Understanding Carbohydrates

Carbohydrates

• Carbohydrates are abundant organic molecules formed through photosynthesis.
• They provide energy in the diet for many organisms.
• They can act as a storage form of energy
• Carbohydrates are structural components.
• They are cell-membrane components for intercellular communication.
• They can also be cell-surface antigens and part of the body's extracellular ground substance.
• Carbohydrates can be associated with proteins and lipids.
• They're found in RNA, DNA, and coenzymes like NAD+, NADP+, FAD, and CoA.
• Carbohydrates contain carbon, hydrogen, and oxygen.
• The general formula for carbohydrates is (CH2O)n = CnH2nOn.
• The ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules.

Classification of Carbohydrates

• There are several classifications of carbohydrates.
• Chemically, they are classified as Monosaccharides, Disaccharides, Oligosaccharides (3-10 units), and Polysaccharides.
• Physiologically, they are classified as Simple Carbohydrates and Complex Carbohydrates.
• Monosaccharides supply 3.74 kcal/g.
• Disaccharides supply 3.95 kcal/g
• Starch supplies 4.18 kcal/g
• On average, carbohydrates supply approximately 4 kcal/g

Simple and Complex Carbohydrates

• Simple carbohydrates consist of monosaccharides and disaccharides.
• Complex carbohydrates consist of oligosaccharides and polysaccharides, like Glycogen, Starches and Fibers (Cellulose)

Additional Information

• Carbohydrates can be soluble or insoluble
• Fiber is insoluble and mostly cellulose.
• Monosaccharide names end with -ose.

Classification of Monosaccharides

• Monosaccharides are classified by the number of carbon atoms they possess.
• Trioses have 3 carbons (Glyceraldehyde, dihydroxy acetone)
• Tetroses have 4 carbons (Erythrose)
• Pentoses have 5 carbons (Ribose, Ribulose)
• Hexoses have 6 carbons (Glucose, Fructose, Mannose, Galactose)
• Heptoses have 7 carbons (Sedoheptulose)
• Nonoses have 9 carbons (Neuraminic acid/Sialic acid)

Monosaccharides - Functional Groups

• Monosaccharides can be classified by their functional group as aldehydes or ketones.
• Polyhydroxy aldehydes or ketones are simple sugars.
• Aldoses are carbohydrate with an aldehyde group
• Ketoses are carbohydrate with a ketone group.

Monosaccharides: Single Sugars

• Glucose is known as blood sugar and is an essential energy source.
• Glucose is a basic sub-unit of larger carbohydrate molecules and is found in fruits, vegetables, and honey.
• Fructose is the sweetest of sugars, naturally occurring in fruits & honey (fruit sugar).
• Fructose combines with glucose to form sucrose and is commercially produced via cornstarch hydrolysis.
• Galactose combines with glucose to form lactose, known as milk sugar.
• Galactose is formed from the digestion of lactose.
• Ribose is a constituent of coenzyme A, ATP, cyclic AMP, RNA, nucleic acids and is involved in oxidizing and reducing agent coenzyme.
• Deoxyribose is part of DNA.

Monosaccharides: Aldoses

• Aldoses (e.g., glucose) have an aldehyde group at one end.

Monosaccharides: Ketoses

• Ketoses (e.g., fructose) have a keto group, usually at C2.

Properties of Carbohydrates

• Carbohydrates exhibit steroisomerism (D-glucose and L-glucose), optical activity ((+) Glucose and (-) Glucose), diastereomers (Mannose, Galactose), and annomerism (Aldoses and ketoses).

Isomers

• Isomers are compounds with the same molecular formulae but different structural formulae.
• They have the same number of each kind of atom differently arranged, giving rise to different properties.

Stereoisomers

• Stereoisomers are molecules with the same chemical formula, but differ in their arrangement at a chiral center.
• The number of stereoisomers is 2n, where n is the number of asymmetric centers.
• 6-C aldoses have 4 asymmetric centers and 16 stereoisomers.

Steroisomerism

• For sugars with more than one chiral center, D or L refers to the asymmetric C farthest from the aldehyde or keto group.
• Most naturally occurring sugars are D isomers.
• D and L sugars are mirror images and have the same name (D-glucose, L-glucose).

Aldopentoses and Aldohexoses

• Aldopentoses (C5) have three chiral carbons and eight stereoisomers.
• Aldohexoses (C6) have four chiral carbons and sixteen stereoisomers.

Isomerism: Stereoisomers

• Stereoisomers are a type of isomerism.
• They are copmpounds with the same molecular formula, functional groups, and the position of functional groups but with different conformations.
• cis-trans isomers have different conformation around double bonds

Isomerism: Enantiomers

• Enantiomers are special type of isomerism.
• Mirror images of each other

Epimers

• Epimers are Carbohydrate isomers that differ in configuration around only one specific carbon atom.
• Glucose and galactose are C-4 epimers

Annomerism

• It is the spatial configuration with respect to the first carbon atom in aldoses and second carbon atom in ketoses.
• Monosaccharides can exist as a linear chain or as ring-shaped molecules.
• In aqueous solutions they are usually found in ring forms

Cyclization of Glucose

• Cyclization of glucose produces a new asymmetric center at C1.
• The 2 stereoisomers are called anomers, α & β.
• Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1:
  • α (ΟΗ group is below the ring)
  • β (OH group is above the ring).

Reducing Sugars

• If a hydroxyl group on an anomeric carbon of a cyclized sugar isn't linked to another compound via glycosidic bond, the ring can open.
• The sugar can then act as a reducing agent
• The compound is termed a reducing sugar.

Reducing Sugars: C-1 vs C-2

• C-1 is the anomeric carbon in glucose.
• C-2 is the anomeric carbon in fructose.
• Common test reagents are Benedicts reagent (CuSO4 / citrate) and Fehlings reagent (CuSO4 / tartrate)

Disaccharides - Sucrose

• Sucrose ("table sugar") is composed of glucose + fructose.
• It has a glycosidic bond linking the anomeric hydroxyls of glucose & fructose..
• The configuration at the anomeric C of glucose is alpha (OH points down from ring)..
• It has a linkage of is α(1→2) glycosidic bond.
• Sugar beets and sugarcane are the most common sources.
• A 50:50 mix of glucose and fructose (invert sugar) is used as a food additive because it's sweeter than sucrose.
• Sucrose is not a reducing sugar.

Disaccharides - Lactose

• Lactose("milk sugar") is composed of galactose & glucose.
• It has a ẞ(1→4) linkage from the anomeric OH of galactose.
• Human milk contains about 7% lactose
• It is a reducing sugar.
• Beta-lactose is sweeter and more soluble than ordinary alpha-lactose

Disaccharides - Maltose

• Maltose ("malt sugar") is composed of glucose + glucose.
• It's present in fermenting grains and prepared foods as a sweetener.
• It's prepared by enzyme-catalyzed degradation of starch.
• Produced during starch digestion by α-amylase in the small intestine.

Polysaccharides

• Polysaccharides are long chains of monosaccharides (monomers) linked by glycosidic bonds.
• They consist of >12 monosaccharide hydrolyzable polymers
• They have a molecular weight that may be 100,000+ daltons depending on the number of joined monomers.
• The chain may be branched or unbranched
• Poly- = “many”

Homopolysaccharides

• These are polymers consisting of a single type of monosaccharide (glucose).
• Examples: Glycogen, Cellulose, Starch

Heteropolysaccharides

• These complex carbohydrates are polymers of at least 2 types of monosaccharides.
• They contain sugar derivatives (amino sugars, uronic acids, sialic acids like examples: Glycosaminoglycans, proteoglycans and Glycoproteins)

Starch

• Starch is the plant storage form of carbohydrate
• It's composed of amylose and amylopectin.
• These are long branched or unbranched chains of glucose

Amylose

• Plants store glucose in polymeric form as amylose or amylopectin, glucose polymers collectively called starch.
• Amylose is a glucose polymer with a(1→4) linkages.
• It accounts for ≈20% of starch.
• Amylose is soluble in hot water and coils into helices.
• The end of the polysaccharide with an anomeric C1 is not involved in a glycosidic bond so its called the reducing end.

Amylopectin

• Accounts for 80% of starch
• It is not water soluble.
• Its a glucose polymer with mainly α(1→4) linkages
• Is is also has branches formed by α(1→6) linkages, also generally longer than shown in the diagram
• Branches result in a compact structure and multiple chain ends where enzymatic cleavage can occur

Glycogen

• Glycogen(the glucose storage polymer in animals) is similar in structure to amylopectin.
• Glycogen has more α(1→6) branches.
• The highly branched structure is able to permit rapid glucose release from glycogen stores, e.g., in muscle during exercise.

Cellulose

• Plant Cellulose contains β-D-glucose units joined by β-1,4-links.
• Cellulose is the fibrous substance that provides structure in plants.
• They impart strengh and rigidity to plant cell walls withstand hydrostatic pressure gradients.
• Hydrogen bonds form between chains within contribute rigidity and toughness to cellulose fibers.

Sugar Derivatives

• Sugar alcohols lack an aldehyde or ketone (e.g., ribitol).
• Sugar acids have have aldehyde at C1, or the OH at C6 that is oxidized to a carboxylic acid( e.g., gluconic/glucuronic acid).
• Amino sugars are when an amino group substitutes for a hydroxyl and example is glucosamine.
• When teh Amino group is acetylated, it becomes what is known as N-acetylglucosamine.
• N-acetylneuraminate (N-acetylneuraminic acid/sialic acid) occurs as a terminal residue of oligosaccharide chains of glycoproteins.

Glycosaminoglycans (GAGs)

• These are also called mucopolysaccharides.
• They are Large, negatively charged, unbranched heteropolysaccharide chains composed of a repeating disaccharide unit [acidic sugar-amino sugar]n.
• They are Modified monosaccharides (acidic/amino groups, sulfated hydroxyl) associated with a small amount of protein (<5%) form proteoglycans.

Glycosaminoglycans (GAGs)

• Proteoglycans are = Protein + 95% carbohydrates
• Glycoproteins are = more protein + small amount of carbohydrate
• Glycosaminoglycans: Hyaluronic acid (Hyaluronate) and Heparin/Heparan Sulfate

Glycosaminoglycans Functions

• Proteoglycans provide a ground/packing substance of connective tissue.
• They hold large quantities of water and occupy space (cushioning or lubricating synovial fluid and the vitreous humor.
• Negatively charged glycosaminoglycans cover animal cell surfaces and interact with lots of extracellular signalling molecules.

Hyaluronate

• Consists of 2 glucose derivatives: glucuronate (glucuronic acid) & N-acetyl-glucosamine
• It is a gylcosaminoglycan
• Examples of functions: Signalling activity, Wound healing, Lubricating the eye, Help in joint movement, Space filler, Cartilage protector

Heparin

• Functions as an anti-coagulant to treat thrombophlebitis(vein inflammation) related to a thrombus (blood clot)

Glycoproteins

• Proteins containing short carbohydrate chains (oligosaccharides).
• These are found in situations to do with functions including the cell memobrane, and oligosaccharides as receptors, drugs, molecules and signals.
• Are are responsible for blood group