Carbohydrates: True or False Statements

• Carbohydrates are organic substances containing carbon, hydrogen, and oxygen.
• The empirical formula of simple carbohydrates is Cn(H₂O)n, where 'n' is the number of carbon atoms.
• Defined as polyhydroxy aldehyde or ketone derivatives, and compounds that yield these derivatives upon hydrolysis.
• Simple carbohydrates have many hydroxyl groups (polyhydroxy) and carbonyl functional groups either as aldehyde or ketone.

Functions of Carbohydrates

• Serve as a primary energy source, like glucose, providing 4 Cal/g.
• Have structural roles as glycosaminoglycans in humans, cellulose in plants, and chitin in insects.
• Act as precursors for organic compounds including fats and amino acids.
• Used as drugs, like cardiac glycosides and antibiotics.
• Serve as a storage form of energy such as glycogen in animals and starch in plants.
• Non-digestible carbohydrates like cellulose function as dietary fibers.
• They are constituents of nucleic acids RNA and DNA, for example, ribose and deoxyribose sugar.
• They are involved in lubrication, cellular intercommunication, and immunity.
• They are also involved in detoxification, like glucuronic acid.

Classification of Carbohydrates

• Divided into 4 groups based on the number of sugar units: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

Monosaccharides

• Are the simplest group of carbohydrates.
• Often referred to as simple sugars.
• Cannot be further hydrolyzed.
• All are reducing sugars and strong reducing agents.
• Have the general formula Cn(H2O)n.

Monosaccharides Sub Classification

• Subdivided based on the number of carbon atoms they possess (trioses, tetroses, pentoses, hexoses).
• Classified by the presence of aldehyde (aldoses) or ketone (ketoses) groups.
• Glyceraldehyde is the simplest carbohydrate and called reference sugar.
• Triose sugars contain 3 carbon atoms.
• Ketose sugars contain a ketone group.
• Ketotriose sugars contain a ketone group and 3 carbon atoms.
• Hexose sugars contain 6 carbon atoms.
• Aldose sugars contain an aldehyde group,
  • Aldohexose sugars contain an aldehyde group and 6 carbon atoms.
• Glucose is an important aldohexose.
• Fructose is an important ketohexose.

Biologically Important Monosaccharides

• Glyceraldehyde is an intermediate in glycolysis.
• Dihydroxyacetone serves as a precursor of glycerol, required for triacylglycerol and phospholipid formation.
• D-Erythrose is an intermediate product of carbohydrate metabolism in the hexose monophosphate pathway.
• D-Ribose is a constituent of nucleic acids (RNA) and coenzymes such as ATP, NAD, NADP, and FAD.
• D-Ribulose and D-Xylulose are intermediates in the pentose phosphate pathway.
• L-Xylulose is a constituent of proteoglycans and glycoproteins.
• D-Glucose serves as the primary sugar utilized by the body for energy.
• D-Fructose can be converted to glucose in the liver and used for energy.
• D-Galactose can be converted to glucose in the liver and metabolized. Synthesized in the mammary gland to make lactose of milk.
• D-Mannose is a constituent of glycolipids, proteoglycans, and glycoproteins and blood group substances.
• Sedoheptulose is an intermediate in the pentose phosphate pathway.

Disaccharides

• Contain two molecules of the same or different monosaccharide units.
• Upon hydrolysis, yield two monosaccharide units.
• Two monosaccharide units are joined by a glycosidic bond.
• Lactulose is a synthetic disaccharide containing galactose and fructose. It is neither digested nor absorbed in the intestine

Examples of Disaccharides

• Maltose consists of glucose + glucose with an α1-4 glycosidic bond, and is found in malt sugar.
• Lactose consists of galactose + glucose with a β 1-4 glycosidic bond, and is found in milk sugar.
• Sucrose consists of glucose + fructose with an α 1- β2 glycosidic bond, and is found in sugarcane.
• Isomaltose consists of glucose + glucose with an α 1-6 glycosidic bond, and comes from digestion of amylopectin.
• Cellobiose consists of glucose + glucose with a β 1-4 glycosidic bond, and is derived from the hydrolysis of cellulose.

Oligosaccharides

• Contain 3 to 10 molecules of monosaccharide units, such as maltotriose which is composed of glucose + glucose + glucose.

Polysaccharides

• Contain more than ten molecules of monosaccharide units.
• Further classified into homopolysaccharides and heteropolysaccharides.
• Homopolysaccharides (homoglycans) are polymers of the same monosaccharide units.
• Heteropolysaccharides (heteroglycans) are polymers of different monosaccharide units or their derivatives, such as amino sugars and uronic acids.

Homopolysaccharides

• Starch contains glucose and is found in plants and rice.
• Dextrin contains glucose and comes from starch hydrolysis.
• Glycogen contains glucose and is found in the liver and muscle.
• Cellulose contains glucose and is found in plant fibers.
• Inulin contains fructose and is found in dahlia roots.
• Chitin contains N-acetyl glucosamine and is found in shells of arthropods.

Starch

• The main storage form of carbohydrates in plants.
• An important dietary source for higher animals, including humans.
• Homopolysaccharide composed of α-D-glucose units held by (α-1,4) and (α-1-6) glycosidic bonds.
• Also known as glucosan or glucan.
• Starch granules contain two forms:
  • Amylose (15-20%) in the inner part and amylopectin (80-85%) in the outer part.
• Starches are hydrolyzed by α-amylase (pancreatic or salivary) to produce dextrins, and finally maltose and glucose units.
• Amylase acts specifically on D (α-1,4) glycosidic bonds.
• Amylose constitutes 15-20% of starch, has a linear unbranched structure, a molecular weight of 60 kDa, and 250-300 glucose residues joined by α 1-4 glycoside links that form a blue color with iodine.
• Amylopectin constitutes 80-85% of starch, has a highly branched structure with branch points occurring every 20-30 glucose units, a molecular weight of 500 kDa, and α 1-4 linkages between glucose residues with α 1-6 glycosidic linkages at branch points, and a reddish-violet color with iodine.

Glycogen

• Is the storage form of energy in animals (animal starch).
• Mainly present in skeletal muscles and liver.
• Has a primary glycosidic bond that connects the carbon 1 atom of one glucose molecule to the carbon 4 atom of the next glucose molecule (α 1-4 linkage).
• Has branching points where carbon 1 connects to carbon 6, and the chain branches approximately every 11 to 18 glucose resides.
• Similar in structure to amylopectin, but has more glucose branches.

Cellulose

• Exclusively present in plants, the most organic substance in the plant kingdom.
• Is a key component of plant cell walls.
• Absent in animal bodies
• Composed of β-D glucose units linked by β (1-4) glycosidic bonds.
• Cellulose isn't digestible by humans.

Biomedical Importance of Cellulose

• Major fiber constituent, a non-digestible carbohydrate.
• Decreases glucose and cholesterol absorption from the intestine.
• Increases the bulk of feces and it acts as a stool softener to avoid constipation.
• Reduces the chance of diseases like cardiovascular disease, colon cancer, diabetes, and diverticulosis.

Heteropolysaccharides

• Contain more than 10 different repeating units.
• Also known as mucopolysaccharides (MPS) or glycosaminoglycans (GAGs).
• Acidic non-sulfated examples include hyaluronic acid.
• Acidic sulfated examples include heparin, heparan sulfate, chondroitin sulfate, dermatan sulfate, and keratan sulfate

Blood Group Substances

• Blood groups (A, B, AB, and O) possess antigens (of erythrocyte membranes) containing carbohydrates as glycoproteins or glycolipids.
• N-Acetylgalactosamine, galactose, fucose, and sialic acid are found in blood group substances.
• Carbohydrate content plays a determining role in blood grouping.

Structure of Glucose

• The structure of glucose may be represented in the straight chain structural formula (Fisher projection) or cyclic form (Ring structure or Haworth projection). Monosaccharides are mainly present in the ring form when in solution.
• Aldehyde (CHO) or ketone (C=O) groups of monosaccharides react with a hydroxy (OH) group forming a bond hemiacetal or hemiketal respectively.
• The aldehyde group of glucose at C-1 reacts with alcohol (OH) group of C-5 or C-4 to form either six-membered rings (glucopyranose) or smaller five-membered rings (glucofuranose)
• In glucose the glucopyranose is much stable than the glucofuranose ring
• In fructose the more stable form is fructofuranose.

Fischer Projections of Glucose

• Representations showing configurations around chiral centers.
• Used to depict the stereo chemistry of carbohydrates.

Haworth projection

• These formulae show six-membered (pyranose) or five-membered (furanose) ring structures of monosaccharides.
• Cyclic forms of glucose are known as α-D-glucopyranose and α-D-glucofuranose.
• Blood Glucose is more thermodynamically stable in the β-D Glucopyranose form

Optical Activity and Stereoisomerism

• Nearly all carbohydrates, with the exception of Dihydroxyacetone (DHA), possess asymmetrical carbon atoms within their structures.
• The presence of asymmetric carbon atoms leads to two key properties: optical activity and stereoisomerism.
• Asymmetric (Chiral) carbon is a carbon atom bonded to four unique atoms or groups of atoms.

Optical Activity

• Occurs when a substance has the ability to rotate the plane of polarized light either to the right or to the left.
• Refers to how solutions behave when placed in a polarimeter.
• If the solution rotates polarized light towards the right, it is dextrorotatory (d/+)
• Substances that rotate polarized light toward the left are laevorotatory (l/-).

Stereoisomerism

• Caused by the presence of chiral carbon atoms.
• Stereoisomers refer to compounds that have the same chemical and molecular formula, but show different configurations.
• Types of stereoisomerism include D and L isomers (enantiomers), epimers, and anomers.

D- and L-Isomers

• The position of the H and OH groups around the carbon atom closest to the terminal primary alcohol carbon determines whether a monosaccharide is a D- or L-isomer.
• If the OH group on this carbon is on the right, it's a D-series carbohydrate; if it's on the left, it's part of the L-series.

Enantiomers

• Mirror images of each other, differing in their spatial arrangement around chiral carbons.
• D-Glucose and L-Glucose are enantiomers.

Number of Isomers

• Number of isomers= 2 to the power of n, where 'n' equals the number of chiral carbon atoms (Van't Hoff's rule).
• Glucose has 4 asymmetric carbons, thus, it has 16 isomers.
• Only D-glucose or D-sugars are used by the human body, and enzyme machinery of cells metabolize D-series of monosaccharides

Anomers

• Relate to the cyclic structure of monosaccharides.
• Hydroxyl group of monosaccharides can react with aldehyde or keto groups making hemiacetal and hemiketal, respectively. Therefore, the aldehyde group of glucose reacts with the alcohol at C5 to form the α and β configurations of cyclic hemiacetals.
• Carbon 1 after ring formation becomes asymmetric and it is called as anomeric carbon.
• The configuration of glucose is conveniently represented by Fischer formulae or Haworth projection formulae.
• The designation α means the OH-group attached to carbon 1 is below the plane of the ring, whereas β signifies it being above the plane.

Epimers

• If two monosaccharides are in different configurations around a single asymmetric carbon (other than anomeric ones), they are known as epimers to each other.
• Glucose and Galactose are C4-epimers.
• Glucose and mannose are epimers with regard to carbon 2.

Racemic Mixture

• When equal concentrations of D- and L-isomers are present.
• Does not show any optical activity.

Mutarotation

• Is the automatic change in the specific optical rotation to a steady fixed rotation.
• Characterized by the interconversion of α and β forms of D-glucose.
• An equilibrium mixture will contain 63% β-anomer and 36% α-anomer of glucose, including a 1% open chain form

Inversion

• Sucrose is dextrorotatory (+66.5°).
• Hydrolyzed mix of glucose and fructose displays levorotatory activity; called 'inversion.'
• Optical activity of fructose is (-92°), and glucose is (+52.7°). Sucrase, converts sucrose, is also referred to as invertase

Monosaccharide Derivatives

• Include sugar acids, sugar alcohols, deoxysugars, aminosugars, aminosugar acids, and ester formation.
• Sugar acids are produced through oxidation of the aldehyde group (C1) to - COOH to make gluconic acid.
• Oxidation of the terminal alcohol group (CH2OH to - COOH) makes Glucuronic acid.
• Reduction of monosaccharides by reducing agent the aldehyde or keto group is reduced to corresponding alcohol making sugar alchols.
• Reduction of fructose produces compounds like D-sorbitol or D-mannitol.
• Sorbitol and dulcitol in high volume can cause swelling of tissue and can cause other pathological conditions such as cataract, peripheral neuropathy, and nephropathy.
• Deoxysugars possess a hydroxyl group replaced by a hydrogen atom- most importantly 2-deoxyribose present in structure of DNA.
• Aminosugars have an amino group (NH2) replacing the hydroxyl group on the second carbon e.g. glucosamine (GluN), galactosamine (GlaN) and mannosamine (ManN).
• Aminosugars are important constituents of glycosaminoglycans (GAGs) and some types of glycolipids and glycoproteins.
• Several antibiotics contain aminosugars which are important for their antibiotic activity.
• Aminosugar acids are formed by the addition of acids to aminosugars.
• Addition of pyruvic acid to mannosamine yields neuraminic acid.
• The N-acetyl derivatives of the aminosugar acids are called sialic acids e.g. N-acetyl neuraminic acid (NANA) which are found in glycolipids and glycoproteins.
• Hydroxyl groups of monosaccharides form esters with phosphoric acids.

Glycosidic Bond

• Is the linkage formed between the -OH group of anomeric carbon of one sugar and another -OH or -NH group of another compound involving the loss of water.

Physiologically Important Glycosides

Include the following:

• Glucovanillin (vanillin-D-glucoside) is the natural flavoring for vanilla.
• Cardiac glycosides are steroidal glycosides with digoxin and digitoxin as examples, and are used to treat congestive heart failure by stimulating muscle contraction.
• Streptomycin is an antibiotic glycoside used to treat tuberculosis.
• Ouabain, an inhibitor of the Na+ – K+ ATPase, which is used to treat congestive heart failure by blocking active transport of Na+.
• Anthracycline glycosides are used to treat different cancers. Daunorubicin is used to treat leukemia, and doxorubicin is used to treat a wide range of cancers.

Test your knowledge of carbohydrate chemistry. See if you can identify the incorrect statements about carbohydrates, heteropolysaccharides, and monosaccharides. Challenge yourself with these true or false questions.