Waseem Abu Obeida. Salsabeel Fleifal. Mamoon Ahram

Transcription

1 8 Waseem Abu Obeida Salsabeel Fleifal Mamoon Ahram

2 Anomers Anomers cyclic monosaccharides or glycosides that are epimers, they differ from each other in the configuration of C-1 if they are aldoses or in the configuration at C-2 if they are ketoses(depending on the direction of OH attached to the anomeric carbon). The epimeric carbon in anomers is known as anomeric carbon. How to locate the anomeric carbon? - Locate the oxygen inside the ring,the anomeric carbon is next to it,the one that is attached to OH group. Example: α-glucose and β-glucose are anomers because they differ at C-1. The different orientation of the resulting OH group has different names: Alpha(α):OH group below ring/on the opposite side to the the CH2OH group Beta(β):OH group above ring/on the same side as the CH2OH. α and β forms interconvert by a process called mutarotation. Glycosidic bonds a glycosidic bond is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate. a glycosidic bond should contain an anomeric carbon.there are many types of the glycosidic bond depending on the group attached to the anomeric carbon,there are two types - N-glycosidic bond - O-glycosidic bond.

3 Glycosidic bonds between monosaccharide units are the basis for the formation of disaccharides,oligosaccharides and polysaccharides Different forms of glycosidic bonds: - A: alpha(1-4) glycosidic bond - B: alpha(1-6) glycosidic linkage - C: β β(1-1) glycosidic bond of a β-d-glucose disaccharide is non-reducing sugar because both anomeric carbons are occupied (refer to slide 3 page 47). Note: every glycosidic bond should contain at least one anomeric carbon.

4 Disaccharides -Disaccharides are sugars formed by the linkage of two monosaccharides by a glycosidic bond. -Disaccharides could be hetero-disaccharides(formed by two different monosaccharides) or homo-disaccharides (the two monosaccharides are the same). -Disaccharides are very stable, they are formed enzymatically by glycosyltransferase enzymes,and also they are hard to break so they need special enzymes to break-up. -after the monosaccharides are joined by glycosidic bonds,these monosaccharides cannot go back to their chain form,and they cannot interconvert by mutarotation. -monosaccharides are called residues when found in larger molecules (disaccharides, oligosaccharides and polysaccharides). -there are many exmples of disaccharides,most common: Maltose(glucose+glucose), Sucrose (fructose+glucose), Lactose(glucose+galactose). - How to differentiate between different type of disaccharides? the type of residue forming them, the stereocongifration of the sugars if D or L structure.(all the sugars in our body is D- sugars,our body cant metabolize the L-sugars). the order of the residues ( from left to right),we consider the number of the two carbons in the glycosidic bond and determine if the the sugars joined by the bond are alpha or beta. Maltose Formed by two alpha glucose molecules joined by α(1-4) glycosidic bond. (we named the bond alpha according to the anomeric carbon in the first molecule) it is a reducing sugar. (free anomeric carbon on the second glucose). maltose systematic name: α-d-glucopyranosyl-(1-4)-α-d-glucopyranose,you need to understand what this name indicates: gluco for glucose. pyranose for six membered ring. (1-4) for the glycosidic bond. The anomeric form of the sugar is alpha-alpha. Lactose

5 Formed by glucose (α or β) and β galactose. systematic name: β-d-galactopyranosyl-(1 4)-D-glucose. it is a reducing sugar.(free anomeric carbon on Glucose). Sucrose Formed by α-d- glucose and β-d fructose,by α (1-2) glycosidic bond. systematic name: α-d-glucopyranosyl-(1 2)-β-D-fructofuranose. it is a non-reducing sugar because both anomeric carbons are involved in the glycosidic bond. - Compare Alpha and Beta Fructose: The Beta position is defined as the -OH being on the same side of the ring as the C # 6. In the ring structure this results in a upwards projection for the -OH on carbon # 2. The Alpha position is defined as the -OH being on the opposite side of the ring as the C # 6. In the ring structure this results in a downward projection for the -OH on carbon # 2.

6 Examples of artificial sweetners: (modified sugars) 1-Aspartame: an artificial, non-saccharide (dipeptide) sweetener used as a sugar substitute in some foods and beverages. 2-Splenda: an artificial sweetener made by modifying sucrose to sucralose. (it has chlorides instead in hydroxyl groups which are above the ring) Clinical cases: -Lactose Intolerance When some people drink milk, they suffer from digestive problems (bloating and diarrhea). This is due to a deficiency of the enzyme lactase in the intestinal villi allows lactase of intestinal bacteria to digest it producing hydrogen gas, carbon dioxide, and organic acids and leading to digestive problems. -Galactosemia Missing the enzyme that metabolizes galactose can result in galactosemia where nonmetabolized galactose accumulates within cells and is converted to the hydroxy sugar galactitol, which cannot escape cells. Water is drawn into cells by osmosis and the swelling causes cell damage, particularly in the brain, resulting in severe and irreversible retardation. It also causes cataract (a medical condition in which the lens of the eye becomes progressively opaque, resulting in blurred vision). Oligosaccharides Raffinose

7 -Raffinose is a hetero-oligosaccharide consists of 3 different monosaccharides (glucose+galactose+fructose) - It is found in beans and vegetables like cabbage, brussel, sprouts, broccoli, asparagus. - Humans cant digest raffinose very well because they lack the enzyme α-galactosidase in their digestive tract,this enzyme hydrolysis Raffinose to D-galactose and sucrose, but intestinal bacteria can ferment it into hydrogen, methane, and other gases. Oligosaccharides as drugs -Streptomycin and erythromycin (antibiotics) -Doxorubicin (cancer chemotherapy) -Digoxin (cardiovascular disease) Polysaccharides long chains of monosaccharides (residues) linked by glycosidic bonds. could be hetero-polysaccharides (different residues) or homo-polysaccharides (same type of residues) as in starch and glycogen. diversity of polysaccharides is due to Monosaccharides types, chain Length, linkage type and branching patterns. there are two types of polysaccharides according to function: 1-Storage for energy usage (glycogen, starch, dextran). 2-Structural (cellulose, pectin, chitin). Glycogen -it is a homo-polysaccharide made of glucose residues -glycogen is stored in the human body as energy source,90% of glycogen is in the liver and 10% in the muscles. -highly branched -Glycogen is short-term energy storage(about one day to run out from the body, proteins and lipids will be used instead).

8 -the glycosidic bonds in the chains are alpha (1-4) and at branching points, they are alpha (1-6). Starch -it is a homo-polysaccharide made of glucose residues. - starch is stored in plants as energy source. -there are two types of starch : amylose(10-20)% amylopectine(80-90)% The difference is that amylose is not branched and amylpoectine is branched and more abundant. -The glycosidic bond in amylose is alpha (1-4) as in glycogen. - The glycosidic bond in amylopectine is alpha(1-4) along the chain and is alpha(1-6) at the branching as in glycogen. Glycogen vs amylopectin -Both are made from the same monomer and both are branched. -Glycogen exists in animals and amylopectin in plants. -Glycogen is more highly branched. -Branch points occur about every 10 residues in glycogen and about every 25 residues in amylopectin.

9 Why is branching important? - It makes it more water-soluble and does not crystallize. - Glycogen is more branched than amylopectine because there is more water in plants than our cells. - Easy access to glucose residues. Glycogen(right) is more accessible than starch(left) because there are more free ends. (the red points in the image). Dextran -A storage homo-polysaccharide (glucose residues). -found in Yeast and bacteria. -differ from glycogen and starch in linkage and branching for dextran) branching pattern.(more variable -glycosidic bond in the main chain is alpha(1-6), at branching points it could be alpha(1-2), alpha(1-3) or alpha (1-4).