Introduction to Carbohydrates (CHO)

• Biochemistry is a science focusing on chemical reactions within living cells.

Importance of CHO

• Carbohydrates (CHO) serve as a source of energy.
• CHO are components of DNA, RNA, and nucleotides.
• CHO are the source of certain vitamins, hormones, and enzymes.
• CHO are present in the cell walls of plants and bacteria.
• CHO are components of glycoproteins, glycolipids, and mucopolysaccharides.
• CHO are major antigens in nature.

Learning Outcomes

• Define and classify carbohydrates.
• Recognize the importance of carbohydrates.
• Differentiate the types of monosaccharides.
• Explain the different types of isomerism.

Carbohydrates (CHO)

• Carbohydrates are organic substances composed of carbon, hydrogen, and oxygen.
• The general formula for carbohydrates is Cn(H2O)n.
• The ratio between hydrogen and oxygen is 2:1, similar to water.
• Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones and their derivatives.

Classification of Carbohydrates by Hydrolysis Products

• Monosaccharides contain one sugar unit and cannot be hydrolyzed; examples include glucose, mannose, galactose, and ribose.
• Disaccharides contain two sugar units; examples include lactose, sucrose, and maltose.
• Oligosaccharides contain 3-10 sugar units; raffinose is an example.
• Polysaccharides contain more than 10 sugar units; examples include starch and glycogen.

Monosaccharides

• Monosaccharides are the simplest carbohydrate units, containing one sugar unit with a general formula of Cn(H2O)n.

Classification of Monosaccharides by Functional Group:

• Aldose: Monosaccharides containing an aldehyde group (CHO).
• Ketose: Monosaccharides containing a ketone group (C=O).

Monosaccharides by Carbon Number

• Trioses contain 3 carbons.
• Tetroses contain 4 carbons.
• Pentoses contain 5 carbons.
• Hexoses contain 6 carbons.

Trioses

• Trioses contain 3 carbons.
• Aldotriose, like Glyceraldehyde, is an example of a triose.
• Ketotriose, like Dihydroxyacetone, is another example of a triose.

Tetroses (4C)

• Tetroses are monosaccharides containing four carbon atoms.
• The suffix "-ulose" indicates a keto group.

Pentoses (5C)

• Pentoses are monosaccharides containing 5 carbon atoms.
• Ribose is used to construct nucleic acids.
• Ribose is part of the structure of ATP and GTP.
• Ribose can be found in the structure of coenzymes like NAD, NADP, and FAD.

Hexoses (6C)

• Hexoses are monosaccharides containing 6 carbon atoms.

Asymmetric (Chiral) Carbon Atom

• An asymmetric carbon atom is a carbon atom attached to 4 different groups or atoms. Any substance containing an asymmetric carbon atom exhibits optical activity and isomerism.

Optical Activity

• Optical activity refers to a substance's ability to rotate plane-polarized light to the right or left.
• If a substance rotates light to the right, it is dextrorotatory, denoted as (d) or (+).
• If it rotates to the left, it is levorotatory, denoted as (l) or (-).
• Glucose, containing 4 asymmetric carbon atoms, is dextrorotatory and named dextrose.
• Fructose, with 3 asymmetric carbon atoms, is levorotatory and sometimes called levulose.

Optical Isomerism

• Optical Isomerism: compounds that share the same chemical structure but differ in the arrangement of group(s) or atom(s) attached to chiral carbon atoms.
• Sugars with one asymmetric carbon atom will have 2 isomers.
• The máximum number of isomers equals 2^n, where n equals the number of chiral carbon atoms.
• Half the isomers are D and half are L.

Isomerism Types: Configuration Isomers (Enantiomers)

• Configuration isomers (Enantiomers) can be D or L isomers.
• They are mirror images and cannot be superimposed due to the presence of two spatial arrangements of groups linked to the asymmetric atom.
• In D-Glyceraldehyde, the hydroxyl group (OH) is attached to the asymmetric carbon on the right side.
• In L-Glyceraldehyde, the hydroxyl group (OH) is attached to the asymmetric carbon on the left side.
• Naturally occurring sugars are in the D series.

D and L Isomers of Monosaccharides

• Sugars contain more than one asymmetric carbon atom. D or L denotes the location of the OH group to the right or left of the subterminal carbon atom.
• Half of stereoisomers are D, and the other half are L.

Epimers

• Epimers are isomers with more than one asymmetric carbon, where all carbons are the same except one.
• Glucose and mannose are epimers at carbon 2.
• Glucose and galactose are epimers at carbon 4.

Aldose & Ketose Isomerism

• Aldose & Ketose isomers share the same molecular formula but differ in their functional groups with examples glucose and fructose.

Anomeric Carbons & Anomers

• Anomeric carbons are asymmetric carbon atoms obtained from the active carbonyl sugar group, either C1 in aldose or C2 in ketose.
• Anomers are isomers resulting from a change in the position of the OH group attached to the anomeric carbon.
• Alpha and beta glucose are 2 anomers
• Alpha and beta fructose are 2 anomers.

Monosaccharide Properties: Ring (Cyclic) Structure of Sugars

• The cyclic form results from a reaction between the active group, C=O (carbonyl), of the aldehyde group in aldoses or the ketone group in ketoses and a hydroxyl group at carbon number 5.
• Furanose contains a 4 carbon ring.
• Pyranose contains a 5 carbon ring.

Haworth Structure

• Haworth structures result from the reaction between a carbonyl and hydroxyl group in the same molecule.
• All OH groups on the right side of the Fischer projection are written downwards in the Haworth structure.
• All OH groups on the left side of the Fischer projection written upwards in the Haworth structure.
• In cyclic glucose, a new asymmetric center forms at C1 (anomeric carbon). Alpha and beta are stereoisomers produced.
• α (OH below the ring)
• ß (OH above the ring)

Importance (Functions) of Hexoses

• Glucose "grape" is the most important sugar of carbohydrates.
• Glucose is the main sugar in the blood.
• Glucose is one of the major energy sources in the body.
• Galactose: It can be converted into glucose in the liver and is synthesized in the mammary gland to make lactose of milk (milk sugar).
• Fructose (fruit sugar): It can be converted into glucose in the liver and is the main sugar of semen.
• Mannose is a constituent of many glycoproteins.