Carbohydrates Structure and Function PDF

Carbohydrate Structure and Function  Carbohydrates ❖ Saccharides ❖ The molecule is comprised of carbon (C), hydrogen (H) and oxygen O) ❖ They occur as starch (provides energy) and cellulose (mechanical strength) in plants and glycogen (stored form of energy in liver and muscles) i...

Carbohydrate Structure and Function  Carbohydrates ❖ Saccharides ❖ The molecule is comprised of carbon (C), hydrogen (H) and oxygen O) ❖ They occur as starch (provides energy) and cellulose (mechanical strength) in plants and glycogen (stored form of energy in liver and muscles) in animals ❖ Plants contain a significantly higher percentage of carbohydrate (30 %) than animals (1%) ❖ They are a source of C for metabolic reactions ❖ They are an important source of energy for the brain ❖ They form part of the structural framework for RNA and DNA causing flexibility of the rings and allowing for storage and expression of these genetic molecules  Carbohydrates ❖ They help in maintaining the structure of cell wall of bacteria and plants ❖ They are linked to proteins (glycoprotein) and lipids (glycolipid) which helps in cell-to-cell recognition, e.g. The sperm cell is able to search, find and bind to an egg for fertilization to take place ❖ Fibres – Increase bowel movement ❖ Carbohydrates are classified into four groups monosaccharides disaccharides oligosaccharides polysaccharides  Monosaccharides ❖ They are referred to as simple sugars ❖ Simple sugars cannot be broken down into smaller groups ❖ They have the general formula (CH2O)n ❖ Monosaccharides can be further classified into groups depending on the (a) # of carbon atoms (b) The position of the carbonyl group i.e. –C=O (c) The chirality of the molecule  (a)The number of C atoms ❖ The name of the monosaccharide is # of C atoms Name Formula dependent on the number of C atoms present 3 Triose C3H6O3 ❖ In naming a carbohydrate the first part of 4 Tetrose C4H8O4 the name represents the number of carbon atoms present and the suffix – ose is then 5 Pentose C5H10O5 added 6 Hexose C6H12O6 ❖ Monosaccharides include molecules with up to 7 carbons 7 Heptose C7H14O7  (b) The position of the carbonyl group ❖ Carbohydrates can be classified into aldehydes and ketones depending on the placement of the carbonyl group (–C=O) ❖ If the –C=O group is located at the end of the molecule then this molecule is an aldehyde and the monosaccharide is called an aldose R = hydrocarbon ❖ If the –C=O group is located within the structure of the molecule, then this molecule is a ketone and the monosaccharide is called a ketose R and R' - different hydrocarbon chains  (c) Chirality of the molecule ❖ Chiral is used to describe an object that cannot be super-imposed on its mirror image ❖ The letter D- or L- is written before the name of the monosaccharide to distinguish between the two isomers Stereochemistry of Carbohydrates ❖ Glyceraldehyde (a chiral molecule), the simplest carbohydrate, exists in two isomeric forms that are mirror images of each other: ❖ These forms are stereoisomers of each other.  Structure of Monosaccharides ❖ Sugars can arrange themselves into two structural forms i.e. linear and ring forms ❖ Linear – Fischer Projection Ring – Haworth Projection  Structure of Monosaccharides ❖ Hexoses - the predominant form is the ring structure ❖ A six membered ring is called a Pyranose e.g. glucose and galactose ❖ A five membered ring is called a Furanose e.g. fructose Mutarotation  The gradual change in rotation at the anomeric carbon is called mutarotation.  Mutarotation causes a conversion of α form into β form and vise versa  The different forms are referred to anomers  This rotation occurs in all reducing sugars (except a few ketoses)  The α- and β- anomers of carbohydrates are typically stable solids  However, in aqueous solution, they quickly equilibrate to an equilibrium mixture of the two forms  For example, in aqueous solution, glucose exists as a mixture of 36% α- and 64% β- (>99% of the pyranose forms exist in solution)  Hemiacetals  When the sugar forms a ring through mutarotation, the product is called a cyclic hemiacetal and it opens easily back to the aldehyde chain, so there is always a small amount of the aldehyde chain present  The glucose chain is 0.01% Mutarotation of Glucose Converting from Fischer to Haworth Projection 1. Draw the Fischer projection of the molecule 2. Draw the basic ring structure 3. D-sugars –CH2OH is placed above the ring on C #5. For L-sugars –CH2OH is placed below the ring on C#5 4. α –sugars, –OH is placed below the ring on C #1. For β-sugars –OH is placed above C #1  Fructose Structures ❖ Fructose can have the same structure interconversion as glucose:- 2 pyranose forms and two furanose forms, so that in a solution of glucose or fructose there will be four rings and one chain. ❖ But glucose is nearly all (99%) pyranose while fructose has 30-40% furanose, 60- 70 % pyranose 20 Properties of Monosaccharides ❖ The presence of the hydroxyl (-OH) functional group makes the molecule soluble in polar solvents (solvents having –OH functional group) e.g. Water H-OH and Ethanol C2H5OH ❖ They are reducing sugars because they have free aldehyde and ketone groups ❖ Glucose, fructose and galactose are some common monosaccharides Glucose: Found in sweet fruits, aldose sugar, sweet, crystalline, the end product of digestion of polysaccharides Fructose: Found in cane sugar, ketose sugar, 50 % sweeter than glucose, crystalline Fructose does not covert to energy as efficiently as glucose and is usually converted to fat stores Galactose: Found in milk, aldose sugar, sweet, crystalline  Disaccharides ❖ They consist of two sugars linked together by an o-glycosidic bond (ether link –O– ) ❖ There exist two types 1-4 glycosidic bond 1-6 glycosidic bond http://academic.brooklyn.cuny.edu/biology/bio4fv/page/1-6branch2.JPG Disaccharides ❖ They are formed via the condensation of two monosaccharide molecules  Disaccharides ❖ There exists three abundant disaccharides Sucrose, lactose and maltose Sucrose: Common name – table sugar, obtained from sugar cane or beets, sweetest disaccharide, it is made up of the monosaccharide glucose and fructose During the condensation reaction, both carbonyl groups are involved in the formation of the glycosidic bond- sucrose is not a reducing agent http://classconnection.s3.amazonaws.com/527/flashcards/807527/png/picture_23131847 9245617.png  Sucrose ❖ When sucrose is treated with acid or invertase (enzyme) the molecule breaks down to give glucose and fructose (hydrolysis) ❖ This is referred to as the inversion of the sugar ❖ Invert sugar is sweeter than sucrose because there is some fructose (sweetest monosaccharide) present ❖ Honey is sweeter than sucrose due to the presence of more invert sugars  BIOLOGICAL IMPORTANCE OF SUCROSE ❖ Sucrose - provides a quick source of energy, provoking a rapid rise in blood glucose upon ingestion ❖ Overconsumption of sucrose - tooth decay, in which oral bacteria convert sugars (including sucrose) from food into acids that attack tooth enamel. The acid dissolves minerals in teeth Low sucrose, proper oral hygiene and maintenance by dental professionals help to preserve teeth ❖ When large amounts of food that contain high percentages of sucrose are consumed, beneficial nutrients can be displaced from the diet, which can contribute to an increased risk for chronic disease  Lactose ❖ Also referred to as milk sugar ❖ It is solely of animal origin and is found in the milk of mammals (except seals) ❖ Human milk contain a higher % of lactose than animal milk. Human milk is therefore sweeter than cow’s milk ❖ It is made up of the monosaccharides - glucose and galactose http://academic.brooklyn.cuny.edu/biology/bio4fv/page/lactose4.JPG  Lactose ❖ The molecule is a reducing sugar due to the presence of a free carbonyl group on C#1 on the glucose ring ❖ It is non fermentable – cannot be broken down by yeast cells → ethanol ❖ It is treated with lactase the molecule breaks down to give glucose and galactose (hydrolysis) ❖ Lactase if found in the intestinal mucosal cells and is responsible for the digestion of lactose  BIOLOGICAL IMPORTANCE OF LACTOSE ❖ Cells in the human body use the chemical building blocks of lactose as a source of chemical energy. ❖ Lactose is the main source of energy supplied to the newborn mammalian in its mother's milk ❖ Lactose intolerance: no lactose or small amounts produced More than ¾ of the world’s adult are lactose intolerant. Up to 90 % of adults of African and Asian decent are lactase deficient.  Maltose ❖ Also referred to as malt sugar ❖ It is not found abundantly in nature ❖ It is the simplest disaccharide ❖ It is made up of two glucose molecules http://bio1151.nicerweb.com/Locked/media/ch05/05_05Maltose.jpg  Maltose ❖ The molecule is a reducing sugar due to the presence of a free carbonyl group on C#1 on the glucose ring ❖ Fermentable –broken down by yeast cells → ethanol ❖ Maltose can be hydrolyzed into glucose molecules by dilute acid or by the enzyme maltase (found in the intestine) and diastase (found in sprouting barley)  Oligosaccharides ❖ Oligosaccharides: 3 – 10 molecules of the same or different molecules of monosaccharides ❖ They are found linked to proteins and lipids (glycoproteins, glycolipids) ❖ E.g. raffinose  Oligosaccharides ❖ They are found on cell membrane surfaces attached to proteins ❖ They may function as receptors ❖ Oligosaccharides on blood cells give the groups A, B, O and AB ❖ On the surface of red blood cells three different types of oligosaccharides may be found ❖ All have a chain of sugars: N-acetyl galactosamine, N-acetylglucosamine, galactose and L- fucose 36  Glycoprotein ❖ One major component of saliva is a glycoprotein called mucin ❖ The highly glycosylated properties of mucins make them resistant to proteolysis and able to hold water, giving them the gel-like properties found in mucosal barriers ❖ Mucins have many functions, due mainly to viscosity, for example lubrication, tissue coating and protection as in some antimicrobial functions ❖ Mucins play a role in the non-immune protection against oral cavity such as the formation of protective coatings covering tooth enamel and oral mucosa, which act as a dynamic functional barrier capable of modulating the untoward effects of oral environment ❖ Mucins are also over expressed in lung diseases such as asthma, bronchitis, COPD and cystic fibrosis Polysaccharides ❖ These are complex sugars of high molecular weight ❖ They may be straight chain or branched ❖ There exists two groups, (a) homopolysaccharides and (b) heteropolyssacharides (a) Homopolysaccharides ❖ Produce only one kind of monosaccharide on hydrolysis ❖ E.g. Starch, glycogen and cellulose Starch ❖ It is an important food storage in plants ❖ Consist of two components (i) amylose (straight chain)  (ii)amylopectin (branched) http://academic.brooklyn.cuny.edu/biology/bio4fv/page/1-6branch2.JPG ❖ Hydrolysis of amylose by α - amylase (found in the digestive tract of animals) → maltose and glucose ❖ β–amylase (found in plants) → maltose ❖ The combination of α - amylase and α-1-6 glucosidase hydrolyzes amylopectin to produce maltose and glucose  BIOLOGICAL IMPORTANCE OF STARCH ❖ Starch helps to control body weight, especially when combined with exercise…very little dietary starch is converted to body fat mainly because it is a very inefficient process for the body. Instead starches tend to be preferentially burnt for fuel ❖ Vital for proper gut function ❖ Serve as an important fuel for the brain and active muscles  Glycogen ❖ It is the storage form of carbohydrate in animals ❖ Structurally similar to amylopectin, however glycogen has more branching http://themedicalbiochemistrypage.org/images/glycogen.jpg ❖ Soluble in water ❖ Hydrolysis using α-1,6-glucanmaltohydrolase → maltose; Acid hydrolysis → glucose ❖ Non reducing sugar  BIOLOGICAL IMPORTANCE OF GLYCOGEN ❖ The liver stores glycogen that the body can easily and rapidly convert to energy ❖ Muscles also store glycogen, which they use during periods of intense physical activity ❖ The uterus also stores glycogen during pregnancy to nourish the embryo ❖ High glycogen levels improve endurance, whereas depletion of glycogen is often associated with fatigue  Cellulose ❖ Most abundant extracellular polysaccharide ❖ Structurally similar to amylose ❖ Glucose linked together by β-1,4-glycosidic linkages http://science.cabot.ac.uk/wp-content/uploads/2010/10/ethanol-cellulose.gif  Cellulose ❖ Insoluble in water ❖ Absorbs water ❖ Hydrolysis produces glucose molecules ❖ Cellulase is present in some insects, enabling the digestion of cellulose  BIOLOGICAL IMPORTANCE OF CELLULOSE ❖ In humans cellulose forms a major part of the dietary fiber that is needed for proper digestion ❖ Cellulose cannot be broken down so it passes through our systems basically unchanged, acting as bulk or roughage that helps the movements of our intestines ❖ Among mammals, only those that are ruminants (cud chewing animals like cows and horses) can process cellulose with special bacteria and microorganisms in their digestive tracts ❖ Ruminants are able to absorb the broken-down cellulose and use its sugar as a food source  (b) Heteropolysaccharides ❖ Produce more than one kind of monosaccharide on hydrolysis  Chondroitin ❖ Found in cartilage and a component of cell coat. ❖ It is a major component of extracellular matrix, and is important in maintaining the structural integrity of the tissue ❖ Loss of chondroitin sulfate from the cartilage is a major cause of osteoarthritis ❖ Along with glucosamine, chondroitin sulfate has become a widely used dietary supplement in treating osteoarthritis  Heparin ❖ An anticoagulant and is present in the liver and lung arterial wall. ❖ Heparin binds to a number of plasma proteins which reduces its anticoagulant activity at low concentrations ❖ Heparin has been widely used as clinical anticoagulant drugs for decades during surgery and kidney dialysis  Hyaluronic acid ❖ It is found in animals as components of various tissues: vitreous body, umbilical cord, synovial fluid ❖ Hyaluronic acid helps in promoting the process of wound repair ❖ It can function as scavenger of free radicals (antioxidant) ❖ Hyaluronic acid involvement in tissue repair has led to the development of a series of biomaterials that are widely used to make advanced dressings…used for healing wounds, burns, skin ulcers ❖ Used as a lip filler in plastic surgery ❖ Used in eye surgeries to help replace natural fluids ❖ Anti-aging cream???