Carbohydrates - nleaders.orgnleaders.org/.../20182019_first_semester_slides/Carbohydrates.pdf · Carbohydrates • Mono- (one), Oligo- (a few) and (many) Polysaccharides • Monosaccharides

1Dr. Diala Abu-Hassan

Dr. Diala Abu-Hassan, DDS, PhD

[email protected]

All images were taken from McMurry et al except where noted

Carbohydrates

Carbohydrates

• Carbohydrates are members of a large class of naturally occurring polyhydroxy ketonesand aldehydes

Dr. Diala Abu-Hassan 2

Importance of carbohydrates

• major energy sources

• cell–cell interactions

• immune recognition

• Major structural components of plants and bacterial cell walls


Carbohydrates

• Mono- (one), Oligo- (a few) and (many) Polysaccharides

• Monosaccharides (simple sugar) a carbohydrate with three to seven carbon atoms. General formula Cn(H2O)n.

• A monosaccharide can be a polyhydroxy aldehyde (aldose) or a polyhydroxy ketone (ketose)

• Aldose A monosaccharide that contains an aldehyde carbonyl group.

• Ketose A monosaccharide that contains a ketone carbonyl group

• Have many hydroxyl groups on adjacent carbons together with either an aldehyde or ketone group



Aldotriose versus ketotriose

Example on Aldotrioses and ketotrioses

Nomenclature: aldo- or keto- + latin number + ose


Most common sugars are aldoses rather than ketoses

1

2

Common sugars


Optical isomers (stereoisomers)• Steriosomers = non-superimposable mirror images =

enantiomers

• Chiral (asymmetric) carbon

• More carbons = more steriosomers

• Ex. Glyceraldehyde

Dr. Diala Abu-Hassan 8Fischer projection

Structure of Glucose


Structure of Glucose


Glucose

• The most important simple carbohydrate in human metabolism.

• It is the final product of carbohydrate digestion and provides acetyl groups for entry into the citric acid cycle as acetyl-SCoA


Galactose

• Plant gums and pectins

• A component of the disaccharide lactose (milk sugar)

• Is produced from lactose during digestion.

• Is converted to glucose to provide energy

• Is synthesized from glucose to produce lactose for milk and compounds needed in brain tissue.


Jam with galactose in the pectin that stiffens it

Galactose


An aldohexose

It differs from glucose only in the orientation of the OH group at C4.

Galactose vs Glucose


An aldohexose

It differs from glucose only in the orientation of the OH group at C4.

Galactose

Glucose

Fructose


• D-Fructose, often called levulose or fruit sugar

• honey and many fruits.

• one of the two monosaccharides combined in the disaccharide sucrose.

• Fructose is produced commercially in large quantities by hydrolysis of cornstarch to make high fructose corn syrup (HFCS).

Fructose


• Like glucose and galactose, fructose is a 6-carbon sugar.

• Aketohexose

• It forms a five-membered rings in solution

Ribose and 2-Deoxyribose

• Both are 5-carbon aldehyde sugars.

• Important as parts of larger biomolecules, such as coenzyme A , ATP, cyclic AMP and in oxidizing and reducing agent coenzymes.

• 2-deoxyribose differs from ribose by the absence of one oxygen atom, that in the OH group at C2.

• Both exist as mixtures of open-chain and cyclic hemiacetal forms.


Reactions of Monosaccharides with Oxidizing Agents: Reducing Sugars

• Carbohydrates that react with oxidizing agents are classified as reducing sugars (they reduce the oxidizing agent).

• Open-chain form of aldose monosaccharides (aldehydes -RCHO) can be oxidized to carboxylic acids (RCOOH)

• The open-chain form continues to be produced, and then oxidized


Ketoses can become aldoses

• In basic solution, ketoses are also reducing sugars.

• Rearrangement.


Reaction with Alcohols: Glycoside Formation

• Hemiacetals react with alcohols with the loss of water to yield acetals


• Monosaccharides are cyclic hemiacetals, thus, they react with alcohols to form acetals, which are called glycosides.

• The OH group is replaced by an OR group.

• A glucoside is a cyclic acetal formed by glucose. A cyclic acetal derived from any sugar is a glycoside.

• The glycosidic bond is the bond between the anomeric carbon atom of the monosaccharide and the oxygen atom of the group OR

• Glycosides are not reducing sugars because they do not contain hemiacetalgroups


Reaction with Alcohols: Glycoside Formation


Reaction with Alcohols: Disaccharide Formation

• In disaccharides and polysaccharides, monosaccharides are connected to each other by glycosidic bonds.

• A disaccharide forms by the reaction of the anomeric carbon of one monosaccharide with an OH group of a second monosaccharide.

Hydrolysis of a disaccharide

• Hydrolysis reaction takes place during digestion of all carbohydrates.


Formation of Phosphate Esters of Alcohols

• The OH groups of sugars can add PO4-2 groups to form phosphate esters.

• The resulting phosphate esters of monosaccharides appear as reactants and products throughout the metabolism of carbohydrates.

• Glucose phosphate is produced by the transfer of a PO4-2 group from ATP

to glucose in the first step of glycolysis.


Disaccharides

• The two monosaccharides in a disaccharide are connected by a glycosidic bond.

• The bond may be α or β

• α points below the ring and β points above the ring

• Examples: α glycosidic bond (maltose), β glycosidic bond (lactose), or a bond that connects two anomeric carbon atoms (sucrose).


Maltose

• Malt sugar

• present in fermenting grains

• Can be prepared by enzyme-catalyzed degradation of starch.

• used in prepared foods as a sweetener.

• produced during starch digestion by α-amylase in the small intestine

• hydrolyzed to glucose by maltase.


Maltose


• Two -D-glucose molecules are joined in maltose by an α-1,4 link

• It is both an acetal (at C1 in the left-side glucose) and a hemiacetal (at C1 in the right-side glucose).

• Since the acetal ring on the left does not open and close spontaneously, it cannot react with an oxidizing agent.

• The hemiacetal group on the right can react with an oxidizing agent, thus maltose can be a reducing sugar.

Lactose (milk sugar)• Human milk is about 7% lactose.

• A disaccharide composed of β-D-galactose and β-D-glucose.

• The two monosaccharides are connected by a β-1,4 link.

• A reducing sugar because the glucose ring (on the right) is a hemiacetal at C1.


Clinical hint-Lactose intolerance

• Is prevalent in adults in all populations.

• The activity of lactase gradually diminishes over the years.

• Because lactose remains in the intestines rather than being absorbed, it raises the osmolarity, which draws in excess water. Bacteria in the intestine also ferment the lactose to produce lactate, carbon dioxide, hydrogen, and methane. The result is bloating, cramps, and diarrhea.

• Lactose free food, the use of commercial enzyme preparations and Lactaid, milk that has been treated with lactase to reduce its lactose content


Sucrose• Common table sugar, cane sugar, beet sugar

• Hydrolysis of sucrose yields one molecule of D-glucose and one molecule of D-fructose.

• The 50:50 mixture of glucose and fructose (invert sugar) is commonly used as a food additive because it is sweeter than sucrose.


• Sucrose differs from maltose and lactose in that it has no hemiacetal group because a 1,2 link joins both anomeric carbon atoms.

• The absence of a hemiacetal group means that sucrose is not a reducing sugar.

• Sucrose is the only common disaccharide that is not a reducing sugar.

Synthetic sugars and sweetness


Polysaccharides• Are polymers of tens, hundreds, or even many

thousands of monosaccharides linked together through glycosidic bonds.


Cellulose

• Cellulose is the fibrous substance that provides structure in plants.

• Each molecule consists of several thousand β-D-glucose units joined in a long, straight chain by β-1,4 links.

• Lying flat side by side


Starch• A polymer of glucose.

• Glucose units are joined by α-1,4 links rather than by the β-1,4 links of cellulose.

• Fully digestible and is an essential part of the human diet.

• Present only in plant material and major sources are beans, the grains wheat and rice, and potatoes.

• Cellulose has only one form but starch has 2 kinds amylose and amylopectin.


Amylase


-Accounts for about 20% of starch

-Somewhat soluble in hot water

-Consists of several hundred to a thousand –D glucose units linked in long chains by the α-1,4 glycosidic bonds. -Tends to coil into helices

Amylopectin


• Accounts for about 80% of starch

• similar to amylose but has much larger molecules (up to 100,000 glucose units per molecule) and has α-1,6 branches approximately every 25 units along its chain.

• A glucose molecule at one of these branch points (blue) is linked to three other sugars.

• Amylopectin is not water-soluble.

Glycogen (animal starch)• Glucose is stored as glycogen for later use.

• Glycogen stores are the liver and muscles.

• Structurally, glycogen is similar to amylopectin in being a long polymer of a-D-glucose with the same type of branch points in its chain.

• Glycogen has many more branches than amylopectin and is much larger (up to one million glucose units per molecule).


Clinical hints- blood groups


Documents

Carbohydrates - nleaders.orgnleaders.org/.../20182019_first_semester_slides/Carbohydrates.pdf · Carbohydrates • Mono- (one), Oligo- (a few) and (many) Polysaccharides • Monosaccharides