Carbohydrates and the Glycoconjugates of Cell Surfaces

1
Chapter 7

• Carbohydrates and the Glycoconjugates of Cell
  Surfaces
• Biochemistry
• by
• Reginald Garrett and Charles Grisham

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Essential Question

• What is the structure, chemistry, and biological
  function of carbohydrates?
• (CH2O)n or (C H2O)n
• Breakdown of carbohydrates provides energy.
• Glycolipids and glycoproteins are glycoconjugates
  involved in recognition between cell types or
  recognition of cellular structures by other
  molecules.

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Outlines

• How Are Carbohydrates Named?
• What Is the Structure and Chemistry of
  Monosaccharides?
• What is the Structure and Chemistry of
  Oligosaccharides?
• What is the Structure and Chemistry of
  Polysaccharides?
• What Are Glycoproteins, and How Do They Function
  in Cells?
• How Do Proteoglycans Modulate Processes in Cells
  and Organisms?

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7.1 How Are Carbohydrates Named?

• Carbohydrates are hydrates of carbon.
• Monosaccharides (simple sugars) cannot be broken
  down into simpler sugars under mild conditions.
• Oligo "a few" - usually 2 to 10
• Polysaccharides are polymers of the simple sugars.

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7.2 What Is the Structure and Chemistry of
Monsaccharides?

• An organic chemistry review
• Aldoses and ketoses contain aldehyde and ketone
  functions, respectively.
• Triose, tetrose, etc. denotes number of carbons.
• Aldoses with 3C or more and ketoses with 4C or
  more are chiral.
• Review Fischer projections and D,L system.

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Stereochemistry Review

• Read text on p. 204-207 carefully!
• D,L designation refers to the configuration of
  the highest-numbered asymmetric center.
• D,L only refers the stereocenter of interest back
  to D- and L-glyceraldehyde!
• D,L do not specify the sign of rotation of
  plane-polarized light!
• All structures in Figures 7.2 and 7.3 are D.
• D-sugars predominate in nature.

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More Stereochemistry

• Know these definitions
• Stereoisomers that are mirror images of each
  other are enantiomers.
• Pairs of isomers that have opposite
  configurations at one or more chiral centers but
  are NOT mirror images are diastereomers.
• Any 2 sugars in a row in Figures 7.2 and 7.3 are
  diastereomers.
• Two sugars that differ in configuration at only
  one chiral center are epimers.

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Cyclic monsaccharide structures and anomeric forms

• Glucose (an aldose) can cyclize to form a cyclic
  hemiacetal.
• Fructose (a ketose) can cyclize to form a cyclic
  hemiketal.
• Cyclic form of glucose is mainly a pyranose.
• Cyclic form of fructose is mainly a furanose.

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Cyclic monsaccharide structures and anomeric forms

• Cyclic forms possess anomeric carbons.
• For D-sugars, ? has OH down, ? has OH up.
• For L-sugars, the reverse is true.
• Mutarotation The optical rotation of glucose
  solution could change with time. It involves
  interconversion of ?- and ?-D-glucose.
• ?D20 112.2? for ?-D-glucose
• ?D20 18.7? for ?-D-glucose

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Monosaccharide Derivatives

• Reducing sugars sugars with free anomeric
  carbons - they will reduce oxidizing agents, such
  as peroxide, ferricyanide and certain metals
  (Cu2 and Ag).
• Fehlings reagent CuSO4 (blue) RC(O)H ?
  Cu2O? (red) RCO2-
• Tollens reagent Ag ? Ag0?
• These redox reactions convert the sugar to a
  sugar acid.
• Glucose is a reducing sugar --- so these
  reactions are the basis for diagnostic tests for
  blood sugar.

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More Monosaccharide Derivatives

• Sugar alcohols (alditols) sweet-tasting, from
  mild reduction of sugars
• Deoxy sugars constituents of DNA, etc.
• Sugar esters phosphate esters like ATP are
  important.
• Amino sugars contain an amino group in place of a
  hydroxyl group.
• Acetals, ketals and glycosides basis for oligo-
  and poly-saccharides.

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7.3 What is the Structure and Chemistry of
Oligosaccharides?

• Its not important to memorize structures, but
  you should know the important features.
• Be able to identify anomeric carbons and reducing
  and nonreducing ends.
• Sucrose is NOT a reducing sugar.
• Browse the structures in Figure 7.19 and Figure
  7.20.
• Note carefully the nomenclature of links! Be able
  to recognize ?(1,4), ?(1,4), etc.

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7.4 What is the Structure and Chemistry of
Polysaccharides?

• Functions storage, structure, recognition
• Nomenclature homopolysaccharide vs.
  heteropolysaccharide.
• Lower the osmotic pressure.
• Starch and glycogen are energy storage molecules.
• Chitin and cellulose are structural molecules.
• Cell surface polysaccharides are recognition
  molecules.

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Starch

• A plant storage polysaccharide
• Two forms amylose and amylopectin
• Most starch is 10-30 amylose and 70-90
  amylopectin.
• Amylose has ?(1,4) links and one reducing end.
• Amylopectin has ?(1,6) branches in every 12-30
  residues.

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Starch

• Amylose and amylopectin are poorly soluble in
  water, but form micellar suspensions.
• In these suspensions, amylose is helical and
  iodine fits into the helices to produce a blue
  color. Amylopectin produces a red-violet color
  with I2.
• Salivary ?-amylase, an endoamylase, is
  ?(1?4)-glucan 4-glucanhydrolase.
• ?-amylase is an exoamylase, cleaving maltose
  units.
• ?(1?6)-glucosidase is required for complete
  hydrolysis of amylopepctin.

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Why branching in Starch?

• Consider the phosphorylase reaction...
• Phosphorylase releases glucose-1-P, products from
  the amylose or amylopectin chains.
• The more branches, the more sites for
  phosphorylase attack.
• Branches provide a mechanism for quickly
  releasing (or storing) glucose units for (or
  from) metabolism.

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Glycogen

• --- the glucose storage device in animals
• Glycogen constitutes up to 10 of liver mass and
  1-2 of muscle mass.
• Glycogen is stored energy for the organism.
• Only difference from amylopectin number of
  branches.
• ?(1,6) branches every 8-12 residues .
• Like amylopectin, glycogen gives a red-violet
  color with iodine.
• Hydrolyzed by ?-, ?-amylase, and glycogen
  phosphorylase.

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Dextrans

• A small but significant difference from starch
  and glycogen.
• If you change the main linkages between glucose
  from ?(1,4) to ?(1,6), you get a new family of
  polysaccharides dextrans.
• Branches can be (1,2), (1,3), or (1,4).
• Dextrans formed by bacteria are components of
  dental plaque.
• Cross-linked dextrans are used as "Sephadex" gels
  in column chromatography.
• These gels are up to 98 water!

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Structural Polysaccharides

• Composition similar to storage polysaccharides,
  but small structural differences greatly
  influence properties.
• Cellulose is the most abundant natural polymer on
  earth.
• Cellulose is the principal strength and support
  of trees and plants .
• Cellulose can also be soft and fuzzy - in cotton.

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Other Structural Polysaccharides

• Chitin - exoskeletons of crustaceans, insects and
  spiders, and cell walls of fungi.
• similar to cellulose, but C-2s are N-acetyl
• cellulose strands are parallel, chitins can be
  parallel or antiparallel.
• Alginates Ca2-binding polymers in algae.
• Agarose and agaropectin - galactose polymers
• Glycosaminoglycans - repeating disaccharides with
  amino sugars and negative charges.

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Bacterial Cell Walls

• Composed of 1 or 2 bilayers and peptidoglycan
  shell
• To resist high internal osmotic pressure, to
  maintain cell shape and size of bacteria.
• Gram-positive One bilayer and thick
  peptidoglycan outer shell.
• Gram-negative Two bilayers with thin
  peptidoglycan shell in between .
• Gram-positive pentaglycine bridge connects
  tetrapeptides.
• Gram-negative direct amide bond between
  tetrapeptides.

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More Notes on Cell Walls

• Note the ?-carboxy linkage of isoglutamate in the
  tetrapeptide
• Peptidoglycan is called murein - from Latin
  "murus", for wall
• Gram-negative cells are hairy! Note the
  lipopolysaccharide "hair" in Figures 7.35 and
  7.36.

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Cell Surface Polysaccharides

• A host of important functions!
• Animal cell surfaces contain an incredible
  diversity of glycoproteins (on the dell surface)
  and proteoglycans (in the extracellular matrix).
• In glass dishes, heart myocytes beat and liver
  cells avoid contact with kidney cells. Cancer
  cells grow without contact inhibition.
• These polysaccharide structures regulate
  cell-cell recognition and interaction. They
  contain several points for linkage (-OH) and are
  more informative than linear proteins and nucleic
  acids.
• The uniqueness of the "information" in these
  structures is determined by the enzymes that
  synthesize these polysaccharides.

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7.5 What Are Glycoproteins, and How Do They
Function in Cells?

• Many structures and functions!
• May be N-linked or O-linked.
• N-linked saccharides are attached via the amide
  nitrogens of asparagine residues.
• O-linked saccharides are attached to hydroxyl
  groups of serine, threonine or hydroxylysine.
• See structures in Figure 7.39

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O-linked Saccharides of Glycoproteins

• Function in many cases is to adopt an extended
  and relatively rigid conformation.
• These extended conformations resemble "bristle
  brushes.
• Bristle brush structure extends functional
  domains up out of the glycocalyx.
• See Figure 7.40

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N-linked Oligosaccharides

• Many functions known or suspected
• N-glycosylation of proteins can alter the
  chemical and physical properties of proteins,
  altering solubility, mass, and electrical
  charges.
• N-linked oligosaccharide moieties can (1)
  stabilize protein conformations, (2) protect
  against proteolysis and (3) promote correct
  folding of certain globular proteins (p. 239).
• Cleavage of monosaccharide units from N-linked
  glycoproteins in blood targets them for
  degradation in the liver. - see pages 238, 239

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7.6 - Proteoglycans

• --- Glycoproteins whose carbohydrates are mostly
  glycosaminoglycans.
• Components of the cell membrane and glycocalyx.
• Consist of proteins with one or two types of
  glycosaminoglycan.
• See structures, Figure 7.44

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7.6 How Do Proteoglycans Modulate Processes in
Cells and Organisms?

• Proteoglycans are glycoproteins whose
  carbohydrate moieties are predominantly
  glycosaminoglycans.
• Example syndecan - transmembrane protein -
  inside domain interacts with cytoskeleton,
  outside domain interacts with fibronectin.
• Highly sulfated glycosaminoglycans bind specific
  proteins (e.g. fibronectin) at sites containing
  basic amino acid residues. (charge interactions)
• A particular pentasaccharide sequence in heparin
  finds to antithrombin III. (sequence-specific)

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Proteoglycan Functions

• Modulation of cell growth processes
• Binding of growth factor proteins by
  proteoglycans in the glycocalyx provides a
  reservoir of growth factors at the cell surface.
• Cushioning in joints
• Cartilage matrix proteoglycans absorb large
  amounts of water. During joint movement,
  cartilage is compressed, expelling water!