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Bio 98 - Lecture 11 Carbohydrates a.k.a. Sugars… I. Definition of a carbohydrate General formula: (CH20)n, "hydrated carbon" Example: C6H1206 is glucose Many carbohydrates have more complex formulas & contain amino, phosphate, sulfate & other groups II. Functions 1. Fundamental source of metabolic energy for most life forms. 2. Components of many important biomolecules. 1. Carbohydrates as an energy source sunlight ^—N plants M m—^Z photosynthesis 2 ' carbohydrate + 02 AA 2' CO, + H>0 animals, plants respiration ATP ADP 2. Biological molecules Penicillin Cellulose - wood, plant fiber, etc. Chitin exoskeleton of arthropods Cell walls of bacteria & yeast Glycoproteins, glycolipids - cell membranes DNA, RNA deoxyribose and ribose Carbohydrates are often polymers Monosaccharides: glucose, ribose, fructose, etc. Oligosaccharides: di-, tri-, tetra- etc. Sucrose is a disaccharide: glucose + fructose Polysaccharides: can be linear or branched (i.e. starch) Monosaccharide nomenclature 1. Carbon number: triose, .., pentose, hexose, .., octose 2. Aldoses and ketoses ■VO 0 H C# 1 2 3 4 5 6 D-glucose HC=0 <=^ HC-OH HO-CH HC-OH HC-OH H2C-OH D-fructose H2C-OH i C=0 HO-CH HC-OH HC-OH H2C-OH C an aldohexose a ketohexose Asymmetric (chiral) carbons allow many distinct monosaccharides; compare mannose, glucose, and galactose, which are all aldohexoses. ^HO ^HO ^HO HO—C—H 3I HO—C—H 4I H —C—OH H—C—OH 3I HO—C—H 4I H—C—OH H —C—OH 3I HO—C—H 4I HO—C—H si si si H —C—OH 6 CH2OH D-Mannose (epimer at C-2) H—C—OH 6 CH2OH D-Glucose H—C—OH 6 CH2OH D-Galactose (epimer at C-4) How many chiral carbons are in glucose? O-chem terms relevant to monosaccharide structure 1. diastereomers - identical structures except for configuration (chirality) at one or more carbons; e.g., all aldohexoses are diastereomers of each other. 2. epimers - differ in chirality at only one carbon. Glucose and galactose are epimers at carbon 4. 3. enantiomers - mirror images; designated as D- & L-; does not constitute a name change. How many aldohexose names are possible? 2 configuration choices at each of 4 asymmetric carbons; however (1.), half of these represent enantiomers (3.). Number of unique names = 24/ 2 = 8. Ring structures Most pentoses and hexoses spontaneously form ring (cyclized) structures in solution. ^ rlu-U HC-OH i 5-member ring: furanose u ■ 6-member ring: pyranose -------- When forming a ring, a new asymmetric center is created, giving rise to 2 possible anomers. HO-CH "i HC-OH i •• HC-OH i 2 D-glucose (linear form) V° i i l H—C—OH C n H HO—C—H 2 4I H—C—OH D-Glucose O si H—C—OH H Linear form 11 6 CH2OH OH H H n 4/- l\ 0H f H / HO fl Ha worth projection a and (3 anomers OH a-D-glucopyranose hemiacetal a-D-Glucopyranose /J-D-Glucopyranose In solution rapid mutarotation occurs (3-D-glucopyranose Linear D-glucose (1%) ^ (~66%) a-D-glucopyranose (~33%) Disaccharides 1. Mainly found in plants 2. Three common disaccharides • sucrose - sugar cane, sugar beets • lactose - milk sugar • maltose - malted (germinating) barley, wheat 3. 2 monosaccharides joined covalently by an O-glycosidic bond Formation of hemiacetal and acetals Glycosidic bond 1 HO —R3 OH 0 # R —C 2 +HO —R \ Aldehyde w 1 2 ^^ R—C—OR I Alcohol H Hemiacetal ^________ , N . ----^ ^ HO —R 3 OR3 1 R —C —OR I H Acetal + H20 Common disaccharides are produced by enzyme-catalyzed condensation/dehydration reactions OH H OH a-D-Glucose H OH /3-D-Glucose hydrolysis condensation H->0^ ^HoO 6 CH2OH acetal \J 2 OH \ hemiacetal OH^ H H Can be either a or (3 due to muta rotation Maltose a-D-glucopyranosyl-(1^4)-D-glucopyranose Notice: there is no hemiacetal Polysaccharides - aka Glycans 1. homopolysaccharides vs heteropolysaccharides 2. can be branched or unbranched 3. used by animals and plants as a compact storage form of CHOs 4. common examples • starch - plants, roots and seeds • glycogen - liver of mammals • cellulose - plant fiber, wood Starch = amylose + amylopectin found in corn, rice, potato, wheat and barley a linear polysaccharide of glucose units Amylopectin - a branched form of amylose Structure of starch glycogen (animal starch) is like starch, but more highly branched High-fructose corn syrup High-fructose corn syrup is produced by milling corn to produce corn starch, then processing that starch to yield corn syrup, which is almost entirely glucose, and then adding enzymes that change most of the glucose into fructose. 1. Corn starch is treated with alpha-amylase to produce shorter chains of sugars called oligosaccharides. 2. Glucoamylase which is produced by Aspergillus, a fungus, in a fermentation vat — breaks the sugar chains down even further to yield the simple sugar glucose. 3. The enzyme xylose isomerase (aka glucose isomerase) then converts some of the glucose to a mixture of about 42% fructose and 50–52% glucose with some other sugars mixed in. http://en.wikipedia.org/wiki/High-fructose_corn_syrup Protein glycosylation: A post-translational modification Protein chain • Sugars covalently attached to the polypeptide as oligosaccharide chains containing CH2OH 4 to 15 sugars • Sugars frequently comprise 50% or more of the total molecular weight of a H NHCOCH, H glycoprotein • Most glycosylated proteins are either secreted or remain membrane-bound • Glycosylation is the most abundant form of post-translational modification • Glycosylation confers resistance to protease digestion by steric protection • Important in cell-cell recognition Asparagine CH2OH H NHCOCH Protein chain Blood group antigens on erythrocyte surface • The O substance is a tetrasaccharide which is missing the 5th residue and does not elicit an antibody response (non-antigenic). • The A antigen and B antigen are pentasaccharides which differ in composition of the 5th sugar residue