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Home , Amino sugar, Carbohydrate catabolism, Deoxy sugar

11/15/17

CHAPTER 12: : Structure and Function OUTLINE

• 12.1 Role of Carbohydrates

• 12.2

• 12.3 Complex Carbohydrates

• 12.4

• 12.5 as Markers

CHAPTER 12: Carbohydrates: Structure and Function WHAT ARE CARBOHYDRATES? • and its derivatives are carbohydrates: Ø Carbohydrates are simple organic molecules that have

a shared basic chemical formula: Cn(H2O)n Ø The name “carbo + hydrate” represents that fact that

they are made from CO2 and H2O by • About half of all earth’s solid carbon is found in two polymers of glucose found in plants: Ø = major energy storage molecule Ø = major structural component of the plant cell wall (aka. “fiber”)

CHAPTER 12: Carbohydrates: Structure and Function THE SIMPLEST CARBOHYDRATES • Monosaccharides are carbohydrates that cannot be hydrolyzed into simpler carbohydrates: Ø These are the fundamental building blocks for all other carbohydrates (often called “simple ”)

Ø All have formulas of based on the basic pattern:

Cn(H2O)n • Monosaccharides have specific functional groups: 1. An aldehyde OR a ketone (not both!) 2. Several (two or more) alcohol (-OH) groups

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CHAPTER 12: Carbohydrates: Structure and Function STRUCTURE & NOMENCLATURE OF MONOSACCHARIDES • Monosaccharides are classified by two features: 1. Length of their main carbon chain (utilize standard IUPAC naming for # of carbons) 2. Whether they contain an aldehyde or ketone group • Names always end with –ose • Two common :

Ø Both C6H12O6 Ø Glucose = Ø =

CHAPTER 12: Carbohydrates: Structure and Function CHIRALITY AND D-SUGARS • Note that monosaccarides have multiple chiral carbons in their structures: Hexoses have 4 1 chiral carbons 2 3

4 The position of the OH at the carbon furthest from the carbonyl determines whether the is in the D-or L-form: Naturally occurring sugars are the D-isomer form

CHAPTER 12: Carbohydrates: Structure and Function DIASTEREOMERS • Many simple sugars differ only in the arrangement of –OH groups around chiral carbons: Ø If multiple chiral centers are present, it is possible to get non-superposable isomers that are not mirror images Ø We call these Diastereomers • Common hexoses are a good example: Ø D-glucose and D- are diastereomers Ø Differ only in the sterochemistry around the 4th carbon in their main chain

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CHAPTER 12: Carbohydrates: Structure and Function MONOSACCARIDES ARE TYPICALLY RINGS • In aqueous solution, common monosaccharides usually form 5- or 6-membered ring structures: Ø Caused by an internal chemical rearrangement between the carbonyl carbon & and alcohol Ø The cyclic structure is more stable in water

• For hexoses, two rings are possible: Ø For (glucose) – 6-member ring called a forms

Ø For (fructose) – 5-member ring called a forms

CHAPTER 12: Carbohydrates: Structure and Function HAWORTH PROJECTIONS OF SUGARS • Haworth projections are used to indicate the 3- dimensional orientation of atoms in a ring: Ø In the ring forms of a , a D-sugar always has the C6 carbon CH2OH group positioned above the ring Ø There is a simple way to remember glucose…..

D-Glucose D-Fructose

CHAPTER 12: Carbohydrates: Structure and Function a- AND b- FORMS OF SUGARS • The carbon atom bonded to both the ring atom and a hydroxyl group is known as the anomeric carbon. Ø The –OH group on the anomeric carbon may be either in a down position or an up position.

Down = a Up = b

a-D-Glucose b-D-Glucose • The numbering scheme for carbons found in a monosaccharide starts at the anomeric carbon

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CHAPTER 12: Carbohydrates: Structure and Function STRUCTURAL EQUILIBRIUM OF MONOSACCHARIDES IN SOLUTION • The cyclic forms of sugars continuously open and close in aqueous solution: Ø This results in an equilibrium that favors the ring forms, but also including the open chain form Ø Note that both a- and b- forms are present in solution

a-D-Glucose b-D-Glucose 36% D-Glucose 64% 0.2%

CHAPTER 12: Carbohydrates: Structure and Function MODIFIED MONOSACCHARIDES • The –OH groups on a simple sugar can be easily modified to add different chemical groups: Ø Recall the type of reactions that work well with alcohols! • These modifications include the following: 1. Amino sugars, with an replacing the –OH. 2. Phosphosugars, with a phosphate ester at an –OH 3. Deoxy sugars, which are missing a hydroxyl group, and instead have an extra –H at that position. 4. , which have an –OR (ether) at the anomeric carbon instead of an –OH.

CHAPTER 12: Carbohydrates: Structure and Function EXAMPLES OF MODIFIED SUGARS

Amino sugar

b-D-Glucoseamine b-D-2-

Glycoside sugar Phosphosugar

Methyl b-D-Glucose-6-phosphate

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CHAPTER 12: Carbohydrates: Structure and Function PRACTICE PROBLEM Two forms of galactose are shown below:

• Are these D-sugars or L-sugars? How can you tell? • Name each form of galactose: ____-____galactose and ____ - ____galactose • Do the Haworth projections show these monosaccharides to be or a ? • Are these monosaccharides chiral? • Number the carbon atoms in the ring. Which carbon atom is the anomeric carbon?

CHAPTER 12: Carbohydrates: Structure and Function OLIGOSACCHARIDES & • Simple sugars (monosaccharides) can be joined together into chains of various lengths: Ø Short chains (3-10) = oligosaccarides (oligo = “few”) Ø Long chains (>10) = polysaccharides (poly = “many”) • Monosaccharides are joined by a type of ether bond is called a .

Ø The glycosidic bond can be broken by addition of water in a hydrolysis reaction.

CHAPTER 12: Carbohydrates: Structure and Function GLYCOSIDIC BONDS IN • Disaccharides are carbohydrates composed of two monosaccharides joined by a glycosidic bond: Ø Monosaccharides are joined by a condensation reaction between two alcohol groups:

a-D-Glucose + a-D-Glucose + H2O • Disaccharides can vary by: 1. The identity of the two monosaccharides 2. The type of linkage between the monomers

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CHAPTER 12: Carbohydrates: Structure and Function IDENTIFYING STRUCTURES • The glycosidic bond in a disaccharide is named to reflect its specific HOW it joins two monosaccharides: 1. The numbering of each carbon atom associated with the glycosidic bond. 2. Whether the anomeric carbon in the glycosidic bond is in an a- or b- orientation.

Maltose 2 glucose with a(1®4) 2 glucose with b(1®4) glycosidic linkage glycosidic linkage

CHAPTER 12: Carbohydrates: Structure and Function COMMON BIOLOGICAL DISACCHARIDES

Disaccharide Monosaccharide Linkage Stereochemistry Biological Common components Type of Anomeric Source Name Carbon in Linkage

Lactose galactose and glucose 1® 4 b Milk sugar

Table glucose and fructose 1® 2 a, b sugar Cellobiose glucose 1® 4 b - Maltose glucose 1® 4 a Malt sugar • Common biological dissaccharides are readily hydrolyzed by digestive :

CHAPTER 12: Carbohydrates: Structure and Function POLYSACCHARIDES • The most common form for biological carbohydrates is in the form of polysaccharides: Ø For energy storage Ø To form tough structural (starch & ) fibers (cellulose)

Ø All three of these are polymers of glucose units • Most polysaccharides contain thousands of monosaccharides linked by glycosidic bonds

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CHAPTER 12: Carbohydrates: Structure and Function ENERGY STORAGE POLYSACCHARIDES • The energy storing polysaccharides consist of glucose units primarily joined by a(1®4) linkages. • There are three major storage polysaccharides: 1. Ø Unbranched (linear) chains of glucose 2. Starch Ø Mostly linear chains with occasional branches (plants) Ø Branch points are a(1®6) glycosidic linkages

3. Glycogen (animals) Ø Highly branched chains with a(1®6) glycosidic linkages

CHAPTER 12: Carbohydrates: Structure and Function STARCH & GLYCOGEN STRUCTURE

CHAPTER 12: Carbohydrates: Structure and Function CELLULOSE • Cellulose is the most abundant carbohydrate: Ø Major component of plant cell wall Ø Forms sheet-like structure provides structural support Ø The main component of wood, paper, grass, and cotton

• Cellulose consists of linear, unbranched glucose units joined by b-(1®4) linkages: Ø Animals lack enzymes to digest these linkages

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CHAPTER 12: Carbohydrates: Structure and Function CELLULAR IDENTITY & OLIGOSACCHARIDES • Besides their role in energy, carbohydrates also play a role in cell recognition: Ø Oligosaccharides can be covalently bonded to & on the cell membrane Ø Attachment is via an alcohol or amine on the or to an alcohol from the sugar

• Cell surface sugars serve as “cell markers” allow immune cells to distinguish host cells from foreign cells

CHAPTER 12: Carbohydrates: Structure and Function THE ABO SYSTEM IN BLOOD • Red blood cells (erythrocytes) have specific markers on them that define the blood groups A, B, AB, or O • The difference between these blood type markers is the presence or absence of a single sugar: Key:

CHAPTER 12: Carbohydrates: Structure and Function CARBOHYDRATES AND ENERGY • The role of dietary carbohydrates is primarily— though not exclusively—catabolic: Ø We extract energy from carbs for our bodily needs

Ø Carbohydrates are converted to CO2 + H2O + energy • The overall process of extracting energy from complex carbohydrates has 3 phases: 1. The hydrolysis of dietary starch into monosaccharides 2. Partial oxidation of monosaccharides into pyruvate and ATP by way of . 3. Complete oxidation of pyruvate through oxidative phosphorylation to produce additional ATP.

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CHAPTER 12: Carbohydrates: Structure and Function STAGE 1: • Complex carbohydrates are first hydrolyzed into simpler carbohydrates using salivary enzymes such as amylase in the mouth:

Ø Products include glucose, maltose, and Ø Dextrins are small oligosaccharides of 3-12 glucose monomers)

• Oligosaccharides are hydrolyzed into simple sugars by an array of intestinal hydrolases

CHAPTER 12: Carbohydrates: Structure and Function STAGE 2: GLYCOLYSIS • Glycolysis is a multistep enzymatic process that converts a molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons)

Consider the 10 steps carbon oxidation + Energy (ATP) states before and after….

D-Glucose Pyuvate

• If molecular oxygen (O2) is present, pyruvate is further catabolized into acetyl CoA in mitochondria

CHAPTER 12: Carbohydrates: Structure and Function THE STEPS OF GLYCOLYSIS • There are 10 steps from glucose to pyruvate: Ø Each step is catalyzed by a different Ø The steps can be divided into two separate phases (see next slide) Ø Most of these steps are reaction types that we previously discussed (Chapter 10 or earlier)

• Each compound between glucose and pyruvate is termed an intermediate in the pathway: Ø Each intermediate is a phosphate-containing compound (due to phosphoryl group transfer)

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CHAPTER 12: Carbohydrates: Structure and Function ENERGY IN GLYCOLYSIS Glycolysis can be divided into two distinct phases: 1.Preparatory Phase: -2 ATP Energy (ATP) is “invested” to change glucose into a better substrate for energy harvest 2.Payoff Phase: + 4 ATP + 2 NADH Energy is harvested in the form of ATP & carbons are partially oxidized to produce NADH (NADH will be used later to make more ATP)

CHAPTER 12: Carbohydrates: Structure and Function ENZYME CLASSES IN GLYCOLYSIS 1. KINASES à phosphoryl group transfer • Steps 1, 3, 7,10 2. DEHYDROGENASES à reactions with NAD+ or FAD cofactor • Step 6 3. ISOMERASES à convert b/w structural isomers • Steps 2, 5, 8 (mutase) 4. DEHYDRATASES à dehydration reaction • Step 9 (enolase)

• Note: step 4 (aldolase) is the only “new” reaction type

CHAPTER 12: Carbohydrates: Structure and Function GLYCOLYSIS: Step 1

Description: D-Glucose Hydroxyl at C1 is phosphorylated

Reaction Type: Phosphoryl group transfer reaction Glucose-6- phosphate Enzyme: Hexokinase

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CHAPTER 12: Carbohydrates: Structure and Function GLYCOLYSIS: Step 2

Description: Isomerization from a Glucose-6- pyranose to furanose phosphate Reaction Type: Isomerization reaction (aldoseà ketose) Fructose-6- phosphate Enzyme: Phosphoglucose isomerase

CHAPTER 12: Carbohydrates: Structure and Function GLYCOLYSIS: Step 3

Description: Fructose-6- Hydroxyl at C6 is phosphate phosphorylated

Reaction Type: Phosphoryl group transfer reaction Fructose-1,6- bisphosphate Enzyme: Phosphofructo- Note that this is now a kinase symmetrical molecule!

CHAPTER 12: Carbohydrates: Structure and Function GLYCOLYSIS: Steps 4 & 5

Description: Fructose-1,6- 4. Bond between C3 bisphosphate & C4 is broken 5. Isomerization

Dihydroxyacetone Reaction Type: phosphate 4. Aldol condensation + 5. Isomerization -3- reaction phosphate Enzyme: 4. Aldolase Glyceraldehyde-3- 5. phosphate 2 x phosphate isomerase

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CHAPTER 12: Carbohydrates: Structure and Function KEEPING TRACK AFTER STEP 5

• Steps 4 & 5 of glycolysis convert a 6-carbon molecule into 2 x 3-carbon molecules • Every subsequent step (6-10) is in duplicate: Ø For each of the 3-carbon molecules Ø Each of these molecules will become a pyruvate

CHAPTER 12: Carbohydrates: Structure and Function GLYCOLYSIS: Step 6 Description: Aldehyde is oxidized to a Glyceraldehyde- carboxylic acid & inorganic 3-phosphate phosphate added

Reaction Type: Redox reaction + phosphoryl transfer 1,3-Bisphospho- glycerate Enzyme: Glyceraldehyde-3- phosphate dehydrogenase

CHAPTER 12: Carbohydrates: Structure and Function GLYCOLYSIS: Step 7

Description: Phosphoryl group on 1,3-Bisphospho- glycerate C1 is removed to ADP

Reaction Type: Phosphoryl group transfer reaction 3-Phospho- glycerate Enzyme: Phosphoglycerate kinase

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CHAPTER 12: Carbohydrates: Structure and Function GLYCOLYSIS: Step 8

Description: Phosphoryl group is 3-Phospho- glycerate moved from C3 to C2

Reaction Type: Isomerization reaction (or group transfer) 2-Phospho- glycerate Enzyme: Phosphoglycerate mutase

CHAPTER 12: Carbohydrates: Structure and Function GLYCOLYSIS: Step 9

Description: 2-Phospho- Water is removed glycerate creating a C=C bond

Reaction Type: Dehydration reaction

Phosphoenol- pyruvate Enzyme: Enolase

CHAPTER 12: Carbohydrates: Structure and Function GLYCOLYSIS: Step 10

Description: Phosphoryl group on Phosphoenol- pyruvate C2 is removed to ADP

Reaction Type: Phosphoryl group transfer reaction Pyruvate Enzyme: Pyruvate kinase

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CHAPTER 12: Carbohydrates: Structure and Function GLYCOLYSIS IS ANAEROBIC • Anaerobic pathways do not require oxygen:

Ø Many microorganisms only use glycolysis to get energy

Ø Several disease-causing bacteria (tetanus, botulism & some gut bacteria) are “obligate anaerobes” • Glycolysis can run without oxygen, and is therefore an anaerobic pathway: Ø Some tissues, such as brain tissue and red blood cells, absolutely require oxygen to get enough energy

Ø In other tissues, such as muscle and , glycolysis is sufficient source of energy

CHAPTER 12: Carbohydrates: Structure and Function THE FATE OF PYRUVATE • What happens to pyrvuvate depends on whether

or not oxygen (O2) is available to a cell:

A. If oxygen is present à Aerobic pathway: • Pyruvate is oxidized in the • This third stage of catabolism occurs in the mitochondria

B. If oxygen is absent à Anaerobic pathway: • Pyruvate is reduced into lactate • This uses up the NADH that formed during the 6th step of glycolysis

CHAPTER 12: Carbohydrates: Structure and Function OXIDATION OF PYRUVATE • If oxygen is present, pyruvate is transported to the mitochondria for further oxidation: Ø A one-step reaction prepares pyruvate for use in the citric acid cycle (Chapter 14) Ø This oxidation step requires coenzyme A (CoA) as a reactant and NAD+ as electron acceptor.

• Pyruvate is oxidized with the loss of CO2 and production of the thioester acetyl-CoA.

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CHAPTER 12: Carbohydrates: Structure and Function • If oxygen is depleted (as in active muscle tissues) pyruvate is reduced to L-lactate in a process called : Ø Recall: lacate is the conjugate base of lactic acid

• The purpose of fermentation is to allow regeneration of NAD+: Ø Glycolysis uses up NAD+ Ø Fermentation uses up NADH to make more Ø Allows ATP production to

continue without O2

CHAPTER 12: Carbohydrates: Structure and Function ALCOHOL FERMENTATION • Some microorganisms use an alternative, 2-step anaerobic fermentation pathway to reduce pyruvate to (C2H5OH) by alcohol fermentation: Ø Yeast, and some bacteria do this

Ø Another by-product of this process is CO2

• This process is used for production of brewed &

baked goods: 1 Ø Wine, beer, etc Ø Bread, etc 2

CHAPTER 12: Carbohydrates: Structure and Function OTHER METABOLIC ROLES FOR GLUCOSE • Other pathways using glucose include: 1. – formation of glycogen from glucose • Used to store glucose for energy (increased by insulin) 2. – degradation of glycogen to glucose • Hydrolyzes glycogen to release glucose (increased by glucagon) 3. – formation of glucose from pyruvate • Used to provide glucose to tissues when glycogen and/or other storage molecules are depleted 4. phosphate pathway – formation of pentose phosphate from glucose • Starting point for synthesis of nucleic acids (DNA and RNA)

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CHAPTER 12: Carbohydrates: Structure and Function METABOLIC USES OF GLUCOSE

1 2

Pentose phosphate 4 pathway

3

Chapters 13 & 14

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