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Transcript of (Chapter 7). - Overview - Classification and Structure of Carbohydrates : A) Isomers and epimers B)...
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- Overview
- Classification and Structure of Carbohydrates :
A) Isomers and epimers
B) Enantiomers
1st Lecture: Pages : 83 - 84
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:OVERVIEW
Carbohydrates are the most abundant organic molecules in nature.
They have a wide range of functions including :
- providing a significant fraction of the energy in the diet of most organisms
- acting as a storage form of energy in the body
- serving as cell membrane components mediate some forms of intercellular communication.
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Carbohydrates serve as a structural component of many organisms:
- the cell walls of bacteria - the exoskeleton of many insects - the fibrous cellulose of plants.
The empiric formula for many of the simpler carbohydrates is (CH2O)n.
hence the name “hydrate of carbon”.
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CLASSIFICATION AND STRUCTURE OF CARBOHYDRATES:
Monosaccharides :- Monosaccharides (simple sugars) can be
classified according to the number of carbon atoms they contain as trioses, tetroses, pentoses, hexoses or heptoses.
. - Examples of some monosaccharides
commonly found in humans are listed in (Figure7.1)
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CH
O
reducinggroup
OH
OH
OH
OH
H
H
H
H
C
C
C
C
CH2OH
1
2
3
4
5
6
carbon
D-Glucose(open chain)
Structure of glucose in its linear form
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OH
OH
OH
OH
H
HH
H
H CC
CC
C
O
CH2OH6
1
O
CH2OH
OH
OH
OH
OH
6
1
- anomer of glucose
O
CH2 OHOH
OH
OH
OH
6
1
- anomer of glucose
Glucose in its ring forms
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Aldoses and Ketoses:
• Carbohydrates with an aldehyde group as their most oxidized functional group are called aldoses.
• Carbohydrates with a keto group as their most oxidized functional group are called ketoses (Figure7.2).
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Monosaccharides are classified to Aldose or Ketose depending upon they have an aldehyde or ketone group.
Aldose Ketose Tetrose (C4H8O4) Erythrose ErythulosePentose (C5H10O5) Ribose RibuloseHexose (C6H12O6) Glucose Fructose
• Carbohydrates that have a free carbonyl group have the suffix “-ose “
[ Note: Ketoses (with some exceptions e.g. fructose) have an additional two letters in their suffix: “-ulose” for example, xylulose].
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• Monosaccharides : - Carbohydrates that can not be hydrolyzed into
simpler CHO. - Can be linked by glycosidic bonds to create
larger structures (Figure7.3).
• Disaccharides: contain two monosaccharide units e.g.:
- Lactose ( galactose + glucose ). - Maltose ( glucose + glucose ). - Sucrose ( glucose + fructose ).
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O
CH2OH
O
O CH 2 OH
SUCROSE (glucose-fructose)
-1,2-bond
OH
CH2OH
O O
O
LACTOSE (galactose-glucose)
-1,4-bond
OH
OH CH2OH CH2OH
O O
O
MALTOSE (glucose-glucose)
OH OH
CH2OH CH2OH
-1,4-bond
O O
O OH OH
CH2OH CH2OH
-1,1-bond
TREHALOSE (glucose-glucose)
Structures of the most common disaccharides.
1 2
1 41 1
1 4
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• Oligosaccharides : Contain 3 to 12 monosaccharide units
(Maltotriose , Glycoproteins).
• Polysaccharides : Contain more than 12 monosaccharide units,
and can be hundreds of sugar units in length: - Starch : Amylose, Amylopectin,Dextrins
- Cellulose - Glycogen “ homopolysaccharides” - Glycosaminglycan “ heteropolysaccharides”
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POLYSACCHARIDES
Cellulose: - linear chain with 1,4 glycosidic bonds
Amylose: - Plant CHO storage - Linear chain with 1,4 glycosidic bonds
Amylopectin: - Amylose + branches with 1,6 glycosidic bonds
Glycogen : - Animal CHO storage - Similar to amylopectin except more branches
Oligosaccharides: - 4-10 (average 8) glucose units; - -limit dextrin = branched (1 or 2 1,6 bonds)
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STRUCTURE OF MONOSACCHARIDES:
A) Isomers and polymers:
Isomers : Compounds having the same chemical formula but having different structures e.g. fructose, glucose, mannose, and galactose are all isomers of each other, [ C6H12O6 ].
Epimers : 2 monosaccharides differ in configuration around only one specific carbon atom (with the exception of the carbonyl carbon see “anomers”) [they are also isomers to each other] Example : glucose and galactose are C- 4 epimers , differ only in the position of the - OH group at C- 4]
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The carbons in sugars are numbered beginning at the end that contains the carbonyl carbon - that is, the aldehyde or keto group (Fig7. 4).
Glucose and galactose are C-4 epimers.
Glucose and mannose are C-2 epimers.
Galactose and mannose are NOT epimers, they differ in the position of - OH groups at two carbons (2 and 4) and are therefore defined only as isomers.
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B) Enantiomers:• A special type of isomerism is found in the pairs of
structures that are mirror images of each other.
• The mirror images called enantiomers, and the two members of the pair are designated as a D- and an L-sugar (Figure 7.5).
• The orientation of the - H & - OH groups around the Carbon atom adjacent to the terminal primary alcohol carbon ( C 5 in glucose) determines whether the sugar is D or L sugar.
• D sugar ( -OH group on C5 is on the Right )
• L sugar ( -OH group on C5 is on the Left )
• The majority of the sugars in mammals are D-sugars.
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Enantiomers
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Thank You
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- Classification and Structure of Carbohydrates :
C) Cyclization of Monosaccharides
D) Complex Carbohydrates
- Starch
- Glycogen
- Cellulose
2nd Lecture: Page : 84-85 & 123-124
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C) Cyclization of monosaccharides:• < 1% of the monosaccharides with five or more carbons
exists in the open-chain (acyclic form).
• They are predominantly found in a ring form : [ aldehyde (or ketone) group + an alcohol group on the same sugar] hemiacetal or hemiketal ring.
- Aldose hemiacetal
- Ketose hemiketal
• If the resulting ring has six members ( 5 C + 1 O) pyranose ring.
• If the resulting ring has five members ( 4 C + 1 O) furanose ring.
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Ring with 5 C + 1 O
Ring with 4 C + 1 O
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1) Anomeric carbon:
• Formation of a ring an anomeric carbon at C 1 of an aldose or at C 2 of a ketose.
• These structures or configurations of the sugar, e.g.. -D- glucose and -D-glucose (Figure.7.6).
• These 2 sugars are both glucose, but they are anomers of each other.
• Glycogen is synthesized from - D - glucopyranose, Cellulose is synthesized from - D - glucopyranose.
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• The cyclic and anomers of a sugar in solution are in equilibrium with each other and can be interconverted, a process called mutarotation. (Figure7.6)
• Crystalline glucose is an - D - glucopyranose.
• Glucose in solution keeps its cyclic structure but isomerism occurs at the anomeric carbon atom (C1)
- - glucopyranose (38 %)
- - glucopyranose (62 %)
- and anomers of glucofuranose ( < 0.3 % )
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Cyclization of monosaccharides
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2) Reducing sugars: • If the oxygen on the anomeric carbon (the carbonyl group)
of a sugar is not attached to any other structure reducing sugar
• A reducing sugar can react with chemical reagents (e.g.. Benedict’s solution) and reduce the reactive component. The anomeric carbon becoming oxidized
[ Note: Only the state of the oxygen on the anomeric carbon determines if the sugar is reducing or nonreducing - the other -OH groups on the molecule are not involved ].
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D. Complex carbohydrates:• Carbohydrates attached to non-carbohydrate structures by
glycosidic bonds “Complex carbohydrates”.
• The non carbohydrate portion named [ aglycone ] e.g.:
- Purines and pyrimidines (Nucleic acids).
- Aromatic rings (Steroids and bilirubin).
- Proteins (Glycoproteins and glycosaminoglycans)
- Lipids (Glycolipids)
• The entire molecule by the generic name called [ glycoside ]
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• The C 1 of aldose, or the C 2 of Ketose participating in the glycosidic link, is called [ a glycosyl residue ].
• if the anomeric carbon of glucose participates in such a bond, that sugar is called a glucosyl residue. (Figure. 7.3).
• If the identity of the attached sugar is known glucoside or galactoside. [ If anomeric carbon of a sugar is in a glycosidic linkage no longer a reducing sugar ].
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Polysaccharides:
1. Macromolecules containing hundreds or thousands of monosaccharides linked together.
2. Storage polysaccharides : Starch – made entirely of glucose.
3. Structural polysaccharides : include cellulose and chitin:
- Cellulose most abundant organic compound on earth.
- Cellulose is a polymer of glucose (like starch) but linkage between glucose monomers differ :
- in Starch , alpha () linkage form
- in Cellulose , beta () linkage form
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• The major polysaccharides are of animal origin (Glycogen) and plant origin ( Starch) and (Cellulose).
• Starch composed of amylose and amylopectin and contains ( 1 4) glycosidic bonds .
• Cellulose is the most abundant polysaccharide of plant origin in nature composed of hundreds of glucose residues contains -D- glucopyranose linked by ( 1 4) glycosidic bonds.
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Starch and cellulose have different shapes and therefore, different properties:
• In cellulose , many parallel glucose strands held together by H bonding.
• These parallel strands form microfibrils.
• Microfibrils intertwine to form a cellulose fibril.
• Microfibrils are then super coiled to form cables of cellulose
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• Cellulose not digested by human beings, but aids in production of mucous along the walls of the intestinal tract facilitates passage of food.
• Few organisms (bacteria , molds) produce enzymes which digest cellulose
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1. O- and N-glycosides:
• If the group on the non-carbohydrate molecule (aglycone) to which the sugar is attached is an -OH group O-glycoside.
• If the group is an -N H2 N-glycoside (Figure7.7).
[Note: All sugar-sugar glycosidic bonds are O-type linkages].
• Typical aglycones :
- purines and pyrimidines ( in nucleic acids) - aromatic rings (in steroids and bilirubin) - proteins (in glycoproteins and glycosaminoglycans) - lipids ( in glycolipids).
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Complex CHO : Glycosides
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2) Naming glycosidic bonds:
• Glycosidic bonds between sugars are named according to :
- the numbers of the connected carbons - the position of the anomeric -OH group of the sugar involved in the bond.
• If anomeric -OH group is in the - configuration, the linkage is an - bond.
• If it is in the - configuration, the linkage is a -bond.
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Structure and function of glycogen: • Glycogen is a branched – chain homopolysaccharide made
exclusively from -D-Glucose.
• The primary glycosidic bond is (1 4) linkage.
• After 8-10 glycosidic residues, there is a branch containing an (1 6) linkage.
• A single molecule of glycogen have a molecular mass of up to 108 daltons.
• Glycogen molecules exist in discrete cytoplasmic granules that contain most of the enzymes need for glycogen synthesis and degradation (Figure 11.3).
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• The main stores of glycogen are skeletal muscles and liver
• The function of muscle glycogen is to serve as a fuel reserve for synthesis of ATP during muscle contraction
• The function of liver glycogen is to maintain the blood glucose level during early stages of fasting (Figure 11.2)
• 4oo g of glycogen makes up to 1 – 2 % of weight of resting muscle.
• 1oo g of glycogen makes up to 10 % of fresh weight of well–fed adult liver.
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• In some glycogen-storage diseases (e.g. Pompe disease, VonGierke's disease & McArdle syndrome), the amount of glycogen in the liver and/or muscle can be significantly higher.
• Liver glycogen stores during the well – fed state and are depleted during fasting state
• Muscle glycogen is not affected by short period of fasting and is only moderately in prolonged fasting.
• Muscle glycogen is synthesized to replenish muscle stores after its depletion following strenuous exercise.
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• Lactose, for example, is synthesized by forming a glycosidic bond between C 1 of a -galactose and C 4 of glucose.
• The linkage is, therefore, a (1 4) glycosidic bond. (see Figure 7.3).
• Cellulose contains ( 1 4) glycosidic bonds and is the most abundant polysaccharide of plant origin in nature composed of hundreds of glucose residues
[Note: Because the anomeric end of the glucose residue is not involved in the glycosidic linkage it (and Therefore, lactose) remains a reducing sugar].
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Cellulose and Starch have different shapes and therefore, different properties:
• In cellulose , many parallel glucose strands held together by H bonding.
• These parallel strands form microfibrils.
• Microfibrils intertwine to form a cellulose fibril.
• Microfibrils are then super coiled to form cables of cellulose
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• Cellulose not digested by human beings, but aids in production of mucous along the walls of the intestinal tract facilitates passage of food.
• Few organisms ( bacteria , molds) produce enzymes which digest cellulose
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Thank you