Structure and Functions of Carbohydrates
Carbohydrates are a third major group of biomolecules. Glypiation - linkage of a phosphatidyl inositol to link proteins to lipids via glycan. The composition of carbohydrates and their function is explained. well as the DNA (deoxyribonucleic acid) for the genetic structure of an individual as they are . To recognize how carbohydrates function in living cells, we must understand sucrose structural model, showing glycosidic linkages between glucose and fructose glycosidic linkage, rather than the presence of different monosaccharides.
The second attachment sites allow shorter glucose chains to branch away from the main chain, packing more glucose units into the compact coiled structure. Animals initiate enzyme-driven hydrolysis reactions to break down glycogen when energy is needed.
For quick access to energy, glycogen is stored primarily in two locations in humans, the liver for easy delivery into the bloodstream and muscles for direct use as needed. Plants synthesize two types of polysaccharides, starch and cellulose. The glycosidic bonds between glucose units in plant starch are similar to those in animal glycogen.
Accordingly, starch molecules are structurally similar, forming compact coils, and play a similar role in energy storage for plants. Unlike glycogen, starch molecules vary widely in the level of branching. Most plants form a mixture of starch polymers with little to no branching and polymers with extensive branching.
Structural Biochemistry/Carbohydrates - Wikibooks, open books for an open world
In addition to providing energy for the plants that synthesize them, starches serve as the main food source for many animals. Humans and other animals produce enzymes that degrade starch molecules into small fragments during digestion.
In humans, this digestion begins in the mouth by an enzyme called amylase, which degrades starch polymers into disaccharides maltose.
To experience starch digestion yourself, try chewing an unsalted cracker for a long time. After a while, did the cracker begin to taste sweet?
Structure-function relations of carbohydrates by neoglycolipid arrays.
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.
Importance of Carbohydrates in Cell Communication Carbohydrates have found to be capable of containing an abundance of information as a result of the various composition and structures that are possible. These diverse compounds result from the fact that monosaccharaides contain many OH groups available for linkage. This further allows for extensive branching. The presence of these various carbohydrates on cell surfaces allows for effective cell-to-cell communication.
The calcium ion helps bind the protein and carbohydrate by interacting with the OH groups found on the carbohydrate. Calcium can also form a linkage between the carbohydrate and glutamates in the lectin. Binding is further strengthened through hydrogen bonds that form between the lectin side chains and the OH groups of the carbohydrate. Carbohydrate recognition and binding is made possible by a homologous domain consisting of amino acids.
These amino acids determine the specificity of carbohydrate binding.
2.7: Structure and Function: Carbohydrates
Examples of Lectins Embryos are attached to the endometrium of the uterus through L-Selectin. This activates a signal to allow for implantation. Malectin[ edit ] Malectin is a carbohydrate-binding protein present in the endoplasmic reticulum of a cell. It plays important roles in the early stages of N-Glycosylation. Selectin[ edit ] Selectins are transmembrane glycoproteins that form a single chain.
They mediate the attachment between leukocytes circulating white blood cells and the surfaces of endothecial cells the cells that line blood vessels. Selections are members of the C-type lectins. The selectins recognize and bind specific oligosaccharides on cell-surface glycoproteins. Reciprocal selectin-oligosaccharide interactions between the two cells types allow the endothelial cells to "capture" circulating leukocytes, which then crawl past the endothelial cells on their way to eliminate the infection or help repair damaged tissues.
Selectins bind specific immune cells to sites of injury and inflammation. There are various types of selectins. For example, the L-type selectin refers to lymph-node vessels, the E-type selectin refers to endothelium, and the P-type is for platelets.Biomolecules (Updated)
Viruses[ edit ] Adhesion to carbohydrates on the cell's surface grants some viruses access to the cell. Viruses can bind to certain residues on glycoproteins and once within the cell, release proteins which will break certain bonds within the glycoprotein to grant the virus free access.
Structure and Function: Carbohydrates - Biology LibreTexts
The influenza virus binds to sialic acid residues which is present on cell surface glycoprotein. The viral protein that's binds to the siliac acid residues is called hemagglutinin. After the virus penetrates the cell membrane, another viral protein, neuraminidase sialidasecleaves the glycosidic bonds to the sialic acid residues, freeing the virus to infect the cell.
Inhibitors of this enzyme such as Tamiflu and Relenza are important anit-influenza agents. Diversity of Carbohydrates[ edit ] Carbohydrates are very diverse due to these reasons: Different monosaccharides can be joined to one another through any of several OH group. The C-1 linkage can have either an alpha or a beta configuration, and extensive branching is possible. Unreleased particles will tend to aggregate and not function.
Some viral glycoproteins from hepatitis C virus may attach to lectins on the surface of liver cells in their infectious cycle. The bacterium Helicobacter pylori uses a cell surface lectin to bind oligosaccharides on epithelial cells lining the stomach. One lectin known as ricin is a very powerful toxin. It is produced in the endosperm of seeds of the castor oil plant and is of concern as a bioterrorism weapon as a result of its acute toxicity when inhaled or ingested.
Lectins were discovered originally in plants and have been most studied in legumes, but lectins are now known to be widely dispersed in nature. In the immune system, a mannan binding lectin MBL helps mediate the first defenses against microorganisms.