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BIOC12Fall2012 Lecture Week 8 Notes

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University of Toronto Scarborough
Biological Sciences
Rongmin Zhao

1 BIOC12Fall2012 Lecture 8: Carbohydrates and Glycoproteins (Chapter 7) Carbohydrates o Called sugars, saccharides o One of the 4 major molecules in life o (CH2O) whnre n is greater or equal to 3 o highly hydrophilic  hydroxyl group o just because they are highly hydrophilic it does not mean that they are water soluble o many carbohydrates are osmotic pressure regulators but not necessary soluble o monossaccharides, polysaccharides (longer than oligosaccharides) o oligosaccharides = 2-20 repeating units Functions o mainly energy source, protection as structural materials, intercellular communication, stabilization of protein structures (example: there is a sugar attached to antibodies which stabilizes their structures) Monosaccharides o derivatives or aldehyde or ketones  aldoses, ketoses o straight chain and contains polyhydroxy alcohols (at least 3 carbon atoms) Aldoses o aldoses contain 3- 6 carbon atoms  example Glucose, mannose o Ketoses o D, L configurations follow Fischer convention o D sugars have the same absolute configuration at the asymmetric center farthest from their carbonyl group as D-glyceraldehyde o It is mostly D enantiomers that are synthesized in living cells o Sugar molecules that differ in configuration at only one of several chiral centers are called epimers o Example: D-fructose and D-psicose, D-fructose and D-tagatose, D-galactose and D-glucose o Cyclic monosaccharide structures and anomeric forms o glucose (an aldose) can cyclize to form a cyclic hemiacetal called pyranose o fructose (a ketose) can cyclize to form a cyclic hemiaketa called furanose 2 o the alcohol group and the carbonyl carbon interact o o cyclic sugars are represented as Haworth projections o heavier bonds project in front of the plane of the paper and the lighter ring bonds projects behind it o o there are 2 anomeric forms of cyclic sugars o planar carbonyl carbon becomes tetrahedral carbon o in aquous solution, there is only minute amounts of linear form o o O can attack the carbonyl carbon from above or below o o hydroxyl down = alpha, hydroxyl up = beta o D-glucose can cyclize in 2 ways froming either furanose of pyranose 3 o o if the hydroxyl on the 5 carbon reacts with the carbonyl carbon then a 6 member ring is formed  pyranose o if the hydroxyl on the 4 carbon reacts with the carbonyl carbon then a 5 membered ring is formed  furanose o in most monosaccharides there are 2 or more hydroxyl groups that can react with an aldehyde or ketone at the other end of the molecule to form a hemiacetal or a hemiketal o a 4membered ring or lower cannot be formed because there is too much steric hinderance, and they will not be structurally stable o D-ribose (5 carbons) readily forms a 5 or 6 memebered ring  both would show up in a water solution, but there will most likely be more of one than the other Monosaccharides are not in planar conformation o All non hydrogen groups are in equatiorial position o Equatorial position is more stable o Only B-D-glucopyranose can adopt equatiorial position (left) o all non hydrogen groups are in a more croded axial position on the right Monosaccharides can be converted to several derivative forms o a variety of chemical and enzymatic reactions produce derivatives of the simple sugars o some of the most common are: sugar acids, sugar alcohols, deoxy sugars, sugar esters, amino sugars, acetals, ketals and glycosides Oxidation of sugars o oxidation of aldose = aldonic acid o glucose to gluconic acid o oxidation of the primary alcohol group of aldoses yields uronic acids  can adopt linear, pyranose or furanose form o if both aldehyde and primary alcohol are oxidized it becomes glucaric acid 4 o Modifications of Sugars o aldose and ketose can be reduced under mild conditions yielding alditols  in cells used as anti- freezing (protects cells) o o the OH group of a sugar can be replaced by H yielding deoxy sugars o o one or more OH groups can be replaced by amino groups and the amino group is often acetylated o o sugars can also be phosphorylated  phosphate is added at the end of the name 5 o o there are also more complex modifications o o N-acetylneuraminic acid is an important component of glycoproteins o Inside cells, most monosaccharides are modified The anomeric group of sugar can condense with an alcohol to form alpha or beta-glycoside o o this reaction is catalyzed by acids o acid will bind the alcohol to form water o all of the hydroxyl groups can be used to form glycosidic bonds o N-glycosidic bonds are bonds formed between the anomeric carbon and an amine  it is part of RNA structured because it contains a deoxysugar (deoxyribose) o 6 Polysaccharides o composed of monosaccharides linked together by glycosidic bonds o disaccharides contain 2 monosaccharides o lactose: O-b-D-galactopyranosyl-(14)-D-glucopyranose o O: linked by O atom o 1 4 indicates the positions linked: C1 to C4 o sucrose : O-a-D-glucopyranosyl-(12)-b-D-fructofuranoside o 1 is the anomeric carbon o the reducing end represents the original aldehyde group o the reducing end (anomeric carbon) does not form glycosidic bond) o Cellulose o composed of glucose residues o liner polymer of up to 15000 D-glucose residues o form B- (14) glucosidic bonds o the primary cell wall component of plant cells o high degree of interchain and intrachain hydrogen bond formation o vertebrate do not have enzyme to break the glucosidic bond o therefore they cannot digest cellulose but for some it can be digested by the enzymes secreted by microorganisms symbiotically existing in the intestine o Chitin o principal structural component of exoskeletons of invertebrates o composed of N-acetylglucosamine B(14) glucosidic bond o similar to cellulose except that the C2 OH is preplaced by acetamido group 7 o Starch o main storage polysaccharide in plants o synthesized and stored in chloroplasts o mixture of a-amylose and amylopectin o a-amylose composed of glucose residue a(14) glucosidic bond o linear form, easy to digest by vertebrate o forming irregular aggregating helically coiled structure o o amylopectin  a(14) linked glucose reside but branched with a(16) glucosidic bond eveyry 24-30 residues o Glycogen o storage polysaccharide in animal o similar structure to starch amylopectin o even higher degree of branching  every 6-10 residues o glycogen phosphorylase breaks down glycogen from the non reducing end o glycogen debranching enzymes are used to break down the glycogen branches o provides energy, especially for skeleton muscles o 8 Glycoproteins o glycoproteins are a type of glycoconjugates o glycoconjugates: the carbohydrates linked to protein or peptide covalently or non covelantly o proteoglycans  complexes of proteins with a glycosaminoglycan, predominantly found in extracellular matrix (connective tissue) of multicellular animals o peptidoglycan  major component of bacterial cell wall made of glycan linked by peptide o glycosylated membranes and secreted proteins Peptidoglycans o bacterial cell wall o gram positive bacteria has thick cell wall  peptidoglycan while gram negative bacteria has a thin cell wall which is easily broken (but have double membrane) o gram positive cell wall can be stained with crystal violet and
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