Chapter 10 – Carbohydrates
Most carbohydrates are produced photo-synthetically
by bacteria, algae and plants
Carbohydrates (“hydrate of carbon”) have the empirical formula
of (CH2O)n , where n ≥ 3.
- Monosaccharides: one monomeric unit
- Oligosaccharides: ~2-20 monosaccharides
- Polysaccharides: > 20 monosaccharides
- Glycoconjugates: linkages to proteins or lipids
Saccharides of varying length
Aldoses and Ketoses
Trioses – three carbon sugars
Ketotriose
Aldotriose
Saccharides of varying length
Aldoses and Ketoses
Tetroses – 4 carbon sugars
Chiral designation comes from
the most distant chiral carbon
from the carbonyl
D-sugars dominate in nature
Saccharides of varying length
Aldoses and Ketoses
Figure 10.2
Pentoses – 5 carbon sugars
Hexoses – 6 carbon sugars
Chiral designation comes from
the most distant chiral carbon
from the carbonyl
Figure 10.1 Isomeric forms of carbohydrates
Epimers
- Epimers – sugars that differ at
only one of several chiral
centers.
example: D-Mannose is an
epimer of D-Glucose
Figure 10.4 Cyclization of
D-fructose to form a- and bfructofuranose
anomeric carbon
Fig 10.3 Cyclization of D-glucose
to form glucopyranose
In aqueous solutions hexoses
and pentoses will cyclize
forming alpha (a) and beta (b)
forms
C1 is called the anomeric carbon
In aqueous solutions, it is the ring structures that dominate
These rings are NOT planar
Figure 10.5
Figure 10.7 modified monosaccharides
6-deoxy-L-galactose
Important in structural
glycans
Important in
metabolic pathways
Disaccharides and other Glycosides
Glycosidic bond – the primary structural linkage in all polymers
of monsaccharides
Glycosides – glucose provides the anomeric carbon
Figure 10.8 Structures of disaccharides: maltose
More structures of disaccharides: lactose and sucrose
Major carbohydrate
in milk
Most abundant sugar
Sucrase, lactase and
maltase are found
on the outer surface
of the epithelial cells
lining the small
intestine.
Comes from the breakdown of starch and glycogen
Reducing and Nonreducing ends of sugars
- In linear polymeric chains of monosaccharides there
is usually one reducing end (containing the free anomeric
carbon) and one nonreducing end
- Branched polysaccharides have a number of nonreducing
ends, but only one reducing end
Nonreducing end
Reducing end
(anomeric carbon)
Read Clinical Insight pg 161
Polysaccharides
- Homoglycans – homopolysaccharides containing only one
type of monosaccharide.
- Heteroglycans – heteropolysaccharides containing residues
of more than one type of monosaccharide
- The lengths and compositions of a polysaccharide may vary
within a population of these molecules
e.g.: starch and glycogen – storage polysaccharides
cellulose and chitin – structural polysaccharides
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 branched
polymer also contains a-1,6-bonds
Figure 10.12
Starch is stored by plants and used as fuel.
Prentice Hall c2002
Chapter 8
16
a-amylase on amylopectin
- a-amylase is found in plants and animals and is a hydrolase
it is an endoglycosidase – hydrolyzes internal a-(14)
glycosidic bonds
Glycogen is is stored by animals and used as fuel.
Prentice Hall c2002
Chapter 8
18
Glycogen
- Glycogen is the main storage polysaccharide of humans
- Glycogen is a polysaccharide of glucose residues connected
by a-(14) linkages with a-(16) branches (one branch every
8 to 12 residues)
- Glycogen is present in large amounts in liver and skeletal muscle
Cellulose – a structural polysaccharide that is a
Major component of cell walls of plants
Cellulose has b-(14) glycosidic bonds
Each glucose
residue is rotated
180o relative to
the next residue
Extended hydrogen bonding
between chains leads to
bundles or fibrils
Figure 10.14 Glycosidic bond
Determines polysaccharide
Structure.
Bent structure
Straight structure
- Humans digest starch and glycogens ingested in their diet
using amylases, enzymes that hydrolyze a-(14) glycosidic bond
- Humans cannot hydrolyze b-(14) linkages of cellulose. Therefore
cellulose is not a fuel source for humans. It is fiber.
- Certain microorganisms have cellulases, enzymes that hydrolyze
b-(14) linkages of cellulose.
- cattle have these organisms in their rumen
- termites have them in their intestinal tract
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.
or Thr
Mucins or mucoproteins
are proteins which has Nacetylgalactosamine
attached. This glycoprotein
is found in mucus and is a
lubricant.
Two methods to anchor the protein to carbohydrates
Figure 10.15 Glycosidic bonds between proteins and carbohydrates
asparagine
serine
Thr is also
used to make
O-linkages
Two methods to anchor the protein to carbohydrates
Figure 10.16 N-linked oligosaccharides
All N-linkages have a common core shown in grey.
High mannose
Protein residue
Complex structure
Protein residue
Additional sugars can be attached to these cores to make diverse and unique structures
Proteoglycans are glycoproteins were the protein is bound to a
special 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.
Assignment
Read Chapter 10
Read Chapter 11
Topics not covered:
Section 10.4 Lectins