Lecture 3 - Carbohydrates Chemistry PDF

THE CARBOHYDRATES: Sugars, Starches & Fibers  Carbohydrates (CH2O)n Simple carbohydrates –Monosaccharides (single sugars) –Disaccharides (double sugars) Complex carbohydrates – Polysaccharides (many sugars) Copyright 2005 Wadsworth Group...

THE CARBOHYDRATES: Sugars, Starches & Fibers  Carbohydrates (CH2O)n Simple carbohydrates –Monosaccharides (single sugars) –Disaccharides (double sugars) Complex carbohydrates – Polysaccharides (many sugars) Copyright 2005 Wadsworth Group, a division of Thomson Learning Simple Carbohydrates Monosaccharides (C6H12O6) Glucose Fructose Galactose Copyright 2005 Wadsworth Group, a division of Thomson Learning  Monosaccharides Glucose – dextrose or blood sugar 1. Primary fuel for the body 2. Found in all disaccharides & polysaccharides  Monosaccharides Fructose – fruit sugar 1. Found in fruit, honey, syrup 2. Converts to glucose in the body  Monosaccharides Galactose – part of lactose 1. Found in milk 2. Converts to glucose in the body Simple Carbohydrates Disaccharides Maltose Sucrose Lactose Copyright 2005 Wadsworth Group, a division of Thomson Learning  Disaccharides Sucrose – table sugar 1. Glucose + Fructose 2. Refined from sugar beets & cane  Disaccharides Lactose – milk sugar 1. Glucose + Galactose 2. Lactose intolerance – missing digestive enzyme needed to split into two monodisaccharide parts to absorb it  Disaccharides Maltose – malt sugar 1. Glucose + Glucose 2. Found in germinating seeds & used in fermentation to produce malted beverages (beer, whiskey) Condensation Hydrolysis Complex Carbohydrates Polysaccharides Glycogen Starches Fibers Copyright 2005 Wadsworth Group, a division of Thomson Learning  OVERVIEW Carbohydrates: The most abundant organic molecules in nature The empiric formula is (CH2O)n, “hydrates of carbon” Carbohydrates: provide important part of energy in diet Act as the storage form of energy in the body are structural component of cell membranes  OVERVIEW CONT’D  Many diseases associated with disorders of carbohydrate metabolism including: Diabetes mellitus Galactosemia Glycogen storage diseases Lactose intolerance  CLASSIFICATION  Monosaccharides: Simple sugar  Disaccharides: 2 monosaccharide units  Oligosaccharides: 3-10 monosaccharide units  Polysaccharides: more than 10 sugar units Homopolysaccharides & heteropolysaccharides  Monosaccharides Further classified based on: 1. No. of carbon atoms 2. Functional sugar group: Aldehyde group – aldoses Keto group – ketoses  Monosaccharides CONT’D Aldose Ketose Triose Glyceraldehyde Dihydroxyacetone Pentose Ribose Ribulose Hexose Glucose Fructose  Isomerism  Isomers Compounds having same chemical formula but different structural formula Aldo-Keto Isomers Example: Glucose (Aldose) and Fructose (Ketose)  Epimers  Epimers CHO dimers that differ in configuration around only one specific carbon atom -Glucose and galactose, C4 -Glucose and Mannose, C2 Galactose and mannose are not epimers  Enantiomers (D- and L-Forms) Structures that are mirror images of each other and are designated as D- and L- sugars based on the position of –OH grp on the asymmetric carbon farthest from the carbonyl carbon Majority of sugars in humans are D-sugars  α- and β-Forms 1 CHO H C OH 2 HO C H D-glucose ❑ Cyclization of Monosaccharides 3 H C OH (linear form) 4 Monosaccharides with 5 or more H C OH 5 CH2OH 6 carbon are predominantly found in 6 CH2OH 5 O 6 CH2OH 5 O H OH the ring form H H H H 4 H 1 4 H 1 OH OH OH OH OH H 3 2 3 2 H OH H OH -D-glucose -D-glucose -The aldehyde or ketone grp reacts with the –OH grp on the same sugar 1 CH2OH 2C O -Cyclization creates an anomeric carbon HO 3 C H HOH2C 6 O 1 CH2OH H C OH (former carbonyl carbon) generating the H 4 C OH 5 H H 4 HO 3 2 OH α and β configurations 5 OH H 6 CH2OH D-fructose (linear) -D-fructofuranose  Mutarotation In solution, the cyclic α and β anomers of a sugar are in equilibrium with each other, and can be interconverted spontaneously Fischer Projection Fischer Projection Haworth Projection  Sugar Isomers 1. Aldo-keto 2. Epimers 3. D- and L-Forms 4. α- and β-anomers  Disaccharides  Joining of 2 monosaccharides by O-glycosidic bond: Maltose (α-1, 4)= glucose + glucose Sucrose (α-1,2) = glucose + fructose Lactose (β-1,4) = glucose + galactose Disaccharides CONT’D Lactose  Polysaccharides Glycogen – long chains of glucose found in animals 1. Stored in liver & muscles 2. Helps maintain blood glucose and important source of “quick energy”, esp. during exercise (lasts only about 12 hrs)  Polysaccharides Starch – long chains of glucose found in plants 1. Cereal grains (wheat, rice, corn, etc.), legumes (beans & peas), and root vegetables (potatoes, yams)  Polysaccharides Fiber – mostly indigestible CHO; gums, mucilages, lignin 1. Component of plant cell walls 2. Classified according to solubility in water 3. Abundant in whole grains, legumes, fruits and vegetables  Polysaccharides Combinations of more than 2 sugars = oligosaccharides If very large called polysaccharides Added to foods for a variety of reasons – Increase dietary fiber content – Thicken – Starch most common polysaccharide – “Gum” naturally occurring added to food  Polysaccharides-Starch Starch most common polysaccharide Made of glucose units linked together Storage form of energy for plants Glycogen storage form of energy for animals Starch forms granules Vary is size and shape depending on type of plant  Polysaccharides-Starch Two- types: – Amylose and amylopectin o Amylose = 20-30% of most native starches o Some starches only contain amylopectin Example: cornstarch  Polysaccharides-Starch Amylose contributes to gel formation Reversible up to between 140˚F – 158˚F Temperature affects gelatinization (irreversible swelling) Starts between 140˚F – 158˚F Increase in water absorption Gelatinization range – Temp. in which all granules are fully swollen  Polysaccharides  Homopolysaccharides: Branched: Glycogen and starch (α-glycosidic polymer) Unbranched: Cellulose (β-glycosidic polymer)  Heteropolysaccharides: e.g., glycosaminoglycans (GAGs)  Complex Carbohydrates  Carbohydrates attached to non-carbohydrate structures by glycosidic bonds (O- or N-type) e.g., 1. Purine and pyrimidine bases in nucleic acids 2. Bilirubin 3. Proteins in glycoproteins and proteoglycans 4. Lipids found in glycolipids  Glycosidic Bonds  N-Glycosidic  O-Glycosidic  Glycosaminoglycans (GAGs)  Glycosaminoglycans (GAGs) are large complexes of negatively charged heteropolysaccharide chains  are associated with a small amount of protein, forming proteoglycans, which consist of over 95 percent carbohydrate  bind with large amounts of water, producing the gel- like matrix that forms body's ground substance  The viscous, lubricating properties of mucous secretions also result from GAGs, which led to the original naming of these compounds as mucopolysaccharides  Glycosaminoglycans (GAGs)  GAGs are linear polymers of repeating disaccharide units [acidic sugar-amino sugar]n  The amino sugar (usually sulfated) is either D-glucosamine or D-galactosamine  The acidic sugar is either D-glucuronic acid or L-iduronic acid  GAGs are strongly negatively-charged: carboxyl groups of acidic sugars Sulfate groups  Resilience of GAGs Relationship between glycosaminoglycan structure and function  Because of negative charges, the GAG chains tend to be extended in solution and repel each other and when brought together, they "slip" past each other This produces the "slippery" consistency of mucous secretions and synovial fluid  When a solution of GAGs is compressed, the water is "squeezed out" and the GAGs are forced to occupy a smaller volume. When the compression is released, the GAGs spring back to their original, hydrated volume because of the repulsion of their negative charges This property contributes to the resilience of synovial fluid and the vitreous humor of the eye  Members of GAGs Examples of GAGs are: 1. Chondroitin sulfates: Most abundant GAG 2. Keratan sulfates: Most heterogeneous GAGs 3. Hyaluronic acid: Compared to other GAGs, it is unsulfated and not covalently attached to protein 4. Heparin: Unlike other GAGs, Unlike other GAGs that are extracellular, heparin is intracellular and serves as an anticoagulant  Take home Message Structure and function of carbohydrates  Mono-, Di-, and Poly-saccharides  Sugar Isomers: Aldo-keto, epimers, D- and L-, α- and β-anomers  Complex carbohydrates: e.g., Glycosaminoglycans and proteoglycans  Structure and function of GAGs  Examples of GAGs: chondroitin sulfate, keratin sulfate, hyaluronic acid and heparin  Fibers Insoluble – nonviscous; cellulose, lignins Soluble – viscous & fermentable; pectins, gums, mucilages  Digestion Mouth –Salivary amylase Stomach –Fibers and satiety Small Intestine -Maltase, sucrase, lactase  Digestion Pancreas –Pancreatic amylase Large Intestine -Fermentation of viscous fibers Water, gas, short-chain fatty acid production Carbohydrate Digestion in the GI Tract Absorption  Metabolism Glucose in the Body Used for energy – fuels most of the body’s cells Stored as glycogen – 1/3 in the liver and 2/3 in muscles Made from protein – gluconeogenesis Converted to fat – when in excess of body’s needs  Constancy of Blood Glucose Regulating hormones – maintain glucose homeostasis 1. Insulin – moves glucose from the blood into cells 2. Glucagon – signals the liver to release glucose into the blood 3. Epinephrine – released when emergency fuel needed Maintaining Blood Glucose Homeostasis Constancy of Blood Glucose Diabetes –Type 1 diabetes Failure of insulin production –Type 2 diabetes Obesity Hypoglycemia –Rare in healthy people Glycemic response –Glycemic index Glycemic Index  Health Effects of Sugar Sugar in excess 1. Contains no nutrients and may contribute to malnutrition 2. Causes dental caries (tooth decay) 3. Does not cause, but can contribute to: obesity, diabetes, heart disease, & behavorial problems  Accusations Against Sugars Sugar causes obesity Sugar causes heart disease  Accusations Against Sugars Sugar causes misbehavior in children and criminal behavior in adults Sugar causes cravings and addictions –serotonin  Recommended Intakes of Sugars DRI –No more than 25% of total daily energy intake -Limit added sugars to