Carbohydrates and Monosaccharides

Physiological Roles of Carbohydrates

• Crucial for providing cells with energy to fuel physiological processes
• Play a role in storing energy
• Important structural components, examples include cellulose
• Mediate interactions between cells via complex carbohydrate networks
• Act as carbohydrate codes, helping to specify treatments
• Transfer information (DNA & RNA), present in every nucleotide as a sugar
• Involved in cell signaling

Monosaccharides: Aldoses and Ketoses

• Carbohydrates are hydrates of carbon with the empirical formula (CH2O)n, where n is 3 or more
• Each carbon atom is associated with one water molecule
• Aldoses have aldehyde groups
• Ketoses have ketone groups

Monosaccharides: Chiral Carbons

• Have multiple asymmetric carbon atoms
• D or L designation is set by the chiral carbon that is farthest away from the carbonyl oxygen
• The designation of D or L is relative to D-glyceraldehyde

Monosaccharides: Isomers

• Molecules with n chiral centers will have 2n stereoisomers
  • For example, 4 chiral centers can create 16 stereoisomers and 3 chiral centers can create 8 stereoisomers
• Isomers have the same molecular formula, but different structures

Monosaccharides: Epimers

• Sugars that differ in configuration at only one chiral center

Monosaccharides: Common Sugars

• The text highlights the need to know the structures of common sugars
• Includes aldoses such as D-Ribose, D-Deoxyribose, D-Glucose, D-Mannose, and D-Galactose
• D-Fructose is the only ketose
• Aldoses contain one more chiral carbon than ketoses of the same length, leading to more stereoisomers

Monosaccharides: Cyclic Structures

• Longer carbohydrates (5+ carbons) tend to cyclize in solution
• Cyclization occurs via reaction of an internal alcohol (hydroxyl group) with either:
  • An aldehyde (aldoses) to form a hemiacetal
  • A ketone (ketoses) to form a hemiketal

Cyclic Monosaccharides: Pyran and Furan Ring Structures

• Cyclized carbohydrates come in two forms:
  • Six-membered sugar ring is a "pyranose"
  • Five-membered sugar ring is a "furanose"
• Ring formation incorporates an internal oxygen atom from an attacking hydroxyl group

Cyclization of Monosaccharides: Glucose to Glucopyranose

• Cyclization of glucose entails C5 hydroxyl reacting with the C1 aldehyde
• Cyclization makes C1 chiral, creating α and β stereoisomers
• The carbon that becomes chiral after cyclization is an anomeric carbon
• The α and β forms are anomers
• The hydroxyl and carbonyl carbon react, leading to anomeric carbons

Anomeric Carbons: α and β Configurations

• Cyclization yields a new chiral carbon to the anomeric carbon, producing two stereoisomers (α and β)
• If the hydroxyl of an anomeric carbon is below the plane of the sugar, it is α
• If the hydroxyl of an anomeric carbon is above the plane of the sugar, it is β
• All anomers are epimers

Anomeric Carbons: Mutarotation

• α and β configurations interconvert in solution through mutarotation
• Mutarotation occurs through a linear intermediate
• Mutarotation brings a change in configuration
• In solution, glucose is mostly (2/3) β-glucopyranose, with (1/3) α-glucopyranose; less than 1% is linear

Cyclization of Monosaccharides: D-Fructose to Fructofuranose

• Cyclization of fructose involves C5 hydroxyl and C2 ketone
• Cyclization renders C2 chiral (anomeric carbon) producing α and β stereoisomers

Ring Forms: Fructofuranose and Fructopyranose

• Fructose can exist in both furan and pyran rings
• β-fructopyranose is found in honey
• Heating β-fructopyranose promotes conversion to β-fructofuranose
• Corn syrup is sweetened with β-fructopyranose, and used in cold drinks

Sugar Derivatives

• Their carbohydrates break with the empirical formula of (CH2O)n
• Include nitrogen, phosphate, or sulfur groups
• Indicate specialized functions, modifying structure to fulfill other processes

Sugar Derivatives: Mustard Bombs

• Glucosinolate and myrosinase can be found in some plants
• Myrosinase produce glucose and isothiocyanate from glucosinolate
• Myrosinase and glucosinolate are stored separately until tissue damage when they react
• Herbivores are discouraged from these plans due to their bitter tase

Monosaccharides: Reducing Agents

• Linear forms can oxidize by mellow oxidizing agents like iron/copper
• Carbonyl group oxidized to a carboxyl group
• Allows for the quantification of sugar present in bodily fluids
• Also determines the reducing end

Disaccharides: Nomenclature

• Glycosidic bond links together all polymers' monosaccharides
  • Formation can occur whenever there is a free hydroxyl group
• O-glycosidic bonds occur through oxygen
  • Covalent linkages through oxygen
• N-glycosidic bonds occur through nitrogen Nomenclature includes:
  • The monosaccharides involved
  • Their ring types(puran, furan)
  • Their configurations (alpha or beta)
  • Their linkages (C1-C4 etc)

Disaccharides: General

• Multiple means many linkages are possible
• Three (glucose, galactose, mannose) make generate 12,000
• Higher order carbohydrate structures are generated by glycosyltransferases
  • (This is activated with linkage through UDP)

Disaccharides: Nomenclature

• Specifies alpha or beta at at the anomeric carbon of each monosaccharide
• Their ring form (furan or pyran)
• The use of osyl for non-reducing sugars
• The use of ose for reducing sugars
• free anomeric carbon

Disaccharides: Maltose

• Has 6 carbons, in pyran ring from glucose or galactose
• When priority L/D

Disaccharides: Lactose

• Lactose intolerance happens when an individual has insufficient levels of the enzyme
• Lactase, which catalyzed hydrolysis into glucose and galactose
• Causes Bloating, cramps, flatulence, diarrhea and nausea.
• Though babies only utilized it, people can persist their intake through evolution

Polysaccharides: General

• Diverse structures and functions
• Functions include energy storage, structural roles, lubrication, etc

Polysaccharides: General (homo vs hetero)

• Homopolysaccharides
  • Polymers with single type of monosaccharide
• Heteropolysaccharides
  • Polymers with multiple types of of monosaccharide residues
• Both can be unbranched/branched

Energy Storage Polysaccharides: Starch and Glycogen

• Polymers stored in polymeric forms in both plants and animals (gylcogen)
• Also heavily hydrated structures

Starch

• Mix of two molecules (amylose and amylopectin)
• Found in plants and fungi

Glycogen

• Short-term
• Stored in liver and animals' skeletal muscle

Energy Storage Polysaccharides: Starch (Amylose and Amylopectin)

• Alpha (1-4) bonds found in Amylose
  • Linear polymer of glucose residues
• Amylopectin consist of residues linked glucose linked α(1-4) (with a(1-6) branch points 24-30 residues every
• amylase can breakdown – contains (to digest & sugars)