Structural Biochemistry/Carbohydrates - Wikibooks, open books for an open world
Examples of monosaccharides are glucose, fructose, and glyceraldehyde. the stereochemistry of the last stereogenic center the sugar is the α-anomer The difference being that the lone pair being donated is coming from the substituent at C . carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. Overall the sugar has many stereochemistry such as enantiomers (D and L), The glycosidic bond can be broken by hydrolysis, which is the addition of the water They form a glycosidic linkage between the α glucose and β fructose. His successful negotiation of the stereochemical maze presented by the . and its exchange relationship to D-(+)-glucose was demonstrated by oxidation to a . Maltose, sometimes called malt sugar, comes from the hydrolysis of starch.
The fact that monosaccharides have multiple hydroxyl groups means that various glycosidic linkages are possible. Indeed, the wide array of these linkages in concert with the wide variety of monosaccharides and their many isomeric forms makes complex carbohydrates information-rich molecules. Three abundant disaccharides are sucrose, lactose, and maltose Figure Sucrose common table sugar is obtained commercially from cane or beet.
Sucrose can be cleaved into its component monosaccharides by the enzyme sucrase. Sucrose, lactose, and maltose are common dietary components. Lactose is hydrolyzed to these monosaccharides by lactase in human beings Section Maltose comes from the hydrolysis of starch and is in turn hydrolyzed to glucose by maltase.
Sucrase, lactase, and maltase are located on the outer surfaces of epithelial cells lining the small intestine Figure Lactase and other enzymes that hydrolyze carbohydrates are present on microvilli that project from the outer face of the plasma membrane of intestinal epithelial cells.
Glycogen and Starch Are Mobilizable Stores of Glucose Large polymeric oligosaccharides, formed by the linkage of multiple monosaccharides, are called polysaccharides. Polysaccharides play vital roles in energy storage and in maintaining the structural integrity of an organism.
If all of the monosaccharides are the same, these polymers are called homopolymers. The most common homopolymer in animal cells is glycogen, the storage form of glucose. As will be considered in detail in Chapter 21, glycogen is a very large, branched polymer of glucose residues. The nutritional reservoir in plants is starch, of which there are two forms. More than half the carbohydrate ingested by human beings is starch. Cellulose, the Major Structural Polymer of Plants, Consists of Linear Chains of Glucose Units Cellulose, the other major polysaccharide of glucose found in plants, serves a structural rather than a nutritional role.
Cellulose is one of the most abundant organic compounds in the biosphere.
Some kg of cellulose is synthesized and degraded on Earth each year. Fibrils are formed by parallel chains that interact with one another through hydrogen bonds.
Complex Carbohydrates Are Formed by Linkage of Monosaccharides - Biochemistry - NCBI Bookshelf
A hollow helix is formed instead of a straight chain Figure Pyranoses typically adopt a chair conformation, similar to cyclohexane. Mutarotation is the change in the optical rotation that occurs by epimerization that is the change in the equilibrium between two epimerswhen the corresponding stereocenters interconvert. Mutarotation was discovered by Augustin-Pierre Dubrunfaut inwhen he noticed that the specific rotation of aqueous sugar solution changes with time.
The optical rotation of the solution depends on the optical rotation of each anomer and their ratio in the solution. Therefore one can use a polarimeter to measure the rotation of a sample and then calculate the ratio of the two anomers present from the enantiomeric excess, as long as one knows the rotation of each pure anomer.
One can monitor the mutarotation process over time or determine the equilibrium mixture by observing the optical rotation and how it changes. Cyclic isomers[ edit ] A monosaccharide often switches from the acyclic open-chain form to a cyclic form, through a nucleophilic addition reaction between the carbonyl group and one of the hydroxyls of the same molecule. The reaction creates a ring of carbon atoms closed by one bridging oxygen atom.
The resulting molecule has an hemiacetal or hemiketal group, depending on whether the linear form was an aldose or a ketose. The reaction is easily reversed, yielding the original open-chain form. In these cyclic forms, the ring usually has 5 or 6 atoms.
These forms are called furanoses and pyranoses, respectively — by analogy with furan and pyran, the simplest compounds with the same carbon-oxygen ring although they lack the double bonds of these two molecules.
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.
Principles of Biochemistry/The Carbohydrates: Monosaccharides, Disaccharides and Polysaccharides
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.