Carbohydrates: Structure, Types and Biological Roles

• Carbohydrates include monosaccharides, disaccharides, polysaccharides, and glycoproteins.

Learning Outcomes

• Recognize the main classes of carbohydrates and their structural and chemical relationships.
• Understand the basic structure and important reactions of monosaccharides.
• Appreciate the biological significance and function of monosaccharides.
• Describe the structure and biological roles of disaccharides.
• Understand key polysaccharides and glycoproteins and how their structure relates to biological function.

Introduction to Carbohydrates

• Carbohydrates (CHO) define simple and complex sugars.
• Along with lipids, CHO provides energy for all biochemical processes.
• CHO comprises carbon, hydrogen, and oxygen and has many -OH hydroxyl groups.
• The ratio of H and O is the same as in water (H₂O), with the empirical formula CH₂O.
• Carbohydrates share a basic structure.
• Compositional groups dictate functional role and importance.
• Plants store CHO as starch, while animals store it as glycogen.

Types of Carbohydrates

• Simple carbohydrates include monosaccharides and disaccharides.
• Complex carbohydrates include oligosaccharides and polysaccharides.
• Polysaccharides include glycogen, starches, and fibers.

Carbohydrate Classification and Examples

• Monosaccharides examples: glucose, fructose, and galactose.
• Disaccharides examples: maltose, lactose, and sucrose.
• Polysaccharides examples: starch, glycogen, and cellulose.
• Disaccharides consist of 2 linked monosaccharides.
• Polysaccharides are oligosaccharides.

Monosaccharides: Structure and Properties

• Monosaccharides have many hydroxyl (-OH) groups.
• The empirical formula ratio is 1C:2H:1O, represented as (CH₂O)n.
• They're aldehydes or ketones with one or more -OH groups.
• A monosaccharide with an aldehyde group is an aldose; with a ketone group, it's a ketose.

Monosaccharides: Naming and Carbon Number

• Monosaccharides are named by aldehyde or ketone groups and the number of carbon atoms.
• Glyceraldehyde and dihydroxyacetone, with 3-carbon backbones, are trioses.

Number of Carbons in Monosaccharides

• n=3 (triose).
• n=4 (tetrose).
• n=5 (pentose).
• n=6 (hexose).
• n=7 (heptose).

Naming Analogy with Amino Acids

• n=3 (tripeptide).
• n=4 (tetrapeptide).
• n=5 (pentapeptide).
• n=6 (hexapeptide).
• n=7 (heptapeptide).

Triose Sugars

• Glyceraldehyde contains one chiral carbon and can exist in D- and L- forms as optical isomers.
• Tetroses, pentoses, hexoses, and larger monosaccharides can contain more than one chiral carbon and are diastereoisomers.

Monosaccharides: Isomers and Stereoisomers

• Carbohydrates exist in multiple isomeric forms.
• Monosaccharides with more than 3 carbon atoms have multiple asymmetric carbon atoms.
• They can exist as enantiomers (non-superimposable mirror images) and diastereoisomers (not mirror images).
• Enantiomers/diastereoisomers are designated as D- or L- by convention.
• Stereoisomers are often indistinguishable chemically but differ in optical properties.
• D- and L- forms are dictated by the chiral carbon atom furthest away from C1 (at the top).

D- and L- Configuration

• The -OH group on the chiral carbon atom furthest from the aldehyde or ketone end determines D- or L-configuration.
• If the -OH is on the right, it's D; if on the left, it's L.
• Nearly all naturally occurring carbohydrates are members of the D-family.

Optical Isomers

• Can exist as laevorotatory or dextrorotatory forms.

Fischer Projections

• Horizontal lines represent bonds projecting out of the page.
• Vertical lines represent bonds projecting backwards behind the page.
• The aldehyde or ketone group (most oxidized) is at the top.
• A chiral carbon atom is at the intersection of horizontal and vertical lines (4 different groups attached).

Common Monosaccharides

• Sugars are diastereoisomers and differ only in configuration at a single asymmetric center, named epimers.
• D-Glucose and D-Galactose are epimeric at C-4.
• D-Glucose and D-Fructose are not epimers.

Common Monosaccharides Summary

• Includes triose (glyceraldehyde, dihydroxyacetone), pentose (ribose, ribulose), and hexose sugars (glucose, galactose, fructose).

Cyclic Structure Formation

• Monosaccharides with 5+ carbons usually form cyclic structures.
• Due to aldehyde or ketone group reacting with a -OH group at the other end.
• The reaction naturally forms a hemiacetal or hemiketal.

Anomeric Forms

• α form: the -OH group is below the ring.
• β form: the -OH is above the ring (BUDA - Beta is Up, Down is Alpha).
• Shown with green labels for carbon 1 and 6 in each structure.
• Sugars do not exist predominantly in the open-chain form but cyclize into a ring form.

Isomers of Glucose

• The aldehyde portion reacts with the C-5 hydroxyl group, forming a cyclic intramolecular hemiacetal (5-carbon pyranose ring).
• This process leads to 2 isomers of glucose, called α and β.
• They differ in the location of the -OH group attached to the hemiacetal C (C1), known as anomers.
• In α form, the -OH is below the ring.
• In β form, the -OH is above the ring.

Haworth Projections

• Alpha and Beta-anomeric forms.

Monosaccharides - Reactions

• Fehling's solution: monosaccharides change the solution from blue to a red precipitate (both aldehyde & ketone).
• Benedict's reagent: a blue-copper solution yields a brick-red precipitate (copper II oxide).
• Benedict's reagent changes color with a reducing sugar.
• Cu²⁺ reduces to Cu⁺ by gaining an electron.
• The carbonyl group oxidizes to carboxylic acid (COOH).

Monosaccharides - Reducing Agents

• Tollen's reagent: diamminesilver I complex forms a silver precipitate, creating a 'silver mirror' in the test tube.
• Reactions rely on oxidation-reduction.
• Fehling's, Benedict's, and Tollen's reagents are oxidizing agents.
• Aldehyde-containing monosaccharides can be reduced, making them reducing agents (reducing sugars).
• All monosaccharides and common disaccharides except sucrose are reducing sugars.

Reducing Sugars

• A chemical reaction where the anomeric carbon has an OH group is a reducing sugar.
• All monosaccharides are reducing sugars.

Chemical Reactions and Reduction of Sugars

• Reducing sugars can react with chemical reagents like Benedict's solution and reduce the reactive compound.
• Non-reducing sugars include sucrose and raffinose, in addition to stacchyose.
• Benedict's or Fehling's solution breaks down a reducing sugar to form a red precipitate.

Use of Aldehyde Group

• An aldehyde group can be reduced to alditol.
• Sodium borohydride (NaBH4) converts D-glucose to D glucitol (or D-sorbitol) by converting carbonyl group to an alcohol (alditol).

Aldose Reductase

• In the body, the enzyme aldose reductase acts on glucose to form sorbitol.
• The aldehyde group reduces by the enzyme in a reaction also involving the co-enzyme NADPH.

Monosaccharides: Structure and Cyclization

• Includes whether monosaccharides form cyclic structures, and what is formed when a ketone bonds with alcohol.
• Defines an anomer.
• Glucose cyclization creates isomers.
• Describes intramolecular hemiacetals in discussing sugars.

Hemiacetals

• When a ketone reacts with an alcohol, a hemiketal is formed.
• Anomers are isomers differing in bond arrangement around the hemiacetal carbon.
• D-glucose reacts with the C-5 hydroxyl group, creating a chiral carbon (C1); the alpha-isomer's C1 hydroxyl group is below the ring; beta-isomer's is above the ring.

Intramolecular Hemiacetal

• An aldehyde sugar forms an intramolecular hemiacetal when the carbonyl group of the monosaccharide reacts with a hydroxyl group on one of the other carbon atoms.

Biological Importance of Glucose

• Glucose metabolizes in the body as a fuel to produce energy.
• Regulates in the body by homeostatic mechanisms (e.g., insulin, glucagon, cortisol, GLP-1, GIP).
• Excess glucose is temporarily stored in the liver and muscle as glycogen, useful for fasting.
• Industrially, used to make vitamin C, citric acid, bioethanol, gluconic acid, and sorbitol.
• Sorbitol, a glucose substitute in chewing gum, is sweet-tasting but prevents bacteria and dental plaque.

Biological Importance of Fructose

• Also know as levulose or fruit sugar.
• The sweetest of all simple sugars, found in honey and corn syrup.
• Biologically derived from table sugar (sucrose) digestion.
• Produced anaerobically by yeast/bacteria fermentation (produces ethanol).
• Fructose undergoes Maillard reaction with amino acids, important in food industry.
• Fructose malabsorption leads to increased fructose which can cause Irritable Bowel Syndrome.
• Some plants store fructose as a polymeric form known as inulin (not insulin), a food reserve equivalent to starch.

Biological Importance of Galactose

• Less sweet than glucose, found in hemicellulose as galactan, can be hydrolysed to galactose.
• Important component of blood group antigens.
• Can be metabolized to glucose by the Leloir Pathway.
• Converted to galactose in humans by hexogenesis process.
• Galactosaemia: defects in galactose metabolism, causes build up of blood concentration (very serious genetic disorder in newborns).

Disaccharides: Formation and Examples

• A condensation reaction between two monosaccharides forms a disaccharide and eliminates water (H₂O).
• C₆H₁₂O₆ + C₆H₁₂O₆ yields C₁₂H₂₂O₁₁ + H₂O.

Examples

• Sucrose: glucose + fructose from sugar cane and sugar beet.
• Lactose: glucose + galactose as milk sugar.
• Maltose: glucose + glucose as malt sugar.
• Trehalose: glucose + glucose made by plants and fungi.
• Cellobiose: glucose + glucose as a breakdown product of cellulose.

Maltose Formation and Structure

• Maltose derives from the condensation of glucose and glucose with an α(1-4) glycosidic linkage.
• Water releases during this process.
• The reverse is called hydrolysis.

Lactose Formation and Structure

• Derived from condensation of galactose and glucose (forms a β-1-4 glycosidic linkage).

Sucrose Formation and Structure

• Sucrose derives from the condensation of glucose and fructose.
• The anomeric carbon of both sugars is involved in glycosidic bond formation.
• The -OH of C1 in D-glucose and -OH of C2 in D-fructose participates in bond.
• Free -OH is not on the anomeric carbon, therefore, sucrose is a non-reducing sugar.

Disaccharides

• Sucrose is composed of α D-glucopyranose and β D-fructofuranose (α 1-2 glycosidic bond), is non-reducing, and hydrolyzes to glucose and fructose via sucrase.
• Lactose is composed of β D galactose and α D glucose (β 1-4 glycosidic bond), contains free anomeric carbons, and may show in urine during late pregnancy and lactation.
• Maltose is composed of 2αD-glucose (α 1-4 glycosidic) and contains free anomeric carbons, therefore reducing sugar.
• Trehalose is composed of 2αD-glucose (α 1-1 glycosidic) and does not contain free anomeric carbons, and is therefore a non reducing sugar.

Components and Bonding

• Maltose and sucrose formation.
• The bond between the 2 monosaccharide units in sucrose involves both anomeric carbons, lacking a free -OH, making it a non-reducing sugar.

Four Classes of Large Biomolecules

• Living things are made up of 4 classes of biological molecules: carbohydrates, protein, nucleic acids, and lipids.
• Macromolecules are polymers, built from smaller building blocks called monomers.
• A polymer is a long molecule with similar building blocks.
• Three of the four classes of life's organic molecules are polymers.

Dehydration Synthesis

• Dehydration removes a water molecule, forming a new bond.
• Hydrolysis adds a water molecule, breaking a bond.

Polysaccharides: repeating units of mono- or disaccharides.

• Homopolysaccharides (homoglycans) consist of a single type of sugar (monomeric unit).
• Heteropolysaccharides (heteroglycans) consist of 2+ types of sugar units
• Defined by monomeric units, sequence of sugar units, glycosidic bonds linking monomers, number of sugar units, and structural branching.
• Can store energy or be a structural polysaccharide.

Starch

• Storage polysaccharide with 2 forms which include amylose and amylopectin (homopolymer).
• Amylose is a linear, unbranched chain of α-D-glucose units (up to 4000 units) with α-1,4-glycosidic bonds.
• Has both a reducing and non-reducing end.
• Amylopectin backbone has glucose, branches lead to one reducing and many non-reducing ends.
• Branches attach to the main chain from the C1 of one α-D-glucose to the C6 hydroxyl group of α-D-glucose in the main chain
• Branches are linked via α-1,6 glycosidic bonds
• Main chain are linked via α-1,4 glycosidic linkage

Complex Carbohydrates

• Polysaccharides: consist of long carbohydrate chains of monosaccharides linked by glycosidic bonds, which include alpha bonds (starch) and beta bonds (found in fiber).

Glycogen

• Storage polysaccharide which represents a major energy store in human and animals.
• Is a homopolymer.
• Highly branched molecule structurally similar to amylopectin.
• Has more numerous α-1,6 glycosidic linkages (thus higher molecular weight).
• Contains a single reducing end and numerous non-reducing ends.
• Humans and other vertebrates store in liver and muscle cells.
• Glycogen contains more and shorter branches than amylopectin

Cellulose

• Structural polysaccharide which represents a major structural component of wood and plant fibers.
• Made of a linear chain of B-D-glucose (3000 units).
• They are linked by B-1,4 glycosidic bonds.
• It is the most abundant polysaccharide.

Chitin

• Structural polysaccharide which makes up protective exoskeletons of arthropods (not in humans).
• Is most similar to cellulose structure and protein keratin's function.
• Composed of the glucose derivative N-acetylglucosamine linked by B-1,4 bonds.

Glycoproteins

• Sugars are attached either to amide nitrogen in side chain of asparagine (N-linkage) or carboxyl to -OH group in side chain of serine or threonine (O-linkage)
• This is described as glycosylation
• N-linked glycoproteins have a common pentasaccharide core (3 mannoses and 2 N-acetylglucosamines)

Glycosaminoglycans

• Also known as a proteoglycans.
• Proteins are attached to a special type of polysaccharide called glycosaminoglycans (95% of weight).
• Includes at least one of the two sugars (glucosamine or galactosamine) in the repeating units has a negatively charged COO- or SO42- group.
• Major types include: chondroitin sulphate, keratan sulphate, heparan sulphate, hyaluronate

Mucins

• Also known as mucoproteins.
• Contain a protein component extensivelu glycosylated to serine or threonine residues by N-acetylgalactosamine.
• Defined by the protein backbone known as the variable number of tandem repeats (VNTR). Vntr region is glycosylated.
• Cys-rich domains facilitate the polymerisation.

Glycoproteins in Plasma Membrane

• Glycoproteins present in the plasma membrane
• The cell membrane is composed of a lipid bilayer with some proteins and glycoproteins embedded in it.
• The hydrophobic tails of the lipids are placed towards the middle of the membrane. Cholesterol is also found in the membrane.

Lecture Summary

• Covered main classes of carbohydrates and how they are related structurally and chemically.
• Examined the basic structure and function of monosaccharides and disaccharides.
• Appreciate some key polysaccharides and their functional roles and importance.