Chapter 7 Flashcards
Sugars (saccharides) contain an
aldehyde or ketone group and
two or more hydroxyl groups.
Monosaccharides generally contain
several chiral carbons and
therefore exist in a variety of stereochemical forms, which may
be represented on paper as Fischer projections. Epimers are
sugars that differ in configuration at only one carbon atom.
Monosaccharides with at least six carbons commonly form
cyclic structures, in which the aldehyde or ketone group is joined
to a hydroxyl group of the same molecule. The cyclic structure
can be represented as a Haworth perspective formula. The
carbon atom originally found in the aldehyde or ketone group
(the anomeric carbon) can assume either of two configurations,
α and β, which are interconvertible by mutarotation. In the
linear form of the monosaccharide, which is in equilibrium with
the cyclic forms, the anomeric carbon is easily oxidized, making
the compound a reducing sugar.
Naturally occurring hexoses include some that have —NH2 at
C-2 (amino sugars), often with the amino group acetylated.
Oxidation of the carbonyl carbon of glucose and other aldoses
yields
aldonic acids (gluconic acid); oxidation at C-6 produces
uronic acids (glucuronate). Some sugar intermediates are
phosphate esters (for example, glucose 6-phosphate).
A hydroxyl group of one monosaccharide can add to the
anomeric carbon of a second monosaccharide to form an acetal
called a glycoside. Oligosaccharides are short polymers of several
different monosaccharides joined by glycosidic bonds. At one end
of the chain, the reducing end, is a monosaccharide residue with
its anomeric carbon not involved in a glycosidic bond, and
therefore available to be oxidized.
Disaccharides and oligosaccharides are named as derivatives of
the sugar at the reducing end. Their names provide
the order of
monosaccharide units, the configuration at each anomeric
carbon, and the carbon atoms involved in the glycosidic
linkage(s)
Homopolysaccharides contain only
a single monomeric sugar species;
heteropolysaccharides contain two or more kinds of monomers.
The homopolysaccharides starch and glycogen are s
storage fuels in plant,
animal, and bacterial cells. They consist of D-glucose units with (α1→4)
linkages, and both contain some branches
The homopolysaccharides cellulose, chitin, and dextran serve s
structural
roles. Cellulose, composed of (β1→4)-linked D-glucose residues, lends
strength and rigidity to plant cell walls. Chitin, a polymer of (β1→4)-linked
N-acetylglucosamine, strengthens the exoskeletons of arthropods.
Homopolysaccharides assume stable conformations dictated by
weak
interactions. The chair form of the pyranose ring is essentially rigid, so the
conformation of the polymers is determined by rotation about the C—O
bonds of the glycosidic linkage. Starch and glycogen form helical structures
with intrachain hydrogen bonding; cellulose and chitin form long, straight
strands that interact with neighboring strands.
Bacterial cell walls are strengthened by peptidoglycan in which the
repeating disaccharide is GlcNAc(β1→4) Mur2Ac.
Glycosaminoglycans are extracellular heteropolysaccharides in w
which one
of the two monosaccharide units is a uronic acid (keratan sulfate is an
exception) and the other is an N-acetylated amino sugar. The high density of
negative charge on these molecules forces them to assume extended
conformations. These polymers (hyaluronan, chondroitin sulfate, dermatan
sulfate, and keratan sulfate) provide viscosity, adhesiveness, and tensile
strength to the extracellular matrix.
Proteoglycans are
huge molecules in which one or more large
glycans, called sulfated glycosaminoglycans (heparan sulfate,
chondroitin sulfate, dermatan sulfate, or keratan sulfate), are
covalently attached to a core protein. Bound to the outside of the
plasma membrane through a peptide or lipid, proteoglycans
provide points of adhesion, recognition, and information transfer
between cells, or between a cell and the extracellular matrix. In
the extracellular matrix, enormous proteoglycan aggregates form,
bound to a long hyaluronan molecule.
Glycoproteins contain oligosaccharides covalently linked to
Asn, Ser, or Thr residues. The oligosaccharides are typically
branched and smaller than glycosaminoglycans. Many cell
surface or extracellular proteins are glycoproteins, as are most
secreted proteins. The covalently attached oligosaccharides
influence the folding and stability of the proteins, provide critical
information about the targeting of newly synthesized proteins,
and allow specific recognition by other proteins.
Glycolipids and glycosphingolipids in plants and animals and
lipopolysaccharides in bacteria are components of
f the cell
envelope, with covalently attached oligosaccharide chains
exposed on the cell’s outer surface
