asing (atau bentuk lain dari limbah) yang menyerang sel dan membantu memperbaiki kerusakan pada membran plasma dengan melayani sebagai patch membran, penyegelan luka. Di masa lalu, lisosom dianggap untuk membunuh sel-sel yang tidak lagi diinginkan, seperti yang di ekor dari kecebong atau web dari jari-jari dari 3 - untuk 6-bulan-tua janin. Sementara lisosom mencerna beberapa bahan dalam proses ini, sebenarnya dilakukan melalui sel mati terprogram, yang disebut apoptosis .

Sumber: http://en.wikipedia.org/wiki/Lysosome

lysosome

A membrane-bounded organelle , found in the cytoplasm of eukaryotic cells , which contains digestive enzymes .It acts as the "garbage disposal" of the cell by breaking down cell components that are no longer needed as well as molecules or even bacteria that are ingested by the cell. The interior of a lysosome is strongly acidic, and its enzymes are active at an acid pH . Lysosomes are found in all eukaryotic cells, but are most numerous in disease-fighting cells, such as leukocytes (white blood cells). 

Some human diseases are caused by lysosome enzyme disorders. Tay-Sachs disease , for example, is caused by a genetic defect that prevents the formation of an essential enzyme that breaks down ganglioside lipids . An accumulation of undigested ganglioside damages the nervous system, causing mental retardation and death in early childhood. 

Details of function and structure

Lysosomes break down cellular waste products, fats, carbohydrates, proteins, and other macromolecules into simple compounds, which are then returned to the cytoplasm as new cell-building materials. To accomplish the tasks associated with digestion , the lysosomes use some 40 different types of hydrolytic enzymes, all of which are manufactured in theendoplasmic reticulum and modified in theGolgi apparatus . Lysosomes are often budded from the membrane of the Golgi apparatus, but in some cases they develop gradually from late endosomes, which are vesicles that carry materials brought into the cell by a process known as endocytosis . 

Like other microbodies , lysosomes are spherical organelles contained by a single layer membrane, though their size and shape varies to some extent. This membrane protects the rest of the cell from the digestive enzymes contained in the lysosomes, which would otherwise cause significant damage. The cell is further safeguarded from exposure to the biochemical catalysts present in lysosomes by their dependency on an acidic environment. With an average pH of about 4.8, the lysosomal matrix is favorable for enzymatic activity, but the neutral environment of the cytosol renders most of the digestive enzymes inoperative, so even if a lysosome is ruptured, the cell as a whole may remain uninjured. The acidity of the lysosome is maintained with the help of hydrogen ion pumps, and the organelle avoids self-digestion by glucosylation of inner membrane proteins to prevent their degradation. 

Sumber: http://www.daviddarling.info/encyclopedia/L/lysosome.html

Lisosom berurusan terutama dengan

• ekstraselular protein, misalnya, protein plasma, yang diambil ke dalam sel, misalnya, oleh endositosis
• permukaan sel-protein membran yang digunakan dalam endositosis yang dimediasi reseptor .
• protein (dan makromolekul lainnya) ditelan oleh autophagosomes .

Sumber: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Proteasome.html