Carbohydrates Chapter 27 Larry Emme Chemeketa Community College
Carbohydrates are energy-yielding macronutrients in the same class of nutrients as fats and proteins. These polyhydroxy aldehydes or ketones include simple carbohydrates like glyceraldehyde and dihydroxyacetone. What are carbohydrates? 3
Carbohydrates are important in society because they provide; (1) basic diets in the form of starch and sugar and……. (2) clothing and shelter. Many of the chemical properties of carbohydrates are determined by the chemistry of the hydroxyl and carbonyl functional groups. 4
Fischer projection formulas are drawn with these characteristics: • The keto or aldehyde group is placed at the top of the projection. (2) Each interior carbon atom is shown as an intersection point between two lines ( ) (3) The H atom and –OH group are written to left or right of the projection . 5 5
This is an example of a modified structural formula of glucose written as a Fischer projection formula. 6 6
Types of Carbohydrates The four major types of carbohydrates are…….. 1. Monosaccharides 2. Disaccharides 3. Oligosaccharides 4. Polysaccharides 8
Monosaccharides • A monosaccharide is a carbohydrate that cannot be hydrolyzed to simpler carbohydrate units. • The monosaccharide is the basic carbohydrate unit of cellular metabolism.
Monosaccharides can be classified bythe …. (a) number of carbon atoms in the molecule ( e.g. a pentose versus a hexose) functional group ( aldoses versus ketoses) configuration ( D versus L isomers) (d) optical activity ( (+) versus (–) isomers ) ring structure ( furanoses versus pyranoses) stereochemistry at an anomeric carbon ( versus isomers) Types of Monosaccharides 10
Types of Monosaccharides Number of Carbons The monosaccharides shown below are classified based on the number of carbons in the molecule. 11
Types of Monosaccharides Functional Group Mononosaccharides with a –CHO (aldehyde) group are known as aldoses whilethose with a (keto) group are known as ketoses. 12
Types of Monosaccharides Configuration Monosaccharides with the –OH group on the right of the carbon alpha to the terminal ROH carbon are D isomers while those with the –OH group on the left are L isomers. 13
Types of Monosaccharides Optical Activity Monosaccharides that rotate plane-polarized light to the right are known as (+) isomers while those that rotate it to the left are (–) isomers. Note: The D and L designations do not indicate the direction of rotation, e.g., the D isomer of glucose could be either the (+) isomer or it can be the (–) isomer. 14
Types of Monosaccharides Ring Structure The cyclic form of monosaccharides that have five atoms in the ring are known as furanoses while those with six atoms are known as pyranoses based on the corresponding heterocyclic ring structures furan and pyran. 15
Types of Monosaccharides Anomeric Configuration Monosaccharides that have an –OH below the ring at the anomeric carbon are known as (alpha) isomers while those with the –OH above the ring are (beta) isomers. 16
Disaccharides • A disaccharide yields two monosaccharides – either alike or different – when hydrolyzed: H+ or enzymes disaccharide + water 2 monosaccharides
Monosaccharides & Disaccharides • Disaccharides are often used by plants or animals to transport monosaccharides from one cell to another. • The monosaccharides and disaccharides generally have the ending –ose – for example, glucose, sucrose, and lactose. • These are water-soluble carbohydrates, which have a characteristically sweet taste and are called sugars.
Oligosaccharides • An oligosaccharide has two to six monosaccharide units linked together.
Polysaccharides • A polysaccharide is a macromolecular substance that can be hydrolyzed to yield many monosaccharide units: H+ or enzymes polysaccharide + water monosaccharides • Polysaccharides are important structural supports, particularly in plants, and also serve as a storage depot for monosaccharides, which cells use for energy.
Why are carbohydrates so important? Carbohydrates are important because they are widely available and because they have exceptional utility. 22
The most important monosaccharides are the pentoses and hexoses as shown in the diagram below . 24
Monosaccharides • The hexose monosaccharides are the most important carbohydrate sources of cellular energy. • Three hexoses – glucose, galactose, and fructose – are of major significance in nutrition. • All three have the same formula, C6H12O6, and thus deliver the same amount of cellular energy. • They differ in structure, but are biologically interconvertible.
“Tree” formulas • Shorthand formulas used in carbohydrate configurations where the following symbols are used: Example is
Glucose is the most important of the monosaccharides. • It is an aldohexose and is found in the free state in plant and animal tissue. or
The concentration of glucose in blood is normally about 80-100 mg/100 mL Glucose is known by either of the following names dextrose (from dextrorotatory), grape sugar (found in grapes) , or blood sugar (because it is transported in the blood). Glucose 28
Galactose is also an aldohexose and occurs, along with glucose, in lactose and in many oligo- and polysaccharides such as pectin and gums. or
Fructose, also know as levulose, is a ketohexose that occurs in fruit juices, honey, and along with glucose, as a constituent of sucrose. It is the sweetest common sugar being about two times sweeter than glucose.
Structures of the Pentoses (Ribose and Deoxyribose) 31
Epimers • Any two monosaccharides that differ only in the configuration around a single carbon atom are called epimers. • In the case of glyceraldehyde this atom is carbon 2.
The D-family of aldoses. The red –OH group indicate the new chiral carbon added in case from top to bottom of the diagram. 34
Haworth Formulas Haworth formulasare structural formulas that represent cyclic sugars. In the case of glucose the formula is drawn as a flat hexagon with H and –OH written above and below the plane of the ring. Haworth formulas are sometimes shown in a abbreviated form as shown here. OH’s removed 36
Most naturally occurring monosaccharides occur in the chair conformation shown. -D-glucopyranose
What is an anomer? An anomer is the or form of a monosaccharide as shown here. 38
What is mutarotation? Emil Fischer 1852-1919 Mutarotation is the change in specific rotation of an anomer as itis converted into an equilibrium mixture of the and forms. Fischer projection formulas showing mutarotation of D-glucose. 39
Haworth formulas showing mutarotation of D-glucose. Sir (Walter) Norman Haworth 1883-1950 40
O OH O O H H
H H H H H H H H H H H H H H H H O OH H O O H H
Conversion of Fischer to Haworth Formulas http://faculty.chemeketa.edu/lemme/CH%20123/Handouts/HaworthFormulas.pdf
Hemiacetals arestructures that contain an alkoxy group and a hydroxyl group on the same carbon atom. Hemiacetals 46
The cyclic structures of monosaccharides are intramolecular hemiacetals. Five and six -membered ring hemiacetals are stable but these rings can open in aqueous solution to the straight-chain aldehyde. 47
Acetals Acetals arestructures that contain two alkoxy groups on the same carbon atom. 48
Glycosides Cyclic acetals are known as glycosides and glycosidesare derivatives of hemiacetals. 49
Glycosides Glycosides like the methyl isomers shown below are less reactive than the corresponding monosaccharide. The methyl isomers shown below will not undergo mutarotation. 50