Transcript Carbohydrates - BioChemWeb.net

Carbohydrates
Energy Cycle of Life
Monosaccharides (Monomeric Sugars)
• Formula: (CH2O)n (with n ≥ 3)
–
–
–
–
–
–
Trioses: n=3
Tetroses: n=4
Pentoses: n=5
Hexoses: n=6
Heptoses: n=7
Octoses: n=8
• Sugars contain one aldehyde or ketone carbonyl and at least
two alcohols.
• Pentoses and hexoses can be linear or cyclic (cyclic or ring form
favored under physiological conditions)
Trioses: The Simplest Monosaccharides
Aldehyde-containing sugar = aldose
Ketone-containing sugar = ketose
Aldose-Ketose Interconversion via
Enediol Intermediate
Note: Isomerization of dihydroxyacetone phosphate to
glyceraldehyde-3-phosphate during glycolysis is catalyzed
by triose phosphate isomerase, a “perfect enzyme.”
The Enantiomers of Glyceraldehdye
Most naturally occurring sugars are D-sugars.
RS Nomenclature System (Cahn, Prelog,
Ingold System)
Some Abundant Monosaccharides
Stereochemistry of Aldotetroses
<- Enantiomers ->
<- Enantiomers ->
<- Diastereomers ->
The Two Enantiomers of the Ketotetrose
Erythrulose
Stereochemical Relationships of the DAldoses
Stereochemical Relationships of the DKetoses
Cyclization of Hexoses
Pentoses can cyclize also (e.g., ribofuranose).
Pyranoses vs. Furanoses
Pyranose Ring Sugars in Chair and Boat
Conformations
Chair conformation more stable
Conformations of Chair Form of a
Pyranose
The Two Chair Conformations of b-DGlucopyranose
Conformation on the left more stable since
relatively bulky OH and CH2OH groups all
occupy equatorial positions (in the
conformation on the right they occupy more
crowded axial positions).
Conformational Isomers of Furanoses
a and b Anomers of Glucose
a anomer
b anomer
Anomers: different
configurations at C1
The Four Most Common Hexoses
Abundance of Different Forms of Some
Monosaccharides
Terminology Describing Structure of Sugars
Derivatives of the Monosaccharides
Modified Sugars
Acid and Lactone Derivatives
Oxidation of a monosaccharide yields the
corresponding acid and lactone derivatives.
Alditols
Reduction of the carbonyl group on a sugar
yields the corresponding alditol.
Amino Sugars
Often found in complex oligoand polysaccharides.
Glycosides and the Glycosidic Bond
Two Toxic Glycosides Produced by Plants
Inhibits Na+/K+ ATP-dependent pump
Yields hydrogen cyanide (HCN) on
hydrolysis.
Oligosaccharides
Structures of Some Important
Disaccharides
Reducing sugar
ide
Non-reducing sugars
H
A reducing sugar has a free anomeric carbon not in glycosidic bond (free
hemiacetal or hemiketal).
Reducing sugars
Occurrence of Some Dissaccharides
Formation of the Glycosidic Bond
Formation of Lactose in vivo
Polysaccharides
• Storage polysaccharides
• Structural polysaccharides
• Glycosaminoglycans
Amylose: A Linear Glucose Polymer
(Glucan)
A major storage form of
glucose in plants.
Polymer of a-D-glucopyranose with a(1->4) glycosidic
linkages
Amylopectin: A Branched Glucan
Another storage form of
glucose in plants.
a(1->4) and a(1->6)
glycosidic linkages
R = reducing end
N = nonreducing end
Glycogen: The Major Storage Form of
Glucose in Animals
•
•
Also a branched glucan with a(1->4) and a(1->6) glycosidic linkages linkages,
like amylopectin.
Has more frequent a(1->6) branches.
Glycogen granules in a liver cell
Organization of Plant Cell Walls
Cellulose: A Structural Polysaccharide in
Plants
Polymer of b-Dglucopyranose with b(1->4)
glycosidic linkages
Chitin: A Structural Polysaccharide in
Exoskeletons of Many Arthropods and
Mollusks
Polymer of N-acetyl-b-Dglucosamine (GlcNAc)
with b(1->4) glycosidic
linkages
Repeating Structures of Some
Glycosaminoglycans
Glycosaminoglycans constitute the carbohydrate component of proteoglycans,
conjugates of glycosaminoglycans and proteins, with structural functions in
vertebrate cartilage and connective tissue.
Glycoconjugates
Functions of Oligosaccharides in
Glycoconjugates
•
•
•
•
•
•
•
•
•
Structural
May affect protein stability or activity
Cell markers and cell surface recognition factors
Immunochemical markers (e.g., ABO blood group
antigens)
Cell-substratum and cell-cell adhesion
Sperm/egg recognition
Cell growth signals
Tagging for transport to lysosomes, tagging for
cleavage/destruction
Protein folding
Bacterial Cell Walls
Bacterial Cell Walls: Peptidoglycans
Peptidoglycan Structure of Gram-Positive
Bacteria
Structure of Lipoteichoic Acid
A membrane-anchored glycolipid
that protrudes through the cell
wall, helps to anchor
peptidoglycan to membrane, as
well as playing a role in adhesion
and cell-cell interactions. It is also
a major antigenic determinant in
the immune response against
Gram-positive bacteria.
Ends in diacylglycerol
moiety anchored in
membrane.
Bacterial Cell Walls: Gram-Negative
Bacteria
NAM = N-acetylmuramic acid
NAG = N-acetylglucosamine
Glycoconjugates Associated with Animal
Cells
Some Glycoproteins
Most extracellular, secreted proteins, as well as extracellular domains of
membrane-bound proteins, are glycosylated.
In addition, some proteins resident in the endoplasmic reticulum and
Golgi apparatus are glycosylated.
Glycoprotein Bonds: N- and O-Linked
Oligo- and Polysaccharides
N-linked sugars
Attached in endoplasmic reticulum on
specific asparagine residues.
Attached in Golgi apparatus on specific
threonine or serine residues.
O-linked sugars
Most commonly: b-galactosyl-(1->3)-a-Nacetylgalactosamine.
N-Linked Oligosaccharides
• 14-residue oligosaccharide (9 mannose, 3 glucose, 2 Nacetylglucosamine (GlcNAc)) attached cotranslationally in ER to
asparagine residues in sequence Asn-X-Ser or Asn-X-Thr
(where X is any amino acid except Pro or Asp).
• Some sugars removed in ER and Golgi by glucosidases and
mannosidases.
• All final N-linked oligosaccharides have common
pentasaccharide core:
Mana(1->6)
Manb(1->4)GlcNAcb(1->4)GlcNAcb1->Protein
Mana(1->3)
• Additional sugars (variable) attached to core, such as GlcNAc,
galactose, fucose and sialic acid, by glycosyltransferases in
Golgi.
• A single protein can be both N-glycosylated and O-glycosylated.
Attachment and Processing of N-Linked
Oligosaccharides
Proteoglycans
Proteoglycans are glycosaminoglycanprotein conjugates that are one of the
components of the matrix around cells in
skin and connective tissue.
Proteoglycans are highly hydrated and, in
conjunction with collagen, help give these
tissues their resilience and resistance to
shear forces.
Aggrecan: the major
proteoglycan in cartilage.
Structure of Aggrecan
Syndecan: A Transmembrane
Proteoglycan
Example of a Glycolipid Conjugate: The
ABO Blood Group Antigens
No additional sugar here
= Type O (non-antigenic
oligosaccharide)
Individual makes antibodies
against A
and B antigen.
+ N-Acetylgalactosamine
= Type A antigen
Both A and B antigens
= Type AB
Individual makes antibodies against
B antigen.
Individual makes neither
anti-A nor anti-B antibodies.
+ Galactose
= Type B antigen
Individual makes antibodies
against A antigen.
Structural Dynamics of Oligosaccharides
Attached to Proteins
Cell Surface Recognition Factors
Lectins: Carbohydrate-Binding Proteins
Lectins are carbohydratebinding proteins often
found on the surface of
cells, providing one
mechanism of cell-cell
recognition through
binding of lectin on one
cell to carbohydrate on
other cell.
Sequencing Oligosaccharides
Cleavage of an Oligosaccharide by
Glycosidases