Biochemistry 1 (BASIC-106)

Dr. Tarek El SewedyBiochemistry 1(MGBC-101)

Lecture 3 Biological function and characteristics of carbohydratesBy the end of this lecture, students will learn:Biological function of carbohydrates.Characteristics of carbohydratesIntended Learning OutcomesLecture contentBiological function of carbohydrates.MonosacharidesDisacharidesPolysacharidesPhysical Properties of CarbohydratesChemical Properties of carbohydrates

Biomolecules of lifeCarbohydratesProteins.LipidsNucleic acids

3. Polysaccharides1-Monosaccharides (CH20)n

2-Disaccharides

Carbohydrate FunctionSources of Immediate energy (ATP) as produced by glucose catabolism(Glycolysis and Krebs cycle).Source of stored energy (Glycogen stored in liver).Intermediates in the biosynthesis of other basic biomolecules (fats and proteins).Associated with other molecules such as vitamins and antibiotics.Cont, Carbohydrate functionStructural function: ex. Ribose, deoxyribose and cellulose and starch in plants.Involved in many cell functions such cell-cell recognition and protein folding.

Digestion of carbohydratesa) -amylase (in saliva) randomly hydrolyzes all the glycosidic bonds of starch.b) Digestion continues in the small intestines with pancreatic amylase. This degrades the starch to maltose .c) Further digestion occurs down the digestive tract in the small intestine. The resulting monosaccharides are absorbed by the small intestine and transported into the blood stream.In living organisms, most carbohydrates are found bound to other compounds rather than as simple sugarsGlycoproteins (hormones, antibodies)Glycolipids Nucleic acid.MonosaccharaidesMonosaccharaides are also known as simple sugarsThey are classified by:Number of carbons. Whether they are aldoses or ketoses.D-glyceraldehyde is the simplest of the aldoses.Glucose is the most widely knownGlucoseGlucose is a monosaccharide with formula C6H12O6 It is absorbed directly into the bloodstream during digestion.Bread, rice, pasta are rich in glucose.Glucose is a common medicalanalyte measured in blood samples. The hormone insulin, regulates the concentration of glucose in the blood. A high fastingblood sugarlevel is an indication of diabetic conditionsOrganisms use glucose as a precursor for the synthesis of several important substances.Starch,cellulose, andglycogen("animal starch"). Some of these polymers like starch or glycogen serve as energy stores while others like cellulose have structural roles. In animals, glucose results from the breakdown ofglycogen, a process known asGlycogenolysis. Glucose can also be synthesized in theliverandkidneysfrom non-carbohydrate intermediates, such aspyruvateandglycerol, by a process known as Gluconeogenesis.Glucose is produced commercially via theenzymatic hydrolysisof starch. Many crops can be used as the source of starch.Maize,rice,wheat

Glucose is classified as a monosaccharide, an aldose, a hexose, and is a reducing sugar.

Fructose

FructoseFructose is found naturally in fruits and vegetables.Fructose is metabolized differently than glucose and other sugars, doesnt stimulate insulin and is characteristically low glycemic. For these reasons, its often used in low-glycemic food applications15 to 80% sweeter than sucrose, and tastes particularly sweet when cold or in solution

Disaccharides

DisaccharidesDisaccharides are formed when two monosaccharaides are joined together and a molecule of water is removed. Milk sugar (lactose) is made from glucose and galactose sugar cane sugar (sucrose) is made from glucose and fructose. Maltose is made up of two glucose molecules. The two monosaccharides are bonded via a glycosidic bondCan be degraded to monosaccharides by hydrolysis producing water.

Sucrose

SucroseFormed of Glucose and fructose monomers joined together.known as table sugarCommercially obtained from sugar cane or sugar beetUsed pharmaceutically to make syrups

LactoseLactose is a famous disaccharide, resulting from a galactose joining to glucose.Milk is the most famous source of Lactose.Used in infant formulations, and as a diluent in pharmaceuticals

MaltoseMade up of two glucose molecules.Produced when amylase breaks down starch.Obtained from malt sugar and when glucose is caramelized.

Polysaccharides

StarchStarch or Amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds.produced by all green plants as an energy storeIt is the most common carbohydrate in the human diet like potatoes, wheat, maize (corn), rice.Cont, Starchconsists of two types of molecules: the linear and helical amylose and the branched amylopectin.Glycogen, the glucose store of animals, is a more branched version of amylopectin.Used as an additive for food processing, typically used as thickeners and stabilizers in foods such as puddings, custards, soups, and salad dressings, and to make noodles and pastas.Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin by weight.

GlycogenGlycogen is also known as animal starch.It is stored in muscle and liver tissueComplete hydrolysis yields glucose

Glycogen Glycogen is the glucose storage polymer in animals, is similar in structure to amylopectin, but glycogen has more branches The highly branched structure permits rapid release of glucose from glycogen stores, i.e. in muscle during exercise. The ability to rapidly mobilize glucose is more essential to animals than to plants

A. Physical properties of carbohydratesD and L forms of sugarsAsymmetric carbon atom: It is a carbon atom attached to four different atoms or groups.Any compound has asymmetric carbon posses the following properties:1-Formation of isomers. 2-Having optical activityIsomers exist in two forms:1-D-form (when the hydroxyl group lies on right side to the asymmetric carbon atom) 2-L-form (when the hydroxyl group lies on left side to the asymmetric carbon atom).In nature, glucose is found in the D form only.

Asymmetric carbon atomCyclic and linear formsMonosaccharides of five or more carbon atoms are present in ring forms. In solutions, cyclic forms are in equilibrium with minute amounts of the linear forms.

-D-Glucose -D-Glucose Enantiomers (Anomers) and differ only in configuration of OH of the anomeric carbon compared to C #6

The anomeric carbon atom (C#1) can exist as two different isomers called and .Though almost exactly the same, they operate differently in our bodies.For example: Humans have the -amylase enzyme for the digestion of starch, but not the -amylase enzyme for the digestion of cellulose. We can digest starch products such as pasta (-Form) and not paper or wood (- form).B. Chemical properties of carbohydrates

Reduction of monosaccharides producing sugar alcohol:The aldehyde and keto group of monosaccharides can be reduced by H2 + catalyst(non enzymatically) or by enzymes producing the corresponding alcohol.

Ester formation:The most common esters of monosaccharidesa are phosphate esters. Ex; Adenosine triphosphate (ATP), glyceraldehydes-3-phosphate.

AcidAlcoholEsterReaction with acids:Concentrated acid remove three water molecules from sugar forming furfural from pentose and hydroxyl furfural from hexose. The furfurals further react with phenols (naphthol) present in to produce a purple ring interface.

Reaction with bases:Strong alkali as concentrated NaOH on heating leads to condensation of monosaccharides forming yellow and brown resin and caramel formation.

Deoxygenation of Hydroxyl GroupsThe hydroxyl groups on Carbon atom#2 of Monosaccharides can be replaced with hydrogen atoms to form deoxy-sugars. Deoxygenation of ribose within a ribonucleotide to form the 2-deoxyribonucleotide is a critical reaction in DNA biosynthesis.

Biomolecules Interconversion(Acetyl CoA)

Please Remember

Students selected for assignment ASSIGNMENTS

The students are requested to prepare slides about the following topics and delivered before next lecture:

Importance of cellulose in plants.()Hypoglycemia.()Hyperglycemia.()Carbohydrates and immunity.()Carbohydrates and cell signaling.()Carbohydrates and as building blocks.()Carbohydrate as fuel()

Study QuestionsMention 3 different functions of carbohydrates

Complete:Monosaccharaides can be aldoses or .The starch of human body is called---------.The major carbohydrates in plants is called...Mention 4 functions of carbohydrates.

Harpers Biochemistry 26th editionSuggested readings: