Manual Plant Carbohydrates I: Intracellular Carbohydrates

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Amylose and amylopectin are examples of starch carbohydrates. The branching in Amylopectin is less than in glycogen. The polymer of glucose is not soluble in water. Glycogen, also known as "animal starch", is the storage of glucose as a source of energy to animal cells. Its structure is similar to that of the amylopectin and has even more branches. These branchings occur once for every ten 1,4-linkages.

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Glycogen is primarily produced in the liver and muscles cells. Chitin is a homopolysaccharide made from repeating units of a derivative of glucose, N-Acetylglucosamine. Chitin is a very important structural component making up the cell walls of fungi, the exoskeletons of arthropods and insects, and other such components. Its structure and linkages are similar to that of cellulose, except that the hydroxyl group on the 2' carbon of glucose is replaced by an acetylamine group.

Reducing sugars have access to their open chain form. Reducing sugars are basically sugars with an aldehyde group in their open form or a hemiacetal group in their ring form at the anomeric carbon that is ready to oxidize. In other words, reducing sugars allow for chain formation and elongation. Examples of reducing sugars are glucose, maltose, and lactose. Reducing sugars can form aldehyde or ketone groups under basic conditions as the carboxyl group is reduced to carbonyl group of aldehyde or ketone group and occurs when the anomeric carbon is not bonded to hemiacetal or hemiketal hydroxyl group.

It has only been mentioned that aldoses can form reducing sugars. What about ketoses? Mechanistically, when ketoses isomerize to their ring form, acetals are formed. So no, ketoses do not form reducing sugars. However, ketoses can tautomerize to aldoses, where a hemiacetal can then be formed upon ring closure. Nonreducing sugars do not have access to free aldehyde or ketone such as glycosides. These type of sugar is basically an acetal in its ring form at the anomeric carbon.

Since the anomeric carbon is fixed in a glycosidic linkage, the sugar chain cannot form or be elongated. An example of a nonreducing sugar is sucrose. Glycoproteins, namely organic compounds of a mixture of carbohydrates and proteins bonded by covalent bonds, are formed in the process of glycosylation.

Carbohydrate groups may be covalently bonded to a protein to form a glycoprotein, which is an important part of the cell membrane in such processes as cell adhesion and the binding of sperm to eggs in fertilization. In glycoproteins, the carbohydrate weight percentage is much less than the weight of carbohydrates found in proteoglycans.

The sugar group of glycoproteins help protein folding and increase protein stability. Sugars within glycoproteins are found attached either to the amide nitrogen atom side chain of asparagine by N-linked glycosidic bond or to the oxygen atom in the side chain of serine or threonine by O-linked glycosidic bond. The bond formed between carbohydrate and protein is by bonding to one of four amino acids of asparagine, hydroxylysine, serine, or threonine. Thus protein glycosylation sites can be predicted because an asparagine can only be glycosylated when found in the form: Asn-X-Ser, Asn-X-Thr, or Asn-X-Cys sequence, where X can be any amino acid but Proline.

But not all potential sites are glycosylated. Glycosylated sites have other factors such as the cell type in which the protein is expressed and protein structure. In N-linked oligosaccharides , it has a common pentasaccharide core with three mannose and two N-acetyglucosamine residues. To form a great variety of oligosaccharide pattern, additional sugars are attached to this core.

Integral membrane proteins of glycoproteins are important for interactions between one cell to the other. Glycosylation of the extracellular part of proteins takes place in the Endoplasmic Reticulum and in the Golgi complex.

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Golgi is the major sorting center of the cell. Protein proceed from Golgi to lysosomes, secretory granules, or plasma membrane. Ribosomes on the cytoplasmic face of the rough ER synthesize the protein that is taken into the lumen of the ER. N-linked glycosylation starts in the ER and continues into the Golgi complex. It begins with the addition of an oligosaccharide precursor made up of a chain of 14 sugar molecules.

However, the O-linked glycosylation site is exclusively in the Golgi complex. Also unlike N-linked glycosylation, O-linked glycosylation has the sugar molecules added one at a time, each by a different glycotransferase enzyme. One example includes the addition of N-acetylgalactosamide GalNAc , in the cis -Golgi area, by N-acetylgalactosamide transferase. It should be noted that the Golgi complex is split into three areas the cis , trans , and medial.

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In the trans -Golgi, a galactose residue is attached to N-acetylgalacosamide by a galactosamide transferase specific to the this region of the Golgi complex. An oligosaccharide precursor that is to be attached to the amide side chain of an asparagine residue in a protein is first attached to dolichol phosphate. Dolichol phosphate is a lipid molecule found in the ER lumen and is made of about twenty isoprene units.

The terminal phosphate group of dolical phosphate is the site of attachment of the oligosaccharide. With the help oligosaccharide-protein transferase, the oligosaccharide is transferred from dolichol phosphate to the asparagine molecule. Proteins from the lumen of the ER and the ER membrane are then transferred to the Golgi complex, where the carbohydrate part of the glycoprotein is altered. Since the Golgi has three areas, each with its own set of enzymes, modifications to the precursor oligosaccharide allows for a range of oligosaccaride structures to form.

After the Golgi complex, proteins proceed to either lysosomes, secretory granules, or the plasma membrane, depending on the signals embedded within the amino acid sequences and the three-dimensional structures. Erythropoietin EPO is a glycoprotein hormone that stimulate the production of red blood cells.

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The presence of three Asn residues and one Ser residues allow oligosaccharides to link the protein at the three N-linked glycosylation and one O-linked glycosylation sites. It is secreted by the kidney. The zona pellucida is a glycoprotein membrane, where it appears at multilaminar primary oocytes around the plasma membrane. The zona pellucida structures must initiate the acrosome reaction, in order to binds with the spermatozoa.

Therefore, scientists found four zona pellucidas that are responsible binding the spermatozoa and the acrosome reaction within the mouse. The most important zona glycoprotein is the ZP3, because ZP3 is responsible for sperm binding. The sperm protein is adhering with the plasma membrane of the oocyte.

In addition, the ZP3 is involved with the acrosomal reaction; this lead to the releasing the spermatozoon of the acrosomal vesicle. The ZP2 is responsible of mediating the subsequent of the sperm binding. The ZP4 is the protein that human encodes the genes.

Components of Plasma Membranes

For humans, it takes five days after fertilization that the zona hatching was performed by the blastocyst. On the other hand, the zona pellucida is being replaced by the layer of trophoblastic cells, when zona pellucida is decomposes and degenerate. Overall, the zona pellucida is has a great importance on the egg death and began the fertilization.

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Oligosaccharides can be sequenced by enzymatic analysis and mass spectroscopy. It is hard to know the structure of sugars so remove sugar from glycoprotein. You will use enzyme and mass spectroscopy to find out the order of these sugars that are attached. Carbohydrate attachment to proteins is important for processing, stability, and targeting these proteins.

Improper glycosylation of proteins can lead to inheritable human diseases called congenital disorders of glycosylation. An example involves I-cell disease. I-cell disease is a lysosomal storage disease. A carbohydrate marker is used for directing degradative enzymes. The lysosomes of people with I-cell disease have large inclusions of undigested glycosaminoglycans. These inclusions are present because the lysosomes of I-cell patients lack the enzyme to degrade them. However, these enzymes are present in high volumes elsewhere in the body, thus indicating incorrectly delivered enzymes in I-cell patients.

It has been shown that carbohydrate-protein complexes function in cell-cell recognition processes as well as adhesion of cells to neighboring cells and the extracellular matrix. The diverse carbohydrate structures displayed on cell surfaces are well suited to serve as interaction sites between cells and their environments. A glycoprotein is formed when a carbohydrate group attaches to a protein through a covalent bond.

These glycosidic bonds link carbohydrates to the amino and hydroxy side chains of asparagine and serine or threonine, respectively. An N-linkage is the bond between a carbohydrate and the nitrogen in the asparagine side chain, and an O-linkage is the bond between a carbohydrate and the oxygen of serine or threonine.

An asparagine residue can accept an oligosaccharide only if the residue is part of an Asn-X-Ser or Asn-X-Thr sequence, in which X can be any amino acid, except proline. Thus, potential glycosylation sites can be detected in a proteins primary structure. Not all potential sites are glycosylated, however. Glocosylated sites depend on protein structure within the region and the cell type in which the protein is expressed. All N-linked oligosaccharides have in common a pentasaccharide core consisting of three mannose and two N-acetylglucosamine residues.

Glycoproteins play several roles in terms of the medical world. Modified carbohydrates have the ability to interfere with the interactions between carbohydrates and proteins. This leads to the inhibition of the cell—cell recognition and adhesion that is a major factor contributing to cancerous growth. Thus, these the ligands of the carbohydrate-binding proteins could potentially evolve into new forms of cancer treatment.

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Carbohydrates as recognition molecules are also important during embryonic development. C arbohydrates of the plasma membrane are major recognition and attaching sites for pathogens during infection. Virus, such as the influenza virus, pathogenic E. These pathogens have proteins, known as lectins, that bind to specific carbohydrates of particular cells.

Thus, the type of cell to be infected depends on the carbohydrates they show in the plasma membrane. Vertebrates, invertebrates and protozoa bear different set of carbohydrates in their cells. Curiously, some pathogens are able to "dress" superficial carbohydrates similar to those of the host cells. In this way, they cannot be detected. There are differences in the carbohydrate composition of cells of vertebrate, invertebrate and protozoa.