Download Sugars and Polysaccharides Carbohydrates Saccharide

Carbohydrates
Sugars and
Polysaccharides
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Voet Biochemistry:
Chapter 11, Pages 356 – 369
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Voet Fundamentals:
Chapter 8, Pages 219 – 234
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Lecture 15
complex carbohydrates are heterogeneous in
size and composition
saccharide sequences are not subject to
standard genetic analysis
difficult to establish functional assays as
typically have structural roles
Structural and functional studies of
carbohydrates are less advanced than for nucleic
acids and proteins
Biochemistry 2000
Monosaccharide Classification
according to chemical
nature of carbonyl
group and
number of C atoms
Monosaccharides - the basic unit of carbohydrates
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synthesized from simpler substances in gluconeogenesis
or photosynthesis
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major energy source & components of nucleic acids and
cerebrosides (lipids)
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Oligosaccharides - consist of a few covalently linked
monosaccharides
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associated with proteins and lipids
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serve structural and regulatory roles
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Polysaccharides - consist of many
covalently linked monosaccharides
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structural function in all organism
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nutritional reserve
Lecture 15
polyhydroxyl aldehyde (aldose) or polyhydroxyl
ketone (ketose)
Lecture 15
Biochemistry 2000
Saccharide Classification
“carbon hydrate” - reflecting the chemical
composition of simple monosaccharides
Aldose or Ketose
Triose, tetrose, pentose,
hexose, or heptose
i.e. Aldotetrose,
or Ketopentose
etc.
D-aldose family of
monosaccharides
Biochemistry 2000
Lecture 15
Biochemistry 2000
Monosaccharide Classification
Stereochemistry
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Classification in D and L carbohydrates
D-aldose family of
monosaccharides
Based upon configuration of chiral C in
glyceraldehyde
Determined by chiral C furthest from
carbonyl for sugars
Example of epimers
Stereochemical terminology (review)‫‏‬
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Enantiomers: differ at all chiral centers
(mirror image molecules)
Diastereomers: differ at one or more
chiral centers
Circles:
Epimeric Carbons
Squares:
D-configuration
Epimers: differ at one chiral center
Lecture 15
Biochemistry 2000
Lecture 15
Configurations
Reactivity (review)
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Alcohols react with aldehydes (1:1 ratio) to form hemiacetals
and (2:1 ratio) acetals
Alcohols react with ketones (1:1 ratio) to form hemiketals
and (2:1 ratio) ketals
Lecture 15
Biochemistry 2000
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Biochemistry 2000
Aldoses can react with themselves forming cyclic hemiacetals (left)
and ketoses can react with themselves forming cyclic hemiketals
(right)
saccharides forming 5 and 6 member rings are referred to as a
furanose and pyranose, respectively
Lecture 15
Biochemistry 2000
Biochemistry 2000
Anomers
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Cyclization can generate two
diastereomers or anomers
Mutarotation
Glucose
Fructose
hemiacetal or hemiketal carbon is
referred to as the anomeric carbon
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α and β anomeric forms of saccharides are in equilibrium
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Mutarotation: spontaneous interconversion between anomeric forms
α anomer: anomeric carbon OH is on
the opposite side of the ring from the
CH2OH of the chiral center defining
D- or Lβ anomer: anomeric carbon OH is on the
same side of the ring as the CH2OH of
the chiral center defining D- or LLearn these structures!
Lecture 15
Biochemistry 2000
~33 %
Lecture 15
Conformational Variability
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~1 %
~66 %
Biochemistry 2000
Glycosidic Bonds
Pyranoses adopt “chair” or “boat” conformations due to the
tetrahedral configuration of carbon atoms
Boat form is only observed when bulky substituents are present
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Boat conformation
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Lecture 15
Biochemistry 2000
Condensation reaction
between anomeric carbon
and any other hydroxyl
containing compound
produces a glycosidic bond
chemically formation of an
acetal or ketal
In disaccharides, the
glycosidic bond is typically
between C1 and C4
glycosidic bonds between
C1 & C6, C1 & C1 are also
common
Lecture 15
Biochemistry 2000
Disaccharides
Polysaccharides
Naming of glycosidic bond:
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(α/β number of anomeric carbon –
(α/β if also anomeric) number of
alcohol carbon)
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lactose – milk sugar (β1-4)
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trehalose – insect hemolymph (α1-α1)
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sucrose – common sugar (α1-β2)
Lecture 15
Biochemistry 2000
Homopolysaccharides
Cellulose: unbranched β1-4 linked
glucose; serves structural role in
plants; indigestible by mammals
Amylose: unbranched α1-4 linked
D-glucose
Glycogen: amylose with α1-6
branch every 8-12 residues
Lecture 15
Biochemistry 2000
variable in length, can be composed of one or more types of
monosaccharide and can be linear or branched
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Homopolysaccharides: composed of one monosaccharide type
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Heteropolysaccharides: composed of two or more monosaccharide types
Lecture 15
Biochemistry 2000