3.3: Carbohydrates
unmodified polyglucoses can be roughly divided into two groups:
1. insoluble, structural polysaccharides 2. hydrated, storage polysaccharides.
Proteoglycans
a diverse group of glycoproteins made up from aggregates of proteins with glycosaminoglycans
If lysozyme is so effective in cleaving the heteropolysaccharide of the bacterial cell wall, why isn't it used clinically to fight bacterial infection?
Lysozyme is fairly indiscriminate in cleaving polysaccharides. It cleaves the host's polysaccharides as well as the bacterial cell wall polysaccharides.
All of the eight D-aldohexoses and four D-ketohexoses are recognized as
different compounds by the body
alcohol + aldehyde =
hemiacetal (glucose)
The bacterial cell wall is an example of a
heteropolysaccharide
_____________________________ occur if more than one kind of hexose is used
heteropolysaccharides
The structure of the proteoglycans, together with their polyionic character, causes the complex to be...
highly hydrated
The rigidity of their cell walls allows bacteria to live in a
hypotonic environment
The pattern of sugars linked to individual glycoproteins in cell membranes contributes to
immunological identity
Bacterial cell walls are composed of
linear carbohydrate chains cross-linked by repeating peptide chains
Besides acting as biochemical fuel, sugars are also found...
linked to proteins (glycoproteins) and lipids (glycolipids)
the position of the anomeric hydroxyl group will determine if water can penetrate __________________ or whether the sheets of linked glucoses can hydrogen bond together to exclude water (cellulose).
the polysaccharide structure (starch, glycogen)
the position of the anomeric hydroxyl group will determine whether
the sheets of linked glucoses can hydrogen bond together to exclude water (cellulose)
Why is the chair confirmation of glucose more common?
the steric crowding is minimized
Proteins without attached sugars are not as highly hydrated as
those with bound sugars
insoluble structural polysaccharides are joined by
β(1 → 4) glycoside bonds
In general, insoluble polysaccharides are
β-linked
Proteoglycans compose what is called the _______________________ of the connective tissue.
"ground substance"
Properties of the carbohydrate portion in glycoproteins:
- increases the aqueous solubility of a protein (the carbohydrate groups are always found on the outside of the three-dimensional structure of a protein). - protects the protein portion from enzymatic degradation. - can organize the structure of bulk water (ice-nucleation proteins), or conversely, can disorganize water structure (anti-freeze proteins). - acts as a recognition marker in various biological processes (e.g., the carbohydrate portions of membrane-bound glycoproteins are always located on the external surface of the cell membrane).
When glucose cyclizes, the anomeric hydroxyl group can be in one of two positions:
1. the opposite side of the ring from the free −−CH2OH, (giving α-D-glucose) 2. the same side of the ring as the free −−CH2OH (giving β-D-glucose)
peptidoglycan
A protein-carbohydrate compound that makes the cell walls of bacteria rigid
Why do ketoses have fewer chiral carbons than the equivalent aldoses?
An aldose with n carbons will have an aldehyde group as C1 and a −−CH2OH as Cn. Neither group contains a chiral carbon: the aldehyde group has only three substituents on carbon, and the terminal carbon does not have four different substituents on carbon. However, all the intervening carbons have four different groups attached, and so will be chiral (n-2 chiral carbons). A ketose possesses two terminal −−CH2OH groups (non-chiral) and an internal ketone group (non-chiral). A ketose has n-3 chiral carbons.
________________________ contribute to the ability of the organism to cause disease
Bacterial cell walls
We noted earlier in this lesson that β-linked polysaccharides are normally insoluble. Does this insoluability apply to the β-linked bacterial cell wall? Describe the difference or similarity between cellulose (an insoluble β-linked polysaccharide) and the bacterial cell wall.
Cellulose is insoluble because of interchain hydrogen bonding, which excludes water. NAM and NAG both contain N-acetyl groups side chains. These N-acetyl groups interfere with the close packing between polysaccharide chains that is a necessary condition for interchain H-bonding. (It is not β-linking per se that causes insolubility, but the packing allowed by the symmetry of β-linking.) In addition, the heteropolysaccharide chains of the bacterial cell wall are linked by polypeptide chains. The overall structure of the bacterial cell wall is a covalently bonded net, overlying the plasma membrane, which protects the hypertonic bacterium without impeding passage of biomolecules in and out of the bacterium.
__________________ is one of the least soluble and toughest structural biochemical molecules known.
Chitin
Each NAM contains a lactic acid residue linked to a tetrapeptide:
D-ala-isoglutamate-L-lys-D-ala
Examples of polyglucose chains with side branches
Glycogen and amylopectin
_____________________________ occur if all monomeric sugar units are the same
Homopolysaccharides
Why does chitin hydrogen bond more efficiently to more chitin than cellulose can to cellulose?
In chitin, a hydroxyl group on each glucose has been substituted by an acetamido functional group; hence, chitin can hydrogen bond more efficiently to other chitin chains than cellulose can to other cellulose chains.
What prevents branching from both sides of a glucose unit in a polysaccharide?
In theory, nothing prevents branching from both sides of a glucose unit in a polysaccharide. Steric crowding is probably what prevents one sugar from being attached to more than three others (i.e., the branch and the two parts of the main chain). In fact, branches usually protrude from every sixth to tenth unit of the polysaccharide chain, rather than from every monosaccharide unit.
Microorganisms in the gut can hydrolyse the polysaccharides in beans, but not all humans can do so in the process of digestion. How does this fact explain the well-known production of gas that occurs after eating a meal of beans?
Microorganisms, after hydrolysing the polysaccharides to monosaccharides, will continue to process the monosaccharides before the host organism can absorb them. The end product of metabolism is frequently a gas, so that metabolism can proceed smoothly.
The carbohydrate chains are alternating _______________________________ units that are β(1 → 4) linked
N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)
What is the mechanism of penicillin's action on the bacterial cell wall?
Penicillin specifically binds to and inactivates enzymes that cross-link the peptidoglycan strands of the bacterial cell wall, thus preventing growing cells from synthesizing the cell wall needed for protection from the hypotonic environment. As a result, the bacterial cells lyse.
Outline the structure of peptidoglycan.
The structure of peptidoglycan is composed of linear chains of alternating β(1 → 4)-linked N-acetylglucosamine and N-acetylmuramic acid. The lactic group of N-acetylmuramic acid forms an amide bond with a D-amino acid-containing tetrapeptide to form the peptidoglycan repeating unit (e.g., L-Ala-D-isoglutamyl-L-Lys-D-Ala in the bacterium Staphylococcus aureus.
What common biochemical structure is found in cartilage and hyaluronic acid? Where are cartilage and hyaluronic acid found in the body? Why do they have such different physical properties?
The structures are glycoproteins. The flexibility of the various glycoproteins is related to the degree of hydration of the individual glycoprotein. The degree of hydration of the glycoprotein, in turn, is related to the amount (and accessibility) of the sugar residues. Joint lubricating fluid (hyaluronic acid) consists of a gel-like matrix of modified, highly charged polysaccharides. This matrix is more highly hydrated and more flexible than any of the other glycoproteins. Cartilage is the least hydrated and thus the least flexible of the structures.
What is the difference between the α anomer and the β anomer of D-glucose?
The α anomer and the β anomer of D-glucose differ in the position of the hydroxyl (OH) group at the anomeric carbon. In the α anomer, the OH substituent of the anomeric carbon is on the opposite side of the ring from the CH2OH group at the chiral centre. In the β anomer, the OH group is on the same side as the CH2OH group.
Compare and contrast the structures and functions of cellulose, chitin, starch, and glycogen.
This question refers to the functions of the following polysaccharides: cellulose, chitin, starch, and glycogen. Cellulose and chitin are both structural polysaccharides which mean that they function to provide strength and rigidity. Cellulose is a linear polymer of up to 15, 000 glucose residues that provides strength to plant cells due to its highly cohesive and hydrogen-bonded structure. Chitin is the principal structural component of the exoskeletons of invertebrates. It is also found in the cell walls of most fungi and many algae. Chitin is a homopolymer of N-acetyl-D-glucosamine residues. Both cellulose and chitin have similar functions, and their structures differ only in that the OH group on the second carbon in cellulose has a acetamido functional group in chitin. Starch and glycogen are both composed of glucose residues and are storage polysaccharides that function as an energy source. Starch is composed of α-amylose and amylopectin. α-amylose is a linear polymer of several thousand glucose residues linked by α (1 → 4) bonds, and amylopectin consists of α(1 → 4)-linked glucose residues but with α(1 → 4) branch points every 24-30 glucose residues. Starch is the primary energy reserve for plants. Glycogen has the same structure as amylopectin, but is more highly branched. Glycogen is the primary energy reserve for animals.
If cellulose forms such a compact, water-excluding structure, how can ruminants (e.g., cows) hydrolyze it?
Time is the key to ruminant use of cellulose. Humans get one pass at hydrolysing food as it passes through their systems. Ruminants process foods in a more leisurely manner (which is what they are doing while chewing the cud). Continual mechanical processing and exposure to water and enzymes will hydrolyze the cellulose molecule from the outside in.
Why does paper (which is composed of unoriented cellulose fibres) lose its strength and structure when wet?
Water will penetrate areas of disorder in the cellulose structure. The water molecules will then disrupt some H-bonds by competing with interchain H-bonds.
Chemically, how could you hydrolyze chitin to monosaccharides?
You could hydrolyse chitin to monosaccharides by boiling it with a concentrated base. Base hydrolysis is the most efficient way to disrupt most polymeric biochemical molecules, because OH− is such an effective nucleophile.
glycoprotein
a protein to which one or more oligosaccharides is or are covalently linked
two most common examples of insoluble structural polysaccharides are
cellulose (glucose units) and chitin (N-acetyl-D-glucosamine units)
The cyclic structure of glucose can adjust to a chair or a boat conformation. Which conformation is more prevalent?
chair conformation
In general, soluble polysaccharides are
α-linked
If we consider only D-hexoses (the stereoisomers found in vivo), there are
eight possible aldohexoses and four possible ketohexoses
glycogen function
energy storage in animals
amylopectin function
energy storage in plants
Oligosaccharides are attached to newly formed proteins by
enzymes
example of hypotonic environment
existing within a host
The polysaccharide chains of both cellulose and chitin stack in such a way that
extensive hydrogen bonding can occur between the chains, and water is excluded
The polyglucose chain can also have side branches, usually α(1 → 6) linked, which...
further disorder the structure of the polysaccharide and allow for extensive hydration
most common hexose in vivo
glucose
Bacterial cell walls are a special subset of
glycoproteins
Anomeric hydroxyl group of a cyclic sugar can react with another alcohol group to form a new bond. This is called a
glycosidic bond
The rigidity of their cell walls allows bacteria to live in a hypotonic environment that would otherwise cause them to lyse (burst)
lyse (burst)
N-linkages form between
one of the sugar hydroxyls (not the anomeric one) and the amino group of asparagine.
Which polysaccharides have a more disordered, hydrated structure?
α-linked
the anomeric hydroxyl group is very important in determining
polysaccharide solubility
Enzymes are not nearly as efficient as DNA at producing identical copies. As a result, oligosaccharide structures may
sometimes show subtle changes
Why is glucose the most common hexose in vivo?
the ability to cyclize readily and without having to resort to complex reactions.
Bacterial cell walls contain D-amino acids; therefore, they cannot be cleaved by normal proteolytic enzymes. Why is this?
the amino acids that make up proteins (and therefore proteolytic enzymes) are L-amino acids
O-linkages resemble the glycoside bond, because the linkage is between
the anomeric sugar carbon and the hydroxyl group of one of the alcohol amino acids (serine or threonine).
α(1 → 4) glycoside bonds places the terminal −−CH2OH group of each glucose monomer on the same side of the polysaccharide chain. How is steric hindering minimized?
the glucose monomeric units bend away from each other in a large, left-handed helix
The bacterial cell wall can be cleaved by lysozyme at a more vulnerable site:
the glycosidic bond joining NAG with NAM
Proteoglycans can be...
very rigid, flexible, or viscous
What is a byproduct of the formation of a glycosidic bond?
water
Sugars increase the ____________________ of proteins and lipids
water solubility
The storage polysaccharides are linked by
α(1 → 4) glycoside bonds
Example of storage polysaccharide
α-amylose