Misplaced Pages

#869130

63-783: Addition of N -acetylgalactosamine (GalNAc) to a serine or threonine occurs in the Golgi apparatus , after the protein has been folded. The process is performed by enzymes known as GalNAc transferases (GALNTs), of which there are 20 different types. The initial O -GalNAc structure can be modified by the addition of other sugars, or other compounds such as methyl and acetyl groups. These modifications produce 8 core structures known to date. Different cells have different enzymes that can add further sugars, known as glycosyltransferases , and structures therefore change from cell to cell. Common sugars added include galactose , N -acetylglucosamine , fucose and sialic acid . These sugars can also be modified by

126-449: A Core 1 structure with an additional N -acetylglucosamine (GlcNAc) sugar. A poly- N -acetyllactosamine structure can be formed by the alternating addition of GlcNAc and galactose sugars onto the GalNAc sugar. Terminal sugars on O-glycans are important in recognition by lectins and play a key role in the immune system. Addition of fucose sugars by fucosyltransferases forms Lewis epitopes and

189-416: A complex modification that forms a long sugar chain. This is required to stabilise the interaction between α-dystroglycan and the extracellular basement membrane. Without these modifications, the glycoprotein cannot anchor the cell which leads to congenital muscular dystrophy (CMD), characterised by severe brain malformations. O-galactose is commonly found on lysine residues in collagen , which often have

252-408: A different form of O-glycosylation, as it does not occur on proteins. This forms glycosphingolipids , which are important for the localisation of receptors in membranes. Incorrect breakdown of these lipids leads to a group of diseases known as sphingolipidoses , which are often characterised by neurodegeneration and developmental disabilities. Because both galactose and glucose sugars can be added to

315-473: A hydroxyl group added to form hydroxylysine . Because of this addition of an oxygen, hydroxylysine can then be modified by O-glycosylation. Addition of a galactose to the hydroxyl group is initiated in the endoplasmic reticulum, but occurs predominantly in the Golgi apparatus and only on hydroxylysine residues in a specific sequence. While this O-galactosylation is necessary for correct function in all collagens, it

378-502: A role in diabetes . Additionally, O-GlcNAcylation can enhance the Warburg Effect , which is defined as the change that occurs in the metabolism of cancer cells to favour their growth. Because both O-GlcNAcylation and phosphorylation can affect specific residues and therefore both have important functions in regulating signalling pathways, both of these processes provide interesting targets for cancer therapy. O-mannosylation involves

441-485: A set of glycosylation enzymes that attach various sugar monomers to proteins as the proteins move through the apparatus. The Golgi apparatus was identified in 1898 by the Italian biologist and pathologist Camillo Golgi . The organelle was later named after him in the 1910s. Because of its large size and distinctive structure, the Golgi apparatus was one of the first organelles to be discovered and observed in detail. It

504-481: A stack; however, in some protists as many as sixty cisternae have been observed. This collection of cisternae is broken down into cis , medial, and trans compartments, making up two main networks: the cis Golgi network (CGN) and the trans Golgi network (TGN). The CGN is the first cisternal structure, and the TGN is the final, from which proteins are packaged into vesicles destined to lysosomes , secretory vesicles, or

567-469: Is a fungal metabolite used experimentally to disrupt the secretion pathway as a method of testing Golgi function. BFA blocks the activation of some ADP-ribosylation factors ( ARFs ). ARFs are small GTPases which regulate vesicular trafficking through the binding of COPs to endosomes and the Golgi. BFA inhibits the function of several guanine nucleotide exchange factors (GEFs) that mediate GTP-binding of ARFs. Treatment of cells with BFA thus disrupts

630-449: Is a long-term store of glucose produced by the cells of the liver . In the liver , the synthesis of glycogen is directly correlated with blood glucose concentration. High blood glucose concentration causes an increase in intracellular levels of glucose 6-phosphate in the liver, skeletal muscle , and fat ( adipose ) tissue. Glucose 6-phosphate has role in regulating glycogen synthase . High blood glucose releases insulin , stimulating

693-440: Is also involved in lipid transport and lysosome formation. The structure and function of the Golgi apparatus are intimately linked. Individual stacks have different assortments of enzymes, allowing for progressive processing of cargo proteins as they travel from the cisternae to the trans Golgi face. Enzymatic reactions within the Golgi stacks occur exclusively near its membrane surfaces, where enzymes are anchored. This feature

SECTION 10

#1732884010870

756-404: Is an unusual O-linked modification as it occurs in the endoplasmic reticulum, catalysed by O-glucosyltransferases, and also requires a defined sequence in order to be added to the protein. O-glucose is often attached to serine residues between the first and second conserved cysteine residues of EGF domains, for example in clotting factors VII and IX. O-glucosylation also appears to be necessary for

819-489: Is associated with heart and respiratory failure, defects in skeletal development and increased tumor metastasis. Different types of proteoglycans exist, depending on the sugar that is linked to the oxygen atom of the residue in the protein. For example, the GAG heparan sulphate is attached to a protein serine residue through a xylose sugar. The structure is extended with several N -acetyllactosamine repeating sugar units added onto

882-465: Is especially common in collagen types IV and V. In some cases, a glucose sugar can be added to the core galactose. Addition of fucose sugars to serine and threonine residues is an unusual form of O-glycosylation that occurs in the endoplasmic reticulum and is catalysed by two fucosyltransferases. These were discovered in Plasmodium falciparum and Toxoplasma gondii . Several different enzymes catalyse

945-472: Is essential to the processes of both anaerobic and aerobic respiration , which involve the production of adenosine triphosphate (ATP), the "high-energy" exchange medium in the cell. During aerobic respiration, ATP is synthesized in the mitochondrion by addition of a third phosphate group to adenosine diphosphate (ADP) in a process referred to as oxidative phosphorylation . ATP is also synthesized by substrate-level phosphorylation during glycolysis . ATP

1008-481: Is important in their protective function as it lubricates the tracts so bacteria cannot bind and infect the body. Changes in mucins are important in numerous diseases, including cancer and inflammatory bowel disease . Absence of O-glycans on mucin proteins changes their 3D shape dramatically and often prevents correct function. Addition of N -acetylglucosamine (O-GlcNAc) to serine and threonine residues usually occurs on cytoplasmic and nuclear proteins that remain in

1071-527: Is in contrast to the ER, which has soluble proteins and enzymes in its lumen . Much of the enzymatic processing is post-translational modification of proteins. For example, phosphorylation of oligosaccharides on lysosomal proteins occurs in the early CGN. Cis cisterna are associated with the removal of mannose residues. Removal of mannose residues and addition of N-acetylglucosamine occur in medial cisternae. Addition of galactose and sialic acid occurs in

1134-543: Is known as a dynamic modification and has a lot of similarities to phosphorylation . O-GlcNAcylation and phosphorylation can occur on the same threonine and serine residues, suggesting a complex relationship between these modifications that can affect many functions of the cell. The modification affects processes like the cells response to cellular stress, the cell cycle, protein stability and protein turnover. It may be implicated in neurodegenerative diseases like Parkinson's and late-onset Alzheimer's and has been found to play

1197-481: Is maintained by inorganic phosphate. Each molecule of glyceraldehyde 3-phosphate is phosphorylated to form 1,3-bisphosphoglycerate . This reaction is catalyzed by glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The cascade effect of phosphorylation eventually causes instability and allows enzymes to open the carbon bonds in glucose. Phosphorylation functions is an extremely vital component of glycolysis, as it helps in transport, control, and efficiency. Glycogen

1260-449: Is often elongated by addition of GlcNAc, galactose and sialic acid. Notch is an important protein in development, with several EGF domains that are O-fucosylated. Changes in the elaboration of the core fucose determine what interactions the protein can form, and therefore which genes will be transcribed during development. O-fucosylation might also play a role in protein breakdown in the liver. Similarly to O-fucosylation, O-glucosylation

1323-486: Is such a ligand, and contains a lot of O-glycans that are necessary for its function. O-glycans near the membrane maintain the elongated structure and a terminal sLe epitope is necessary for interactions with the receptor. Mucins are a group of heavily O-glycosylated proteins that line the gastrointestinal and respiratory tracts to protect these regions from infection. Mucins are negatively charged, which allows them to interact with water and prevent it from evaporating. This

SECTION 20

#1732884010870

1386-430: Is synthesized at the expense of solar energy by photophosphorylation in the chloroplasts of plant cells. Phosphorylation of sugars is often the first stage in their catabolism . Phosphorylation allows cells to accumulate sugars because the phosphate group prevents the molecules from diffusing back across their transporter . Phosphorylation of glucose is a key reaction in sugar metabolism. The chemical equation for

1449-459: The Golgi complex , Golgi body , or simply the Golgi , is an organelle found in most eukaryotic cells . Part of the endomembrane system in the cytoplasm , it packages proteins into membrane-bound vesicles inside the cell before the vesicles are sent to their destination. It resides at the intersection of the secretory, lysosomal, and endocytic pathways. It is of particular importance in processing proteins for secretion , containing

1512-485: The trans cisternae. Sulfation of tyrosines and carbohydrates occurs within the TGN. Other general post-translational modifications of proteins include the addition of carbohydrates ( glycosylation ) and phosphates ( phosphorylation ). Protein modifications may form a signal sequence that determines the final destination of the protein. For example, the Golgi apparatus adds a mannose-6-phosphate label to proteins destined for lysosomes. Another important function of

1575-429: The trans-Golgi network (TGN). This area of the Golgi is the point at which proteins are sorted and shipped to their intended destinations by their placement into one of at least three different types of vesicles, depending upon the signal sequence they carry. Though there are multiple models that attempt to explain vesicular traffic throughout the Golgi, no individual model can independently explain all observations of

1638-462: The 1 and 3 N-atoms of the imidazole ring. Recent work demonstrates widespread human protein phosphorylation on multiple non-canonical amino acids, including motifs containing phosphorylated histidine, aspartate, glutamate, cysteine , arginine and lysine in HeLa cell extracts. However, due to the chemical lability of these phosphorylated residues, and in marked contrast to Ser, Thr and Tyr phosphorylation,

1701-406: The Golgi apparatus is in the formation of proteoglycans . Enzymes in the Golgi append proteins to glycosaminoglycans , thus creating proteoglycans. Glycosaminoglycans are long unbranched polysaccharide molecules present in the extracellular matrix of animals. The vesicles that leave the rough endoplasmic reticulum are transported to the cis face of the Golgi apparatus, where they fuse with

1764-399: The Golgi apparatus is made up of a series of compartments and is a collection of fused, flattened membrane-enclosed disks known as cisternae (singular: cisterna , also called "dictyosomes"), originating from vesicular clusters that bud off the endoplasmic reticulum (ER). A mammalian cell typically contains 40 to 100 stacks of cisternae. Between four and eight cisternae are usually present in

1827-461: The Golgi apparatus varies among eukaryotes. In mammals, a single Golgi apparatus is usually located near the cell nucleus, close to the centrosome. Tubular connections are responsible for linking the stacks together. Localization and tubular connections of the Golgi apparatus are dependent on microtubules . In experiments it is seen that as microtubules are depolymerized the Golgi apparatuses lose mutual connections and become individual stacks throughout

1890-425: The Golgi apparatus. Currently, the cisternal progression/maturation model is the most accepted among scientists, accommodating many observations across eukaryotes. The other models are still important in framing questions and guiding future experimentation. Among the fundamental unanswered questions are the directionality of COPI vesicles and role of Rab GTPases in modulating protein cargo traffic. Brefeldin A (BFA)

1953-422: The Golgi apparatus. Galactose, on the other hand, is added to ceramide already in the Golgi apparatus, where the galactosphingolipid formed is often sulfated by addition of sulfate groups. One of the first and only examples of O-glycosylation on tyrosine , rather than on serine or threonine residues, is the addition of glucose to a tyrosine residue in glycogenin . Glycogenin is a glycosyltransferase that initiates

O-linked glycosylation - Misplaced Pages Continue

2016-412: The Golgi membrane and empty their contents into the lumen . Once inside the lumen, the molecules are modified, then sorted for transport to their next destinations. Those proteins destined for areas of the cell other than either the endoplasmic reticulum or the Golgi apparatus are moved through the Golgi cisternae towards the trans face, to a complex network of membranes and associated vesicles known as

2079-471: The Golgi. Until recently, it was believed that the process is restricted to fungi , however it occurs in all domains of life; eukaryotes, (eu)bacteria and archae(bacteri)a. The best characterised O-mannosylated human protein is α-dystroglycan . O-Man sugars separate two domains of the protein, required to connect the extracellular and intracellular regions to anchor the cell in position. Ribitol , xylose and glucuronic acid can be added to this structure in

2142-404: The addition of sulfates or acetyl groups. GalNAc is added onto a serine or threonine residue from a precursor molecule , through the activity of a GalNAc transferase enzyme. This precursor is necessary so that the sugar can be transported to where it will be added to the protein. The specific residue onto which GalNAc will be attached is not defined, because there are numerous enzymes that can add

2205-429: The analysis of phosphorylated histidine (and other non-canonical amino acids) using standard biochemical and mass spectrometric approaches is much more challenging and special procedures and separation techniques are required for their preservation alongside classical Ser, Thr and Tyr phosphorylation. The prominent role of protein phosphorylation in biochemistry is illustrated by the huge body of studies published on

2268-538: The blood. The phosphorylation of glucose can be enhanced by the binding of fructose 6-phosphate (F6P), and lessened by the binding fructose 1-phosphate (F1P). Fructose consumed in the diet is converted to F1P in the liver. This negates the action of F6P on glucokinase, which ultimately favors the forward reaction. The capacity of liver cells to phosphorylate fructose exceeds capacity to metabolize fructose-1-phosphate. Consuming excess fructose ultimately results in an imbalance in liver metabolism, which indirectly exhausts

2331-434: The cell membrane is negatively charged. This reaction occurs due to the enzyme hexokinase , an enzyme that helps phosphorylate many six-membered ring structures. Phosphorylation takes place in step 3, where fructose-6-phosphate is converted to fructose 1,6-bisphosphate . This reaction is catalyzed by phosphofructokinase . While phosphorylation is performed by ATPs during preparatory steps, phosphorylation during payoff phase

2394-409: The cell surface. The TGN is usually positioned adjacent to the stack, but can also be separate from it. The TGN may act as an early endosome in yeast and plants. There are structural and organizational differences in the Golgi apparatus among eukaryotes. In some yeasts, Golgi stacking is not observed. Pichia pastoris does have stacked Golgi, while Saccharomyces cerevisiae does not. In plants,

2457-435: The cell, compared to O -GalNAc modifications which usually occur on proteins that will be secreted. O-GlcNAc modifications were only recently discovered, but the number of proteins with known O-GlcNAc modifications is increasing rapidly. It is the first example of glycosylation that does not occur on secretory proteins. O -GlcNAcylation differs from other O-glycosylation processes because there are usually no sugars added onto

2520-440: The ceramide lipid, we have two groups of glycosphingolipids. Galactosphingolipids are generally very simple in structure and the core galactose is not usually modified. Glucosphingolipids, however, are often modified and can become a lot more complex. Biosynthesis of galacto- and glucosphingolipids occurs differently. Glucose is added onto ceramide from its precursor in the endoplasmic reticulum, before further modifications occur in

2583-424: The conversion of D-glucose to D-glucose-6-phosphate in the first step of glycolysis is given by: Glycolysis is an essential process of glucose degrading into two molecules of pyruvate , through various steps, with the help of different enzymes. It occurs in ten steps and proves that phosphorylation is a much required and necessary step to attain the end products. Phosphorylation initiates the reaction in step 1 of

O-linked glycosylation - Misplaced Pages Continue

2646-568: The conversion of glucose to glycogen, present in muscle and liver cells. All forms of O-glycosylation are abundant throughout the body and play important roles in many cellular functions. Lewis epitopes are important in determining blood groups , and allow the generation of an immune response if we detect foreign organs. Understanding them is important in organ transplants . Hinge regions of immunoglobulins contain highly O-glycosylated regions between individual domains to maintain their structure, allow interactions with foreign antigens and protect

2709-467: The core structure and because the sugar can be attached or removed from a protein several times. This addition and removal occurs in cycles and is performed by two very specific enzymes. O-GlcNAc is added by O-GlcNAc transferase (OGT) and removed by O-GlcNAcase (OGA). Because there are only two enzymes that affect this specific modification, they are very tightly regulated and depend on a lot of other factors. Because O-GlcNAc can be added and removed, it

2772-504: The cytoplasm. In yeast , multiple Golgi apparatuses are scattered throughout the cytoplasm (as observed in Saccharomyces cerevisiae ). In plants , Golgi stacks are not concentrated at the centrosomal region and do not form Golgi ribbons. Organization of the plant Golgi depends on actin cables and not microtubules. The common feature among Golgi is that they are adjacent to endoplasmic reticulum (ER) exit sites. In most eukaryotes,

2835-490: The development of modern microscopes in the twentieth century, the discovery was confirmed. Early references to the Golgi apparatus referred to it by various names, including the Golgi–Holmgren apparatus, Golgi–Holmgren ducts, and Golgi–Kopsch apparatus. The term Golgi apparatus was used in 1910 and first appeared in scientific literature in 1913, while "Golgi complex" was introduced in 1956. The subcellular localization of

2898-399: The elongation of the core fucose, meaning that different sugars can be added to the initial fucose on the protein. Along with O-glucosylation, O-fucosylation is mainly found on epidermal growth factor (EGF) domains found in proteins. O-fucosylation on EGF domains occurs between the second and third conserved cysteine residues in the protein sequence. Once the core O-fucose has been added, it

2961-437: The endoplasmic reticulum. Proteins synthesized in the ER are packaged into vesicles, which then fuse with the Golgi apparatus. These cargo proteins are modified and destined for secretion via exocytosis or for use in the cell. In this respect, the Golgi can be thought of as similar to a post office: it packages and labels items which it then sends to different parts of the cell or to the extracellular space . The Golgi apparatus

3024-458: The fate of the protein. The compartmentalization of the Golgi apparatus is advantageous for separating enzymes, thereby maintaining consecutive and selective processing steps: enzymes catalyzing early modifications are gathered in the cis face cisternae, and enzymes catalyzing later modifications are found in trans face cisternae of the Golgi stacks. The Golgi apparatus is a major collection and dispatch station of protein products received from

3087-565: The heart. This further suggests a link between intermediary metabolism and cardiac growth. Protein phosphorylation is the most abundant post-translational modification in eukaryotes. Phosphorylation can occur on serine , threonine and tyrosine side chains (in other words, on their residues) through phosphoester bond formation, on histidine , lysine and arginine through phosphoramidate bonds , and on aspartic acid and glutamic acid through mixed anhydride linkages . Recent evidence confirms widespread histidine phosphorylation at both

3150-606: The individual stacks of the Golgi apparatus seem to operate independently. The Golgi apparatus tends to be larger and more numerous in cells that synthesize and secrete large amounts of substances; for example, the antibody -secreting plasma B cells of the immune system have prominent Golgi complexes. In all eukaryotes, each cisternal stack has a cis entry face and a trans exit face. These faces are characterized by unique morphology and biochemistry . Within individual stacks are assortments of enzymes responsible for selectively modifying protein cargo. These modifications influence

3213-465: The liver cell's supply of ATP. Allosteric activation by glucose 6-phosphate, which acts as an effector, stimulates glycogen synthase, and glucose 6 phosphate may inhibit the phosphorylation of glycogen synthase by cyclic AMP -stimulated protein kinase . Phosphorylation of glucose is imperative in processes within the body. For example, phosphorylating glucose is necessary for insulin-dependent mechanistic target of rapamycin pathway activity within

SECTION 50

#1732884010870

3276-460: The liver. The liver's crucial role in controlling blood sugar concentrations by breaking down glucose into carbon dioxide and glycogen is characterized by the negative Gibbs free energy (ΔG) value, which indicates that this is a point of regulation with. The hexokinase enzyme has a low Michaelis constant (K m ), indicating a high affinity for glucose, so this initial phosphorylation can proceed even when glucose levels at nanoscopic scale within

3339-408: The preparatory step (first half of glycolysis), and initiates step 6 of payoff phase (second phase of glycolysis). Glucose, by nature, is a small molecule with the ability to diffuse in and out of the cell. By phosphorylating glucose (adding a phosphoryl group in order to create a negatively charged phosphate group ), glucose is converted to glucose-6-phosphate, which is trapped within the cell as

3402-522: The proper folding of EGF domains in the Notch protein. Proteoglycans consist of a protein with one or more sugar side chains, known as glycosaminoglycans (GAGs), attached to the oxygen of serine and threonine residues. GAGs consist of long chains of repeating sugar units. Proteoglycans are usually found on the cell surface and in the extracellular matrix (ECM), and are important for the strength and flexibility of cartilage and tendons. Absence of proteoglycans

3465-476: The region close to the membrane so that the protein extends away from the surface. For example, the low-density lipoprotein receptor (LDL) is projected from the cell surface by a region rigidified by O-glycans. In order for leukocytes of the immune system to move into infected cells, they have to interact with these cells through receptors . Leukocytes express ligands on their cell surface to allow this interaction to occur. P-selectin glycoprotein ligand-1 (PSGL-1)

3528-790: The region from proteolytic cleavage. Alzheimer's may be affected by O-glycosylation. Tau, the protein that accumulates to cause neurodegeneration in Alzheimer's, contains O-GlcNAc modifications which may be implicated in disease progression. Changes in O-glycosylation are extremely common in cancer . O-glycan structures, and especially the terminal Lewis epitopes, are important in allowing tumor cells to invade new tissues during metastasis. Understanding these changes in O-glycosylation of cancer cells can lead to new diagnostic approaches and therapeutic opportunities. Golgi apparatus The Golgi apparatus ( / ˈ ɡ ɒ l dʒ i / ), also known as

3591-731: The scaffold for blood group determinants. Addition of a fucose alone creates the H-antigen, present in people with blood type O. By adding a galactose onto this structure, the B-antigen of blood group B is created. Alternatively, adding a GalNAc sugar will create the A-antigen for blood group A. O -GalNAc sugars are important in a variety of processes, including leukocyte circulation during an immune response, fertilisation, and protection against invading microbes . O -GalNAc sugars are common on membrane glycoproteins , where they help increase rigidity of

3654-429: The secretion pathway, promoting disassembly of the Golgi apparatus and distributing Golgi proteins to the endosomes and ER. Phosphorylation In biochemistry , phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation , are common in biology . Protein phosphorylation often activates (or deactivates) many enzymes . Phosphorylation

3717-411: The sugar and each one will favour different residues. However, there are often proline (Pro) residues near the threonine or serine. Once this initial sugar has been added, other glycosyltransferases can catalyse the addition of additional sugars. Two of the most common structures formed are Core 1 and Core 2. Core 1 is formed by the addition of a galactose sugar onto the initial GalNAc. Core 2 consists of

3780-405: The transfer of a mannose from a dolichol- P -mannose donor molecule onto the serine or threonine residue of a protein. Most other O-glycosylation processes use a sugar nucleotide as a donor molecule. A further difference from other O-glycosylations is that the process is initiated in the endoplasmic reticulum of the cell, rather than the Golgi apparatus. However, further addition of sugars occurs in

3843-414: The translocation of specific glucose transporters to the cell membrane; glucose is phosphorylated to glucose 6-phosphate during transport across the membrane by ATP-D-glucose 6- phosphotransferase and non-specific hexokinase (ATP-D-hexose 6-phosphotransferase). Liver cells are freely permeable to glucose, and the initial rate of phosphorylation of glucose is the rate-limiting step in glucose metabolism by

SECTION 60

#1732884010870

3906-414: The xylose. This process is unusual and requires specific xylosyltransferases. Keratan sulphate attaches to a serine or threonine residue through GalNAc, and is extended with two galactose sugars, followed by repeating units of glucuronic acid (GlcA) and GlcNAc. Type II keratan sulphate is especially common in cartilage. Galactose or glucose sugars can be attached to a hydroxyl group of ceramide lipids in

3969-408: Was discovered in 1898 by Italian physician Camillo Golgi during an investigation of the nervous system . After first observing it under his microscope , he termed the structure as apparato reticolare interno ("internal reticular apparatus"). Some doubted the discovery at first, arguing that the appearance of the structure was merely an optical illusion created by Golgi’s observation technique. With

#869130