Chapter 14 Notes
Fischer and Haworth structures of alpha-D-ribose Note the relationship between the Fischer projections and the cyclic Haworth projections. Ring Structure for Ribose: The chair form of ribose follows a similar pattern as that for glucose with one exception. Since ribose has an aldehyde functional group. Its cyclic form (ribofuranose) is the carbohydrate portion of RNA, DNA, and other nucleic acids. Fischer projection · D-Ribose β-DDeoxyribofuranose.
Glycosylated sites have other factors such as the cell type in which the protein is expressed and protein structure. In N-linked oligosaccharidesit 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 Glycosylation of the extracellular part of proteins takes place in the Endoplasmic Reticulum and in the Golgi complex. 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 GalNAcin 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. 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.
Examples Erythropoietin EPO is a glycoprotein hormone that stimulate the production of red blood cells. 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.
Zona Pellucida 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. 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. Sequence Oligossacrides 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. Glycosylation Errors 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. Carbohydrate-binding proteins[ edit ] O-linkage 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 commmon 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. There has been research on the development of protein serum-based cancer diagnostics.
Also, proper glycosylation of membrane proteins by stabilizing potassium channels prevents degradation of human tissue and muscular dystrophy. Lectins[ edit ] Specific carbohydrate-binding proteins in plants and animals are lectins, which are the partners that bind carbohydrate structures and facilitate cell-cell interaction.
These multiple weak interactions sum together to form a strong linkage. Linkage of lectin is like Velcro, weak interaction but strong composite! Lectins are found in animals, plants and microorganism. Lectins also play an important role in cell recognition since modified carbohydrates and oligosaccharides have the ability to interfere with carbohydrate—protein interactions and therefore, inhibit the cell—cell recognition and adhesion processes.
Lectins are exquisitely specific: Carbohydrates on the surface of one cell bind to the binding sites of lectins on the surface of another cell. If two substituents are near and pointing within the same direction, then there will be a steric hindrance.
In contrast, the equatorial position will have its substituents to form bonds that are parallel to the plane of the ring. This formation produces less crowding and is the most preferred form for the chair conformation. Lastly, the boat form is unlikely, due to the fact that there is crowding and steric hindrance.
Haworth Projection[ edit ] A Haworth projection is a simple way to show cyclic sugars and their glycosidic linkages. It consists of the ring on a horizontal plane but ignores the chair and boat forms so that the ring is flat. If drawn from a Fischer projection with the carbonyl on top, the groups on the right side become the groups on the bottom of the ring and the groups on the left become the groups on the top. The carbon at the very bottom of the Fischer projection is placed on top of the ring by default if it is not a part of the ring itself.
If drawing from a chair form, all groups on top of the chair become the groups on top of the Haworth projection ring. The easiest way to find all these groups is to choose an axial position that is above the ring and then go around the ring, alternating axial and equatorial positions. In the picture on the right, all the groups attached to red bonds would go on top of the ring the Haworth projection and all the groups attached to the blue bonds would go below.
Simple Ketoses[ edit ] A ketose is a sugar that has a ketone group in each of its molecule. Dihydroxyacetone, for example, has 3 carbon atoms in its backbone - it is the simplest ketose among this category. It is also the only optically inactive ketose.
In comparison to other aldoses, ketones will have one less chiral carbon than aldoses even though they share the same number of carbon atoms. Thus, when forming a ring, the ketone at the second carbon will be utilized to form a ring. Similar to aldoses, furanose rings can take up a different conformation than a ring. The other conformation is called the envelope form: Both forms resembles an envelope. Ketotriose List of common ketoses[ edit ] Triose: A triose contain 3 carbon, and ketotriose contains a ketone functional group.
A ketotriose has no chiral center and one stereoisomers.Fischer to Haworth projection HD 720p
An example of ketotriose is Dihydroxyacetone. Dihydroxyacetone has many uses, and it is non-toxic. Many creams had Dihydroyactenone as an active ingredient.
Dihydroxyacteone is also known as DHA. It is also use for suntanning. Chemical and Engineering News Ketotetrose Tetrose: Erythrulose A Tetrose is a monosaccharide that contains 4 carbon atoms. A Keto-tetrose is a tetrose that has a ketone functional group attached to Carbon 2 of the straight chain.
A ketotetrose has 2 stereoisomers because it has one chiral center. An example of a ketotetrose is Erythrulose. Erythrulose has the chemical formula of C4H8O4. It is often used in self-tanning products.
Examples of Ketopentoses Pentoses: Ribulose, Xylulose A Pentose is a general term to define a monosaccharide containing five carbons. When there is the prefix "keto" in front of the pentose, it means that in five carbon containing sugar, there is a ketone functional group attached to the structure. A ketopentose has a total of four stereoisomers. An example of a ketopentose is Ribulose.
The structure of Ribulose has a ketone functional group attached to C-2 of the straight chain figure. The diastereomer of D-Ribulose is D-Xylulose.
- Vitamins and Cofactors, cont.
- File:D-Ribose Haworth.svg
- Structural Biochemistry/Carbohydrates/Monosaccharides
A Hexoses contains 6 carbons. A hexoses containing a ketone functional group is called ketohexose. Ketohexose has 3 chiral centers and 8 different stereoisomers. Examples of ketohexose are Fructose, Psicose, Sorbose, Tagatose. Fructose can react with hydroxyl group to form a hemiketal group, and it can formed pyranose or furanose depending on whether the C-2 keto group reacts with the C-6 or C-5 hydroxyl group.
D-Fructose is the most common ketohexose. Ketoses in Reactions[ edit ] Transketolase Reaction[ edit ] The Transketolase reaction is very similar to the Transaldolase reaction. However, the Transketolase is different because it transfers a two carbon unit instead of Transaldolase's three carbon unit. Thiamine pyrophospate TPP ionizes so that it has a carbanion which is a negatively charged carbon. The importance of carbanion is that they can attack carbonyls, so that carbons are added in a sense to the nucleophile.
Structural Biochemistry/Carbohydrates/Monosaccharides - Wikibooks, open books for an open world
TPP attacks a ketose substrate where it than releases the aldose product to yield an activated glycoaldehyde unit. An activated glycoaldehyde unit is an electron sink because of a positively charged nitrogen atom where a carbonyl of an aldose product is attacked and then separated after some electron movement.
The importance of the transketolase reaction is that it is the mechanism that the enzyme TPP uses to change a ketose substrate to a ketose product that has a different group attached to it. Transaldolase Reaction[ edit ] The transaldolase reaction involves the transfer or a three carbon dihydroxyacetone unit from a ketose donor to an aldose acceptor.