Chapter 24: Carbohydrates and Nucleic Acids

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1 Chem 225 otes Page 138 I. Introduction Chapter 24: Carbohydrates and ucleic Acids A. Definitions and aming Conventions Carbohydrate: a sugar. A polyhydroxy aldehyde or ketone with the molecular formula C n 2n n, or any compound that can be hydrolyzed to this formula. The names of sugar usually end in -ose. Aldehyde or ketone: 1) Aldose: a sugar that contains an aldehyde. 2) Ketose: a sugar that contains a ketone. umber of carbons: 1) Triose: a three carbon sugar 2) Tetrose: a four carbon sugar. 3) Pentose: a five carbon sugar. 4) exose: a six carbon sugar. Saccharide: another name for a sugar. 1) Monosaccharide: a compound that contains a single sugar. 2) Disaccharide: a compound that contains two sugars connected by a covalent bond. 3) Polysaccharide: a compound that contains many sugars connected by covalent bonds. Lecture otes Dr. Thomas Mucciaro. All rights reserved.

2 Lecture otes Dr. Thomas Mucciaro. All rights reserved. B. The Family of D-Aldoses C C 2 D-(+)-glyceraldehyde C C C 2 C 2 D-( )-erythrose D-( )-threose C C C C C 2 C 2 C 2 C 2 D-( )-ribose D-( )-arabinose D-(+)-xylose D-( )-lyxose C C C C C C C C C 2 C 2 C 2 C 2 C 2 C 2 C 2 C 2 D-(+)-allose D-(+)-altrose D-(+)-glucose D-(+)-mannose D-( )-gulose D-( )-idose D-(+)-galactose D-(+)-talose Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 139

3 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 140 C. Important Sugars in Biochemistry (MEMRIZE TESE) C C 2 C C C C 2 C 2 C 2 C 2 D-glucose D-fructose D-ribose D-glyceraldehyde II. Monosaccharides A. D/L Configurations and Epimers Sugars are chiral and exist as pairs of non-superimposable mirror images (enantiomers): We need a convenient way of distinguishing between the two enantiomers of a sugar. Use the configuration of the highest numbered stereocenter in the Fischer projection of the sugar (look at atom C-5 in the above structures). Look at the configuration of the highest priority (by R/S rules) group (usually or ) on the highest numbered stereocenter in the Fischer projection: n the right side of the projection = D enantiomer n the left side of the projection = L enantiomer Lecture otes Dr. Thomas Mucciaro. All rights reserved.

4 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 141 Examples: Classify the following as D or L enantiomers: C C C C C 2 C 2 C 2 C 2 ribose threose erythrose glyceraldehyde In a pair of enantiomers, all stereocenters have the opposite configuration (all stereocenters are mirror images). Some closely related sugars differ in having only one stereocenter of opposite configuration. These are epimers: pairs of chiral molecules that differ in the configuration of only one stereocenter (all other stereocenters have the same configuration). B. Cyclic Forms of Sugars Sugars contain aldehyde/ketone carbonyls and alcohol hydroxyl groups, so they can do an intramolecular reaction to make a hemiacetal (review section 20.5, pp ). Cyclic forms of sugars are called furanose or pyranose, depending on if they have a five- or six-membered ring: Lecture otes Dr. Thomas Mucciaro. All rights reserved.

5 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 142 aworth Projection: draw the cyclic form as a flat ring with groups straight up and down. Rules for Converting Fischer projections to aworth projections: 1) Identify the carbon with the that forms the ring. Count the atoms in the ring, and draw the ring skeleton. Put the ring oxygen in back, and carbon 1 on the right. 2) Write all the groups except the tail and C1 onto the ring. Groups on right of Fischer = down on aworth Groups on left of Fischer = up on aworth 3) Direction of tail depends on direction of the that formed the ring: Ring forming on right = tail up Ring forming on left = tail down 4) The hemiacetal carbon can have the up or down. is cis to the tail = stereoisomer is trans to the tail = stereoisomer If the stereochemistry is not specified, a wavy bond can be written. Lecture otes Dr. Thomas Mucciaro. All rights reserved.

6 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 143 The pyranose (6-membered ring) forms of sugars can also be written as chairs: C. Mutarotation and Anomers Most carbohydrates are chiral compounds, so they rotate plane-polarized light. Recall that the direction and magnitude (angle) of rotation (at a given concentration, cell length, and wavelength) is a physical constant called the specific rotation and symbolized as [] D. D-glucose is unusual because it has two different values for [] depending on the conditions under which it is crystallized: Low temperature crystals: mp = 146 o C, [] D = +112 o igh temperature crystals: mp = 150 o C, [] D = +19 o Furthermore, when either type of crystal is dissolved in water, the initial rotation gradually changes until it reaches a constant value of o. This is called mutarotation (muta is a prefix meaning change). There are two forms of D-glucopyranose (the cyclic hemiacetal form of D-glucose). They differ in the configuration of the that is formed at C1: C C 2 C 2 C 2 -D-glucopyranose D-glucose -D-glucopyranose Lecture otes Dr. Thomas Mucciaro. All rights reserved.

7 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 144 These two different forms of glucopyranose are called anomers, and the carbon that changes configuration is called the anomeric carbon. As shown above, the anomeric pyranose forms are in equilibrium with the open form of glucose. This means that each molecule of glucose is rapidly forming a ring and then re-opening. When the alpha () form of glucopyranose opens, the open-chain glucose can either reclose in the alpha form, or it can close in the beta form. Thus, over time a sample of 100% -D-glucopyranose will change to a mixture of the alpha and the beta form. Thus, the two crystal forms isolated are the 100% pure anomers. The alpha anomer has a specific rotation of +112 o. When either form is dissolved in water, they gradually equilibrate to a mixture of 36% alpha and 64% beta, which has a rotation in between the rotations of the two pure anomers: % alpha form % beta form [] D Low Temp Crystal 100% 0% +112 o igh Temp Crystal 0% 100% +19 o Equilibrium mixture 36% 64% o Lecture otes Dr. Thomas Mucciaro. All rights reserved.

8 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 145 D. Glycoside Formation bserved reaction: Mechanism (review acetal formation) Lecture otes Dr. Thomas Mucciaro. All rights reserved.

9 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 146 Glycosides can be hydrolyzed in aqueous acid: Glycosides are named by naming the alkyl group of the R (alcohol), and changing the -ose ending of the carbohydrate name to -oside. III.Polysaccharides (ligomers of Carbohydrates) ligomer: a molecule constructed from two or more smaller molecules. Monomer: a small molecule that can be connected to other monomers to build up an oligomer. A. Sugars can be connected by forming glycosides: Lecture otes Dr. Thomas Mucciaro. All rights reserved.

10 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 147 B. Sucrose (a disaccharide) C 2 -D-glucopyranose C 2 C 2 C 2 C 2 C 2 -D-fructofuranose sucrose C. Starch (a polysaccharide) C C 2 4' 3' 2' 1' Starch is used to store glucose in plants for later use. Why? Lecture otes Dr. Thomas Mucciaro. All rights reserved.

11 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 148 D. Glycogen (a polysaccharide) C C 2 1 C ' C 2 4' 3' 2' 1' Glycogen is used to store glucose in higher organisms (like mammals). Why? E. Cellulose (a polysaccharide) C C 2 4' 3' 2' 1' Used as a structural compound in plants (cell wall, wood, fiber, cotton). Lecture otes Dr. Thomas Mucciaro. All rights reserved.

12 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 149 IV.ucleic Acids: DA and RA A. Introduction ucleic acids: acidic compounds found in the nucleus of cells. ucleic acids carry the genetic information of the cell. Two main types of nucleic acids: Ribonucleic acids (RA): used mainly for transfer of genetic information to build proteins. Also catalyzes chemical reactions in protein synthesis and stores genetic information in some viruses. Deoxyribonucleic acids (DA): used for long-term storage of genetic information. B. ucleosides and ucleotides: Building Blocks of ucleic Acids RA and DA have a chain of sugars connected by phosphates (the sugar-phosphate backbone). Example: RA backbone: 5' end P - C 2 P 3' end C 2 P - C 2 base 1 base 2 - base 3 5' end P - Ribose P - Ribose P - Ribose 3' end base 1 base 2 base 3 Lecture otes Dr. Thomas Mucciaro. All rights reserved.

13 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 150 In RA, the sugar is ribose. In DA, the sugar is 2-deoxyribose (removing the hydroxyl group at carbon 2 makes the DA chain more stable). The sugars have cyclic amine bases (purines and pyrimidines) attached at the anomeric carbon (the hemiacetal carbon of the furanose form of the sugar). 2 2 purine adenine (A) guanine (G) 2 C 3 pyrimidine cytosine (C) uracil (U) thymine (T) Most of the bases are the same, except that RA uses uracil (U) where DA uses thymine (T), which has an extra methyl group. The extra methyl group helps make DA more stable than RA (so it has better long-term stability). otice that some of the bases exist in the keto forms of tautomer. The enol forms would be aromatic according to uckel s rules: Lecture otes Dr. Thomas Mucciaro. All rights reserved.

14 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 151 The sugar and base together is called a nucleoside. When the nucleoside has a phosphate attached to the C-5 hydroxyl, the entire molecule is called a nucleotide. 2 2 C 2 C 2 2 C 2 C 2 adenosine (A) guanosine (G) cytidine (C) uridine (U) - P P - - P P - C 2 C 2 2 C 2 C 2 adenosine monophosphate, AMP (adenylic acid) guanosine monophosphate, GMP (guanidylic acid) cytidine monophosphate, CMP (cytidylic acid) uridine monophosphate, UMP (uridylic acid) 2 2 C 3 C 2 C 2 2 C 2 C 2 deoxyadenosine deoxyguanosine deoxycytidine deoxythymidine Lecture otes Dr. Thomas Mucciaro. All rights reserved.

15 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 152 C. Structure of ucleic Acid Chain The chains of nucleic acids are built of alternating sugar-phosphate units (for example, the backbone of RA): 5' end P - 5' end P - C 2 base 1 C 2 base 1 P - P - 3' end C 2 base 2 3' end C 2 base 2 The sugars are connected as esters of phosphoric acid (phosphate esters). The connection is from the 3 -hydroxyl to the phosphate to the 5 -hydroxyl of another sugar. Lecture otes Dr. Thomas Mucciaro. All rights reserved.

16 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 153 D. Base Pairing in RA and DA The amine bases of nucleic acids and nucleosides can form hydrogen bonds with each other. ribose ribose ribose ribose guanine G:::C cytosine adenine A::T thymine Guanine bonds with cytosine (G-C), while adenine bonds with thymine (A-T) in DA or uracil (A-U) in RA. 3' end ' end ribose P ribose P ribose P A C G T G C P ribose P ribose P ribose 5' end ' end The G-C pair has three hydrogen bonds, while the A-T pair has only two hydrogen bonds. A greater amount of G-C in a strand of DA makes it more stable. Lecture otes Dr. Thomas Mucciaro. All rights reserved.

17 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 154 E. Double elix of DA Bases of one chain of DA will pair up with bases on a complementary chain of DA, forming two intertwined chains. The chains twist into a helix as they pair. Because there are two chains, this is called a double helix. Lecture otes Dr. Thomas Mucciaro. All rights reserved.

18 Chem 225 otes Ch 24: Carbohydrates and ucleic Acids Page 155 A complementary copy of a DA chain can by made by pairing up complementary nucleotides, then connecting the nucleotides into a chain with a DA polymerase enzyme. Lecture otes Dr. Thomas Mucciaro. All rights reserved.