CH 7: Carbohydrates

First Biochemistry Chapter Biochemistry – study of substances found in living organisms and their interactions with each other Most important branch of science in medicine Substances can be bio-inorganic or bio-organic Bio-inorganic = water, ions, and ionic compounds Bio-organic: Carbohydrates: C, H, O Lipids: C, H, O Proteins: C, H, O, N, S Nucleic Acids: : C, H, O, N, P

Carbohydrates Carbohydrates are the most abundant bio-orainc substance on the planet Made by photosynthetic (PHS) organisms PHS reaction – see board Plants use the carbs made for 2 (really 3) purposes: 1.Source of energy – glucose (not a separate purpose in text) 2.Make starch – storage form of glucose 3.To make cellulose – structural polysaccharide found in cell walls

Functions of Carbohydrates in Animals 1.Source of energy - glucose 2.Storage form of energy – glycogen Made and stored in liver and muscles 3.Use carbons to make other bio-organic substances 4.5 Carbon sugars are key component of DNA and RNA Deoxyribose – DNA Ribose – RNA 5.Short, often branched carbohydrates attach to proteins and lipids on the outside of the plasma membrane Called glycoproteins and glycolipids

Classifying Carbohydrates Carbohydrate = polyhydroxyl aldehyde or ketone Aldehyde carb = aldose sugar* Ketone carb = ketose sugar* * monosaccharides and disaccharides are also referred to as sugars

Classes of Carbohydrates 1.Monosaccharides – single polyhyroxyl ald/ketone unit 2.Oligosaccharides – 2-~10 monos. units joined by covalent bonds, may be branched Disaccharides – 2 monos. units joined by covalent bond Sucrose, lactose, maltose, cellobiose 3.Polysaccharides – MANY monos. units joined by covalent bonds Glycogen Starch – amylose and amylopectin Cellulose

Chiral Compounds Many carbohydrates are chiral compounds Chiral objects have mirror images that are cannot be superimposed (like your hands). Chiral compounds have the same number of atoms arranged differently in space. A chiral carbon atom has four different groups attached Called a chiral center Page 238 7

Determine if there is a chiral carbon in each compound. 8

? Chiral Carbons ?

D and L Notation - Cabohydrates D,L tells which of the two chiral isomers of a carbohydrate we are referring to. If the –OH group on the next to the bottom carbon atom points to the right, the isomer is a D-isomer; if it points left, the isomer is L. The D form is usually the isomer found in nature. 10

D or L ? 11

Fischer Projections Simplified way to show chiral centers

Draw Fischer Projection for…..

Terms Chiral center (carbon) Chiral compound – non-superimposable mirror images chiral center present Achiral compound - superimposable mirror images no chiral centers

New Types of Stereoisomers Enantiomers – stereoisomers with non-superimposable mirror images Diastereomers – stereoisomers that are not mirror images Cis-trans isomers fit here as well as the carb isomers we’re working on Epimers – diastereomers that differ at just one chiral center

Examples…… See board for examples…then from the book, COVER answers 7.2 – example or practice exercise 7.3 – “ “ 7.4 – both example and practice exercise Page 246 may be useful

Properties of Enantiomers Constitutional isomers differ in most chemical and physical properties Enantiomers (a stereoisomer) have the same: Boiling points, melting points Density Solubility and reactivity with achiral compounds Enantiomers differ in: Rotation of polarized light Solubility in chiral solvents Reactivity with chiral reactants

Chiral Compounds and Polarized Light

Enantiomers and Polarized Light Enantiomers are optically active – rotate polarized light May rotate in clockwise or counter-clockwise direction Clockwise rotation = Dextrorotary (+) Counter-clockwise rotation = Levorotary (-) D & L enantiomers rotate light in opposite directions Which direction they rotate is unrelated to the D/L designation and must be determined experimentally The degree of rotation depends on the concentration of the solution and the identity of the substance.

Monosacchrides - describing Aldoses sugars are monosacchrides with an aldehyde group and many hydroxyl (-OH) groups. Ketoses sugars are monosacchrides with a ketone group and many hydroxyl (-OH) groups. 20

Monosacchrides - describing Three Carbons = Triose Four Carbons = Tetrose Five Carbons = Pentose Six Carbons = Hexose Page

Identify each as tetrose, pentose or hexose, and as aldose or ketose, D or L? 22

Hexose Monosaccharides CHO OHH HHO HHO OHH CH 2 OH D-Galactose (aldohexose)

D-Glucose Also called blood sugar or dextrose* mg/100 mL of blood All other nutritionally important sugars are converted to glucose in the liver. Forms a 6 membered cyclic hemiacetal * Name related to the clockwise rotation of light

D-Galactose D-galactose is an epimer of D-glucose Rarely found as free monos. in nature Find in Lactaid milk Not very sweet Find galactose as a component of glycoproteins on brain and other nerve cells, also an important carbohydrate in blood typing

D-Fructose Also known as fruit sugar and levulose Rotates polarized light counter-clockwise Sweetest of all monosacharides High fructose corn syrup Glucose and fructose differ in just the location of the carbonyl group

D-Ribose & D-Deoxyribose Pentose Sugars – not nutritionally important Components of RNA and DNA in hemiacetal (cyclic) forms

Hemiacetal Formation – Cyclic Carbohydrates Monosaccharides form cyclic hemiacetals Formation of the hemiacetal Alpha versus beta form (anomers) Showing structure – Haworth projections Chiral OH on right goes down on structure Chiral OH on left goes up on structure

Copyright © Cengage Learning. All rights reserved 30 2 forms of cyclic D-glucose: Alpha-form: -OH of C1 and CH 2 OH of C5 are on opposite sides Beta-form: -OH of C1 and CH 2 OH of C5 are on same sides Cyclic Hemiacetal Forms of D-Glucose

Glucose in solution Less than.01% 37% glucose 63% glucose

D-Ribose

D-Fructose Forms a 5 membered cyclic form

Skip 7.12 (for now!)

Disaccharides Monosaccharides in their cyclic form react to form acetals……aka…disacharrides or a glycoside Bond is called a glycosidic linkage Disaccharides we will consider: 1.Maltose 2.Cellobiose 3.Lactose 4.Sucrose Goal is to: Know constituent monosaccharides Recognize type of bond Know how body handles – digestible or not? Identify sugar as a reducing sugar or not Page 262

Formation of Maltose

Copyright © Cengage Learning. All rights reserved 37 Maltose  (1-4)  D –glucose -- D-glucose

Copyright © Cengage Learning. All rights reserved 38 Cellobiose  D –glucose -- D-glucose

Copyright © Cengage Learning. All rights reserved 39 Lactose  D –galactose -- D-glucose

Sucrose –  (1->2) linkage  (1->2) linkage

Polysaccharides - Consider 1.Sugars present in polymer Homopolysaccharide or heteropolysaccharide 2.Length of chain 3.Extent of branching 4.Type of linkage between monomers and at branch points

Polysaccharides 1.Starch Amylose Amylopectin 2.Glycogen 3.Cellulose 4.Chitin (not covered)

Copyright © Cengage Learning. All rights reserved 43 Starch: Polymer of glucose Storage form of glucose in plants Two forms Amylose Amylopectin Starch

Amylose % of starch Straight chain polymer coils Up to ~1000 glucose units  (1  4) glycosidic bonds

Amylopectin % of the starch Branched chain polymer of glucose  (1  4) glycosidic bond for straight chain and  (1  6) at the branch Branch every ~25-30 glucose up to 100,000 glucose units Humans can hydrolyze alpha linkages in starch

Glycogen Polymer of glucose, up to 1,000,000 glucose units Excess glucose in blood stored in the form of glycogen – animal storage form of glucose Bonding? Branch every ~8-12 glucose ~2-3x more branched than amylopectin

Cellulose Structural polysaccharide – made by plants, bacteria, and others (not humans) Straight chain polymer of glucose, up to ~ 5,000 glucose units  (1  4) glycosidic bonds between glucose Humans don’t have enzymes that hydrolyze  (1  4) - so humans cannot digest cellulose

Cellulose chains crosslink with H bonds to form bundles/fibers – Cotton ~95% cellulose and wood ~50% cellulose

Reactions of Monosaccharides Oxidation Reactions (3) 1.Mild oxidizing agent such as Tollen’s or Benedict’s o All monosaccharides and disaccharides except sucrose react – called reducing sugars o Hemiacetal group opens to reform the aldehyde or ketone group o Ketone group rearranges to form an aldehyde o Reaction oxidizes the aldehyde to a carboxylic acid o No hemiacetal to open in sucrose

Oxidation of Monosaccharides 2.Strong oxidizing agents can oxidize both ends of a monosaccharide the carbonyl group and the terminal primary alcohol group oxidize to produce a dicarboxylic acid. 3.Enzymes can oxidize the primary alcohol end of an aldose such as glucose, without oxidation of the aldehyde group! VERY difficult to do in the lab

Reduction to a Sugar Alcohol The carbonyl group in a monosaccharide (either an aldose or a ketose) is reduced to a hydroxyl group using hydrogen as the reducing agent. The product is the corresponding polyhydroxy alcohol - sugar alcohol. Sorbitol

Reactions of Monosaccharides Glycoside formation – already covered Hemiacetal of one carb. Reacts with an alcohol group on another carb to produce a glycosidic linkage/bond