Carbohydrates: Structure and Function

Introduction to Carbohydrates

• Carbohydrates are naturally occurring polyhydroxy aldehydes and ketones
• Monosaccharides are simple sugars with 3 to 7 carbon atoms
• Monosaccharides contain one aldehyde or ketone group, resulting in two classifications:
  • Aldose: aldehyde group
  • Ketose: ketone group
• Carbohydrate family names end with "-ose"
• Examples of monosaccharides include glucose, fructose, and ribose
• Aldehyde groups are always at the end of the carbon chain
• Ketone groups are always on the second carbon of the chain
• Carbon chain length varies in aldoses and ketoses:
  • Tri- (3), Tetra- (4), Penta- (5), Hexa- (6), Hepta- (7) carbons
• Relevant functional groups are Aldehyde (C=O), Ketone (C=O), and Hydroxyl (-OH)
• Monosaccharides form disaccharides and polysaccharides (polymers)
• Monosaccharides react with alcohols, lipids, and proteins to form biomolecules

Handedness of Carbohydrates and Fischer Projections

• Glyceraldehyde is an aldotriose and the simplest chiral carbohydrate
• Chiral compounds lack symmetry and exist as enantiomers (mirror images)
• Carbohydrates come in D- (right-handed) and L- (left-handed) forms
• Fischer Projections represent chiral carbon atoms as intersections of two lines
  • Horizontal lines: bonds projecting out
  • Vertical lines: bonds projecting behind the plane

Aldotetroses and Diastereomers

• Aldotetroses have two chiral carbons, resulting in four isomeric forms
• Erythrose and Threose are mirror-image pairs (enantiomers) or non-mirror-image pairs (diastereomers)

D and L Nomenclature

• D and L nomenclature derives from the Latin dextro for "right" and levo for "left"
  • D form: Hydroxyl group on the rightmost chiral carbon points right
  • L form: Hydroxyl group points left

Structure of Glucose and Other Monosaccharides

• D-Glucose, also called dextrose or blood sugar, is the most abundant monosaccharide
• D-Glucose serves as a source of energy in most organisms
• Glucose exists in open-chain and cyclic forms
  • Open-chain: Hydroxy aldehyde
  • Cyclic hemiacetal forms at the anomeric carbon at C1, creating two possible anomers (α and β)
• Mutarotation: Equilibrium between open-chain and cyclic forms of glucose in water
• Cyclic Forms differ at the anomeric carbon C1
  • α-anomer: OH group opposite to CH2OH
  • β-anomer: OH group same side as CH2OH
  • Anomers only differ in the positions of substituents at carbon 1
  • Carbon 1 is the Anomeric Carbon Atom
  • -CH2OH group (the last carbon in the chain) in D sugars is always above the plane of the ring

Monosaccharide Structure Summary

• Monosaccharides are polyhydroxy aldehydes or ketones with three to seven carbon atoms
• D and L isomerism are based on the chiral carbon furthest from the carbonyl group
• Fischer projections: D sugars have the –OH on the right, and L sugars have the -OH on the left
• Common monosaccharides: Glucose, Fructose, and Ribose
• D-Glucose, and other 6-carbon aldoses, form cyclic hemiacetals where –OH groups on chiral atoms are conventionally represented.
• In glucose, the hemiacetal carbon (anomeric carbon) is chiral, and α and β anomers differ in the orientation of the –OH groups on this carbon
  • The α anomer has the –OH on the opposite side from the –CH2OH
  • The β anomer has the -OH on the same side as the -CH2OH

Important Monosaccharides

• Monosaccharides can form multiple hydrogen bonds through their hydroxyl groups and are generally high-melting, white, crystalline solids soluble in water.
• Common Monosaccharides:
  • Aldohexoses: Glucose, Mannose, Galactose
  • Aldopentoses: Ribose, Deoxyribose
  • Ketoses: Fructose
• Predominantly D-sugars are used in biochemistry and are important for cellular metabolism
• Sweet-Tasting and Digestible Sugars: Non-toxic and essential for energy in humans

Reactions of Monosaccharides

• Aldehydes are oxidized to carboxylic acids (RCHO → RCOOH)
• Ketoses can also act as reducing sugars because of keto-enol tautomerism
• Keto-enol tautomerism describes the equilibrium that results from a shift of a hydrogen atom and a double bond, possible with with a hydrogen atom on a carbon adjacent to a carbonyl carbon
• Carbohydrates that react with mild oxidizing agents are reducing sugars
• Glycoside Formation: A hemiacetal reacts with an alcohol to form a glycoside with the loss of water
• Because glucose and other monosaccharides are cyclic hemiacetals, they react with alcohols to form acetals called glycosides
• A glycosidic bond links the anomeric carbon and the -OR group
• In larger molecules, monosaccharides are connected to each other by glycosidic bonds
  • Hydrolysis reverses and breaks these gylcosidic bonds (digestion of carbohydrates)
• Monosaccharides can form phosphate esters for metabolism
• Phosphorylation is the combination of a hydroxyl group (-OH) with PO3-2
• Phosphate esters of alcohols contain a -PO3-2 group bonded to the oxygen atom of an -OH group
• The -OH groups of sugars can add -PO3-2 groups to form phosphate esters the same way

Common Disaccharides

• Disaccharides include sucrose (common table sugar)
• Disaccharide Structure: Two monosaccharides linked by a glycosidic bond (α or β link in cyclic monosaccharides)
• Glycosidic bonds create a 1,4 link between C1 of one monosaccharide and C4 of the second monosaccharide
• Sucrose is made of Glucose + Fructose (common table sugar)
• Maltose is made of Glucose + Glucose
• Lactose is made of Galactose + Glucose (milk sugar)

Important Polysaccharides Based on Glucose

• Polysaccharides are polymers of tens, hundreds, or thousands of monosaccharides linked by glycosidic bonds
• Starch (Amylose and Amylopectin)
  • Amylose: Long chain of D-glucose, coiled structure
  • Amylopectin: Branches every 25 glucose units
• Glycogen
  • Energy storage in animals
  • Highly branched (similar to amylopectin, but has more branches)
• Cellulose
  • Structural polysaccharide found in plants
  • Has β-1,4-glycosidic bonds (humans cannot digest)
• Modified Polysaccharides
• Monosaccharides with modified functional groups are components of a wide variety of biomolecules
  • Hyaluronate: Found in synovial fluid
  • Chondroitin: Found in cartilage and tendons
  • Heparin: Anticoagulant found in the liver

Test your knowledge of carbohydrate structures, including disaccharides like maltose, and polysaccharides like glycogen and cellulose. Questions cover glycosidic bonds, functional groups, anomeric carbons, and mutarotation.