Biochemistry 1 (BASIC-106)

Dr. Tarek El SewedyBiochemistry 1(MGBC-101)Lecture 2 Carbohydrates Structure & classificationAnd Biological function

By the end of this lecture, students will learn:The basic structure and classes of carbohydrates.Biological function of carbohydratesIntended Learning OutcomesLecture contentWhat are carbohydrates?Classification of carbohydrates.Classification of monosaccharides.Reducing & non reducing sugars.Classification of polysaccharides.Biological function of carbohydrates

Biomolecules of lifeCarbohydratesProteins.LipidsNucleic acidsWhat are carbohydrates?In biochemistry, carbohydrates is a synonymous of saccharide.The wordsaccharide comes from theGreekword skkharon, meaning sugar".The term carbohydrate often means anyfoodthat is particularly rich in the comples carbohydratestarch(such ascereals,bread, andpasta) or simple carbohydrates, such assugar(found incandy,jams, anddesserts).Carbo-hydrate is composed of carbon and water (C.H2O)n. For every carbon there is 1 water molecule or 2 hydrogen atoms and 1 oxygen atom (with some exceptions).

Classification of carbohydratesThere are a variety of classification schemes. The most common classification scheme divides the carbohydrates into groups according to the number of individual simple sugar units.Monosaccharides contain a single unitdisaccharidescontain two sugar units (2 monosaccharides)polysaccharidescontain many sugar units as in polymers

Monosaccharides can be further classified by the number of carbons present.Six carbons = Hexose (ex. Glucose, Galactose,fructose) Five carbons = Pentose (ex. Ribose)Four = Tetrose (Erythose) Three = Triose (Glyceraldehyde)

Modified monosaccharidesphosphorylated sugars are one example of modified monosaccharides. An important phosphorylated sugar is glucose 6-phosphate.glucose 6-phosphate provides energy in certain metabolic pathways, and it can be converted and stored as glycogen when blood glucose levels are high. If blood glucose levels are low, glucose 6-phosphate can be converted back into glucose to enter the bloodstream again.glucose 6-phosphate possesses a negative charge. This prevents the molecule from leaving the lipid-bilayer membranes. This allows the cell to easily uses the modified sugar to provide energy, or convert it to glycogen as storage.Classification according to Functional Groups

Aldoses: Are sugars containing an aldehyde functional group on carbon #1 (anomeric carbon)- Monosaccharides in this group are glucose, galactose, ribose, and glyceraldehyde.Ketoses: Sugars containing the ketone group on carbon # 2 (anomeric carbon)- The major sugar in this group is fructose.

Classification according to reducing capabilityReducing capability is defined by the presence of free aldehyde or ketone group. A. Reducing: Contain free groups (contain one free anomeric carbon atom), glucose, maltose, lactose and fructose.All Monosaccharides are reducing sugars. Two of three disaccharides maltose and lactose, have the free groups needed to act as reducing agents.B. Non-reducing: Contain no free groups. Sucrose and polysaccharides are non reducing. (sucrose's anomeric carbon is not "free" since this carbon is used to link fructose and glucose together.

Reducing Disaccharides

LactoseMaltoseNon reducing disaccharide

Classification of polysaccharidesHomo-polysaccharides (all the same type)Hetero-polysaccharides (mixtures of monomer types)Complex carbohydrates (joined to non-carbohydrate molecules)

3. Polysaccharides1-Monosaccharides (CH20)n

2-Disaccharides

Disaccharidesfructoseglucosegalactoseasucrosemaltoselactose(fructose-glucose)(glucose-glucose)(glucose-galactose)monosaccharaidesjoined together to make disaccharides.18

PolysaccharidesGlucoseStarch (unbranched)Starch (branched)GlycogenCellulose19Carbohydrate 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. By the time the chewed food reaches the stomach, where acidity deactivates the amylase, the average chain length of starch has gone from several thousand to fewer than 8 glucose units.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 streamIn 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 insulin reaction, regulate the concentration of glucose in the blood. A high fastingblood sugarlevel is an indication of diabetic conditionsGlucose is used as an energy source in most organisms aerobic or anaerobic respiration. The first step of this is thephosphorylationof glucose byhexokinaseto prepare it for breakdown to provide energy. The major reason for the immediate phosphorylation of glucose by ahexokinaseis to prevent diffusion out of the cell.Organisms 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.Inplants, glucose is a product of photosynthesis. In animals, glucose results from the breakdown ofglycogen, a process known asGlycogenolysis. In animals, glucose is 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. Glucose is synthesized by chlorophyll in plants using carbon dioxide from the air and sunlight as an energy source. Glucose is further converted to starch for storage.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.

Another view of amylose and amylopectin, the two forms of starch. Amylopectinis a highly branched structure, with branches occurring every 12 to 30 residuesGlycogenGlycogen is also known as animal starch.It is stored in muscle and liver tissueComplete hydrolysis yields glucose 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

ASSIGNMENTS

Students selected in the previous slide are requested to prepare slides about any of the following topics and delivered before next lecture:

Importance of carbohydrates in human life. Disease related to high glucose level.Carbohydrate and energy.Carbohydrate digestion.

Students selected for assignmentStudy Questions:Mention the 4 major biomolecules of life.Mention one classification of carbohydrates giving examples.Explain why sucrose is a non reducing while maltose is a reducing sugarHarpers Biochemistry 26th editionSuggested readings: