Carbohydrates slide

1. 2.2. Carbohydrates
• Hydrated carbons of polyhydroxy aldehydes or ketones.
• Composed of carbon, hydrogen and oxygen with
emperical formula, (CH2O)n.
• Function
• Energy sources, 4KCal/g
• Structural and protective (DNA , RNA, cell wall)
• Recognition
• Adhesion

2. • The 2-D extended structure is called Fischer projection
and the 3-D ring one is called Haworth formula.
• The ring structure of aldose and ketose are called
hemiacetal and hemiketal, respectively.
• Three classes
• Monosaccharide
• Oligosaccharide
• Polysaccharide

3. Monosaccharides
• Single polyhyrdoxy aldehyde or ketone unit.
• Glucose is the most abundant monosaccharide.
• Those with more than 4 carbon have cyclic structure.
• Important fuel molecules & building blocks for nucleic acids.
• The smallest monosaccharides, for which n = 3, are
dihydroxyacetone and D & L glyceraldehyde.
– Thus they are trioses.

4. Functional groups
Ketone = R-CO-R Aldehyde = R-CO-H

5. Functional groups

6. The D and L isomers are enantiomers or mirror images of each other

8. • Monosaccharides with 4, 5, 6, and 7 C-atoms are called
tetroses, pentoses, hexoses, and heptoses, respectively.
• Because these molecules have multiple asymmetric
carbons, exist as diastereoisomers (optically active
isomers that are not mirror images) as well as enantiomers.
• Note that D-glucose and D-mannose differ in configuration
only at C-2, such sugars are called epimers.
• Thus, D-glucose and D-mannose are epimeric at C-2; D-
glucose and D-galactose are epimeric at C-4.

10. • Unmodified glucose reacts with oxidizing agents such as
Cu2+ (Fehling's solution) because the open-chain form
has a free aldehyde group that is readily oxidized.
• Sugars that react are called reducing sugars; those that
do not are called non-reducing sugars.
• Non-reducing sugars do not have a free aldehyde group
and so cannot react with Cu2+.
• Monosaccharides are reducing sugars.

11. D-Glucose D-Gluconic acid

12. Ring formation
• The predominant forms of ribose, glucose, fructose, and
many other sugars in solution exist as rings.
• In the process of cyclization in general, an aldehyde can
react with an alcohol to form a hemiacetal.
• For an aldohexose such as glucose, the C-1 aldehyde in
the open-chain form of glucose reacts with the C-5
hydroxyl group to form an intramolecular hemiacetal.

13. • The resulting cyclic hemiacetal, a six-membered ring, is
called pyranose because of its similarity to pyran.
• Analogous to Pyran:
O
pyran
O
H
OH
H
OH
H
OH
H
OH
CH2OH
H
1
2
3
4
5
6

14. • The aldehyde C atom now becomes asymmetric during ring
formation.
• This new asymmetric carbon atom formed during cyclization
is called the Anomeric carbon.
• The cyclic structure containing the -OH group on the right of
the anomeric carbon is known as α-D-glucopyranose; on
the left side is called β-D-glucopyranose.

15. • Haworth projections:
– Can be written from the Fischer projection.
– C1 drawn on the right (anomeric C).
– The cyclic structure of a D-isomer has the last CH2OH
group located above the ring (C6).
– –OH groups on the left are drawn up (C3).
– –OH groups on the right are drawn down (C2, C4).

16. • Haworth projections:
• C atoms are not shown.
• The designation α means that the hydroxyl group attached
to C-1 is below the plane of the ring; β means that it is
above the plane of the ring.
• The C-1 carbon atom is called the anomeric carbon atom,
and the α and β forms are called anomers.

17. Ring formation …
Anomeric C

19. • Similarly, a ketone can react with an alcohol to form a
hemiketal.
• The C-2 keto group in the open-chain form of a
ketohexose, such as fructose, can form an intramolecular
hemiketal by reacting with either the C-6 -OH group to form
a six-membered cyclic hemiketal or the C-5 -OH group to
form a five-membered cyclic hemiketal, respectively.
• The five-membered ring is called a furanose because of
its similarity to furan.

20. Ring formation …

22. Ring formation …

23. Ring formation: Pentose sugars

25. Monosaccharides join with amines and alcohols
through glycosidic bonds
• The anomeric C-atom reacts with the –OH group of methanol to
form two products, methyl α & β -D-glucopyranoside.
• The new bond formed between the anomeric carbon atom of
glucose and the -OH oxygen atom of methanol is called a
glycosidic bond specifically, an O-glycosidic bond.
• The anomeric carbon atom of a sugar can also be linked to the
nitrogen atom of an amine to form an N-glycosidic bond.

27. Modified monosaccharides
Usually expressed on cell surfaces

28. Modified monosaccharides…

29. Complex carbohydrates formed from monosaccharides
• Because sugars contain many hydroxyl groups, glycosidic
bonds can join one monosaccharide to another.
• Oligosaccharides are built by the linkage of two or more
monosaccharides by O-glycosidic bonds.
• The wide array of these linkages in concert with variety of
monosaccharides and their many isomeric forms makes
complex carbohydrates information-rich molecules.

30. • Complex forms include:
– Disaccharides
– Oligosaccharides
– Polysaccharides
Disaccharides
• Consist of two sugars joined by an α-glycosidic bond.
• Most common ones
• Sucrose:
• Lactose:
• Maltose

31. • Sucrose: formed by glycosodic linkage between α-D-
glucopyranose and β-D-fructofuranose.
– The enzyme sucrase breaks the disaccharides.
• Lactose: formed by β-1-4 glycosidic linkage between
galactose and glucose.
– In humans lactase and in bacteria β-galactosidase enzyme breaks
the linkage.
• Maltose: two D-glucose residues are joined by a glycosidic
linkage b/n the α-anomeric form of C-1 on one sugar and
the -OH oxygen atom on C-4 of the adjacent sugar.
– Such a linkage is called an α -1,4-glycosidic bond.
– Hydrolysed by maltase.

32. Disaccharides …
Maltose

33. Disaccharides …

34. Polysaccharides
• Large polymeric oligosaccharides, formed by the linkage
of multiple monosaccharides are called polysaccharides.
• Polysaccharides play vital roles in energy storage and in
maintaining the structural integrity of an organism.
• If all of the monosaccharides are the same, these
polymers are called homopolymers.

35. Glycogen
• The most common homopolymer in animal cells is glycogen.
• This storage form of glucose is a very large, branched
polymer of glucose residues.
• Most of the glucose units in glycogen are linked by α -1,4-
glycosidic bonds.
• The branches are formed by α -1,6-glycosidic bonds, present
about once in 10 units.

36. Starch
• The nutritional reservoir in plants is starch, of which there
are two forms:
• Amylose, the unbranched type of starch, consists of
glucose residues in α -1,4 linkage.
• Amylopectin, the branched form, has about 1 α -1,6
linkage per 30 α -1,4 linkages, in similar fashion to
glycogen except for its lower degree of branching.
• More than half the carbohydrate ingested by human
beings is starch.
• Both amylopectin and amylose are rapidly hydrolyzed by
α -amylase, an enzyme secreted by the salivary glands
and the pancreas.

37. Strach and glycogen: energy storage polysaccharides

38. Amylopectine

39. Cellulose
• Cellulose is major polysaccharide of glucose found in plants,
serves a structural rather than a nutritional role.
• Cellulose is one of the most abundant organic compounds
in the biosphere.
• It is an unbranched polymer of glucose residues joined by β-
1,4 linkages.

40. Cellulose…
• The α configuration allows cellulose to form very long,
straight chains.
• Fibrils are formed by parallel chains that interact with one
another through hydrogen bonds.
• The α-1,4 linkages in glycogen and starch produce a
different molecular architecture from that of cellulose.
• A hollow helix is formed instead of a straight chain in
glycogen and starch.

41. • These differing consequences of the α and β linkages are
biologically important.
• The straight chain formed by β linkages is optimal for the
construction of fibers having a high tensile strength.
• In contrast, the open helix formed by α linkages is well
suited to forming an accessible store of sugar.
• Mammals lack cellulases and therefore, cannot digest
wood and vegetable fibers.
Cellulose…

42. Cellulose: a structural polysaccharide
Favour straight chain
Favour bent structures

43. Chitin
• Same as cellulose, except –OH on C2 replaced with
acetamide.
– Amino sugar
– Homopolymer of N-acetyl-D-glucosamine
• Very strong.
• Structural component of exoskeleton of arthropods

44. Glycoproteins
• Covalent attachment to proteins: glycosylation.
• Linkage through
• N atom of R chain of Asparagine (Asn) (N-linked) to from
GlcNac :N acetyl glucose amine or
• O atom of R chain of Serine or Threonine residues (O-
linkage) to form GalNac: Nacetyl galactoamine.
• Asn accept an oligosaccharide if
• Asn-X-Ser or Asn-X-Thr (X = any residue).
• Thus potential glycosylation sites can be detected within
aa sequences in the polypeptide chains.

46. Glycoproteins …
• All N-linked glycopeptides have in common
• A pentasaccharide core consisting of
– 3 mannose
– 2 N-acetylglucoseamine residues
• Additional sugars attached to this core
– Form great variety of glycoproteins

49. • Carbohydrates are linked to some soluble proteins as well
as membrane proteins.
• In particular, many of the proteins secreted from cells are
glycosylated.
• Most proteins present in the serum component of blood
are glycoproteins (Eg: elastase).
• Furthermore, N-acetylglucosamine residues are O-linked
to some intracellular proteins.
• The role of these carbohydrates, which are dynamically
added and removed, is under active investigation.

50. Elastase

51. • Protein glycosylation takes place inside the lumen of the
endoplasmic reticulum (ER) and the Golgi complex,
organelles that play central roles in protein trafficking.
• Elastase, which is secreted by the pancreas as a zymogen,
is synthesized by ribosomes attached to the cytoplasmic
face of the ER membrane.
• The peptide chain is inserted into the lumen of the ER as it
grows, guided by a signal sequence of 29 amino acids at
the amino terminus.

52. • This signal sequence, is then cleaved from the protein in
the transport process into the ER.
• After the protein has entered the ER, the glycosylation
process begins.
• The N-linked glycosylation begins in the ER and
continues in the Golgi complex, whereas the O-linked
glycosylation takes place exclusively in the Golgi.

54. • The Golgi complex is the sorting center in the targeting of
proteins to lysosomes, secretory vesicles, and the plasma
membrane.
• The cis face of the Golgi complex receives vesicles from
the ER, and the trans face sends a different set of
vesicles to target sites.
• Vesicles also transfer proteins from one compartment of
the Golgi complex to another.

56. Mannose 6-phosphate target lysosomal
enzymes to their destination …
• Active enzymes are synthesized from Golgi
• But exported rather than directed to lysosomes
• Enzymes mislocated in I-cell disease
• These enzymes normally contain
• Mannose 6-phophate residue
• In I-cell the mannose is unmodified
• Mannose 6-phophate is a marker which direct many
hydrolytic enzymes from Golgi to lysosomes.

57. • A glycoprotein destined for delivery to lysosomes acquires
a phosphate marker in the cis Golgi compartment in a two-
step process.
– First, a phosphotransferase adds a phospho-N-acetylglucosamine
unit to the 6-OH group of a mannose residue.
– Then a phosphodiesterase removes the added sugar (Glu Nac) to
generate a mannose 6-phosphate residue in the core
oligosaccharide.
• I-cell patients are deficient in the phosphotransferase
catalyzing the first step in the addition of the phosphoryl
group.