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Chapter 10 – Carbohydrates
Most carbohydrates are produced photo-synthetically by bacteria, algae and plants
Carbohydrates (“hydrate of carbon”) have the empirical formulaof (CH2O)n , where n ≥ 3.
- Monosaccharides: one monomeric unit
- Oligosaccharides: ~2-20 monosaccharides
- Polysaccharides: > 20 monosaccharides
- Glycoconjugates: linkages to proteins or lipids
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Saccharides of varying lengthAldoses and Ketoses
Trioses – three carbon sugars
AldotrioseKetotriose
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Tetroses – 4 carbon sugars
Saccharides of varying lengthAldoses and Ketoses
Chiral designation comes fromthe most distant chiral carbonfrom the carbonyl
D-sugars dominate in nature
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Saccharides of varying lengthAldoses and Ketoses
Pentoses – 5 carbon sugarsHexoses – 6 carbon sugars
Figure 10.2
Chiral designation comes fromthe most distant chiral carbonfrom the carbonyl
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Figure 10.1 Isomeric forms of carbohydrates
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Epimers
- Epimers – sugars that differ at only one of several chiral centers.
example: D-Mannose is an epimer of D-Glucose
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Figure 10.4 Cyclization ofD-fructose to form a- and b-fructofuranose
anomeric carbon
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Fig 10.3 Cyclization of D-glucose to form glucopyranose
In aqueous solutions hexosesand pentoses will cyclizeforming alpha (a) and beta (b)forms C1 is called the anomeric carbon
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In aqueous solutions, it is the ring structures that dominateThese rings are NOT planar
Figure 10.5
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Figure 10.7 modified monosaccharides
Important in metabolic pathways
Important in structuralglycans
6-deoxy-L-galactose
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Disaccharides and other Glycosides
Glycosidic bond – the primary structural linkage in all polymersof monsaccharides
Glycosides – glucose provides the anomeric carbon
Figure 10.8 Structures of disaccharides: maltose
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More structures of disaccharides: lactose and sucrose
Major carbohydratein milk
Most abundant sugar
Comes from the breakdown of starch and glycogen
Sucrase, lactase andmaltase are foundon the outer surfaceof the epithelial cellslining the small intestine.
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Reducing and Nonreducing ends of sugars
- In linear polymeric chains of monosaccharides thereis usually one reducing end (containing the free anomericcarbon) and one nonreducing end
- Branched polysaccharides have a number of nonreducingends, but only one reducing end
Reducing end(anomeric carbon)
Nonreducing end
Read Clinical Insight pg 161
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Polysaccharides
- Homoglycans – homopolysaccharides containing only onetype of monosaccharide.
- Heteroglycans – heteropolysaccharides containing residuesof more than one type of monosaccharide
- The lengths and compositions of a polysaccharide may varywithin a population of these molecules
e.g.: starch and glycogen – storage polysaccharidescellulose and chitin – structural polysaccharides
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Starch- D-glucose is stored intracellularly in polymeric forms
- plants and fungi store glucose as starch- Animals store glucose as glycogen
- Starch is a mixture of amylose (unbranched)and amylopectin (branched every 25 sugars)
(a) Amylose is a linear polymer containing only a-1,4-glycosidic bonds.
(b) Amylopectin is a branchedpolymer also contains a-1,6-bonds
Figure 10.12
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Prentice Hall c2002 Chapter 8 16
Starch is stored by plants and used as fuel.
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a-amylase on amylopectin
- a-amylase is found in plants and animals and is a hydrolaseit is an endoglycosidase – hydrolyzes internal a-(14) glycosidic bonds
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Prentice Hall c2002 Chapter 8 18
Glycogen is is stored by animals and used as fuel.
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Glycogen
- Glycogen is the main storage polysaccharide of humans
- Glycogen is a polysaccharide of glucose residues connectedby a-(14) linkages with a-(16) branches (one branch every8 to 12 residues)
- Glycogen is present in large amounts in liver and skeletal muscle
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Cellulose – a structural polysaccharide that is aMajor component of cell walls of plants
Cellulose has b-(14) glycosidic bonds
Each glucoseresidue is rotated180o relative to the next residue
Extended hydrogen bondingbetween chains leads to bundles or fibrils
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Figure 10.14 Glycosidic bondDetermines polysaccharideStructure.
Bent structure
Straight structure
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- Humans digest starch and glycogens ingested in their dietusing amylases, enzymes that hydrolyze a-(14) glycosidic bond
- Humans cannot hydrolyze b-(14) linkages of cellulose. Thereforecellulose is not a fuel source for humans. It is fiber.
- Certain microorganisms have cellulases, enzymes that hydrolyzeb-(14) linkages of cellulose.
- cattle have these organisms in their rumen- termites have them in their intestinal tract
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Carbohydrates attached to proteins form glycoproteins
Many glycoproteins are found as components of cell membranes (chapter 11) and take part in cell adhesion and binding.
Two methods to anchor the protein to carbohydrates
or Thr
Mucins or mucoproteins are proteins which has N-
acetylgalactosamine attached. This glycoprotein is found in mucus and is a
lubricant.
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Figure 10.15 Glycosidic bonds between proteins and carbohydrates
asparagine serine
Two methods to anchor the protein to carbohydrates
Thr is also used to makeO-linkages
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Figure 10.16 N-linked oligosaccharides
All N-linkages have a common core shown in grey.
Protein residue Protein residue
High mannoseComplex structure
Additional sugars can be attached to these cores to make diverse and unique structures
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Proteoglycans are glycoproteins were the protein is bound to aspecial class of polysaccharides called a glycosaminoglycan.
This class of glycoprotein are used as structural components and lubricants.
-In proteoglycans nearly 95% of the mass comes from the polysaccharide.
-Proteoglycans function as lubricants, structural components in tissue and mediate the adhesion of cells to the extracellular matrix.
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Assignment
Read Chapter 10Read Chapter 11
Topics not covered:Section 10.4 Lectins