Macromolecules and Carbohydrates

• Macromolecules are large, carbon-based organic molecules that are created by the polymerization of smaller subunits.
• Carbohydrates, proteins, nucleic acids, and lipids are the four major types of macromolecules.
• Carbohydrates, lipids, and proteins provide energy to the body, but carbohydrates are the preferred source if available.

Carbohydrates Overview

• Carbohydrates are the primary source of energy for the body.
• They serve as structural components, such as in the cell membrane.
• They make up part of the backbone of nucleic acids like DNA and RNA.
• Carbohydrates play a role in cell identification and signaling.
• Carbohydrates are carbon-based molecules rich in hydroxyl groups.
• Carbohydrates are known as polyhydroxy aldehydes and ketones.
• Glucose (C6H12O6) is the most abundant carbohydrate and the most important simple carbohydrate in human metabolism.
• The general empirical formula for carbohydrates is Cn(H2O)n.

Classification of Monosaccharides

• Monosaccharides are classified by the number of carbon atoms they contain or by their characteristic carbonyl group.
• Classification based on number of carbon atoms: trioses, tetroses, pentoses, hexoses, heptoses, and octoses.
• Classification based on carbonyl group: aldehyde group or ketone group.

Classification of Carbohydrates

• Simple carbohydrates include monosaccharides (single sugar unit) and disaccharides (2 sugar units).
• Complex carbohydrates include oligosaccharides (3 to 10 sugar units) and polysaccharides (10 or more sugar units).
• Homopolysaccharides consist of the same monosaccharide, such as starch and cellulose.
• Heteropolysaccharides consist of different monosaccharides, such as hyaluronic acid.

Monosaccharides Details

• Aldoses contain an aldehyde group e.g. glucose and ribose.
• Ketoses contain a ketone group e.g. fructose and ribulose.
• An aldose has the carbonyl on the first carbon (C1), which is an aldehyde.
• A ketose has the carbonyl on the second carbon (C2), which is a ketone.
• An aldotetrose is a 4-carbon sugar with an aldehyde functional group.
• The most common monosaccharides have 3-8 carbon atoms.
• The suffix "-ose" indicates a molecule is a carbohydrate.
• Prefixes like tri-, tetr-, and pent- indicate the number of carbon atoms.
• Glyceraldehyde (an aldotriose) and dihydroxyacetone (a ketotriose) are the simplest monosaccharides as two 3-carbon trioses.
• Most carbohydrates of interest in biochemistry are aldohexoses or aldopentoses, excluding glyceraldehyde and fructose.
• Monosaccharides cannot be hydrolyzed into any simpler form.
• Monosaccharides can exist in linear form (Fischer's projection), cyclic form (Haworth's formation), or chair conformations.
• Pentoses and hexoses tend to form rings to improve stability as the linear form is not stable.
• Isomers are molecules with the same molecular formula but different structures.
• Aldose-ketose isomerism, D- and L- forms, enantiomers, diastereomers, epimers, pyranose and furanose ring structures, and α- and β-anomers are classifications of isomerism in carbohydrates.
• Monosaccharides possess stereogenic centers, typically carbon atoms that bind four different groups and are called 'asymmetric (chiral)' carbon atoms.
• All carbohydrates contain at least one asymmetrical (chiral) carbon, except dihydroxyacetone.
• Glyceraldehyde (a chiral molecule) is the standard reference molecule, and its asymmetric carbon atoms confer optical activity.
• Molecules rotate polarized light to the right (Dextrorotatory) or to the left (Levorotatory).
• D-glyceraldehyde and L-glyceraldehyde are stereoisomers.
• They differ in the configuration about the chiral carbon furthest from the carbonyl carbon.
• Glyceraldehyde molecules are enantiomers, which are mirror images of each other.
• Diastereomers are pairs of isomers with opposite configurations at one or more chiral centers but not mirror images.
• Epimers are diastereomers that differ in configuration at only one chiral (stereogenic) center, and are not mirror images
• D-glucose and D-mannose are epimers that differ in configuration at C-2.
• D-glucose and D-galactose are epimers that differ in configuration at C-4.
• Monosaccharides are either pyran or furan.
• Six-membered ring monosaccharides are pyranoses.
• Five-membered ring monosaccharides are furanoses.

Important Monosaccharides

• Glucose (C6H12O6) is the most important simple carbohydrate in human metabolism.
• Hormones like insulin and glucagon regulate blood glucose.
• Diabetes mellitus is related to blood sugar problems.
• Fructose (C6H12O6) is a fruit sugar that is a structural isomer of glucose and tastes sweeter.
• Fructose intolerance can lead to fructose accumulation and hypoglycemia.
• Galactose (C6H12O6) comes from dairy products like milk.
• Galactose is a component of lactose (disaccharide) and glycoproteins in brain and nerve tissue.
• Galactosemia is when excess galactose results in cataracts because of enzyme deficiency.
• Most monosaccharides in humans are D-sugars.
• L-arabinose and L-fucose are examples of important sugars that occur in L-form and are found in glycoproteins of plants and mammalian cells.

Disaccharides

• Disaccharides are formed via condensation/dehydration reactions, removing a H2O molecule to join two monosaccharide molecules.
• A glycosidic bond (O-glycosidic bond) links two monosaccharides together.
• Hydrolysis breaks down disaccharides into monosaccharide subunits.
• Two monosaccharides joined form a disaccharide.
• Examples of disaccharides include sucrose (glucose + fructose or table sugar), maltose (glucose + glucose or malt sugar, used to make malted candy), and lactose (glucose + galactose or milk sugar).
• Sucrose, which is commonly called “table sugar”, is composed of glucose and fructose linked through an α(1→2)β glycosidic bond, and is highly sweet and soluble.
• Maltose consists of two glucose molecules joined by an α(1→4) glycosidic linkage, is named “malt sugar”, and is an intermediate product of starch hydrolysis.
• Lactose is exclusively found in the milk and it consists of galactose and glucose in a β(1→4) glycosidic bond; it is less sweet and soluble.

Oligosaccharides

• Oligo means “few."
• Contains 3-10 monosaccharide molecules liberated on hydrolysis.
• Functions include cell recognition and cell adhesion in a cell membrane.
• Often present as glycans: oligosaccharide chains linked to proteins or lipids via N- or O-glycosidic bonds.
• ABO substances are examples of oligosaccharides that are present in most cells of the body and in secretions.
• Oligosaccharides help provide the ABO blood group determinants on the surface of the red blood cells.
• All have a chain of four sugars, which are written in four-letter code of gal - nag – gal – fuc.
• These represent galactose, n-acetylglucosamine, and fucose.
• Fucose is a 6-carbon L-sugar.

Polysaccharides

• These are large molecules with more than 10 monosaccharide units.
• They are joined by O-glycosidic linkages in a chain that may be continuous or branched.
• Storage polysaccharides contain only α-glucose units, e.g., starch and glycogen.
• Structural polysaccharides contain only β-glucose units, e.g., cellulose and chitin.

Polysaccharides details

• Starch serves as the main food storage molecule of plants.
• Starch is a mixture of two polymers named amylose (20%) and amylopectin (80%).
• Glucose units in amylose are linked by α-1,4 glycosidic bonds and the chains are long and unbranched.
• Glucose units in amylopectin are linked in short chains by α-1,4 glycosidic bonds forming branched structure every 24-30 glucose units.
• 1,6 glycosidic bonds form Amylopectin branches .
• Glycogen is a large, branched polymer of glucose residues as a homopolysaccharide readily broken down to yield glucose molecules when energy is needed by the body.
• Glycogen chains linked mainly by α(1→4) glycosidic bonds with α(1→6) glycosidic bonds at branch points, and stored as glycogen predominately in liver and muscle cells.
• The structure of glycogen is identical to amylopectin except that α-(1,6) branching occurs about every 12 glucose units.
• D-ribose and D-2-deoxyribose are examples of five-carbon aldehyde sugars that are present in nucleic acids.
• Deoxyribose is the sugar that lacks an OH group at C-2 (β-deoxyribose).
• Ribose has a five-carbon chain and makes a furanose ring.
• Amino sugar is an amino group substitutes for a hydroxyl e.g Glucosamine.
• N-acetylglucosamine where the amino group is acetylated.
• Heteropolysaccharides, the commonly called glycosaminoglycans (GAGs) or mucopolysaccharides.
• GAGs are long, unbranched chains generally composed of a repeating disaccharide unit consists of amino sugar and acidic sugar.
• Amino sugars include D-acetyl glucosamine and D-galactosamine.
• Acidic sugars include D-glucuronic acid, and L-iduronic acid.
• EX: Chitin, hyaluronic acid, chondroitin sulfate, dermatan sulfate, keratan sulfate and heparin.
• Chitin is the second most abundant polysaccharide next to cellulose in nature.
• Exoskeleton of crustaceans and insects that include lobsters, beetles and spiders contains Chitin.
• N-acetylglucosamine is the building blocks of Chitin which contains β (1--→4) glycosidic bonds.
• Chitin is used as surgical thread that biodegrades to close wounds.
• Hyaluronic acid (Hyaluronate) consist of Long chains of modified glucose units found in synovial fluid in joints, eye and in connective tissues like ligaments, cartilage and skin that provides lubrication and shock absorption properties.
• Heparin is a polysaccharide polymer consisting of two types of monosaccharides: uronic acid and glucosamine; it is an anticoagulant used in preventing blood clotting after surgery and in blood sample tubes.
• Chondroitin 6-sulphate are Repeating disaccharide units composed of glucuronic acid and N-acetylgalactosamine, found in connective tissues, tendons, and cartilage, and used to prevent and manage osteoarthritis.