Chapter 7 Exam 3 Flashcards
What are carbohydrates?
carbohydrates = aldehydes or ketones with at least two hydroxyl groups, or substances that yield such compounds on hydrolysis
many carbohydrates have the empirical formula (CH2O)n
What are the classes of carbohydrates?
monosaccharides = simple sugars, consist of a single polyhydroxy aldehyde or ketone unit
example: D-glucose
disaccharides = oligosaccharides with two monosaccharide units
example: sucrose (D-glucose and D-fructose)
oligosaccharides = short chains of monosaccharide units, or residues, joined by glycosidic bonds.
polysaccharides = sugar polymers with 10+ monosaccharide units
examples: cellulose (linear), glycogen (branched)
How does stereoisomerism work in monosaccharides?
backbones of monosaccharides:
-unbranched carbon chains with single bonds linking all carbon atoms
-one of the carbon atoms is double-bonded to an oxygen atom to form a carbonyl group
-other carbon atoms are bonded to a hydroxyl group
Stereoisomers arise because many of the carbon atoms to which the hydroxyl groups are attached are chiral centers
enzymes that act on sugars are stereospecific
What is a aldose?
aldose = carbonyl group is at an end of the carbon chain (in an aldehyde group)
What is a ketose?
ketose = carbonyl group is at any other position (in a ketone group)
What makes sugar sweet?
TAS1R2 and TAS1R3 encode sweet-taste receptors
binding of a compatible molecule generates a “sweet” electrical signal in the brain
requires a steric match
(The amino acids glycine, alanine, and serine are mildly sweet and harmless; nitrobenzene and ethylene glycol have a strong sweet taste but are toxic.
Stevioside, a sugar derivative isolated from the leaves of the stevia plant (Stevia rebaudiana Bertoni), is several hundred times sweeter than an equivalent amount of sucrose.
Small protein brazzein (54 amino acids), isolated from berries of the Oubli vine (Pentadiplandra brazzeana Baillon) in Gabon and Cameroon, is 17,000 times sweeter than sucrose on a molar basis. Presumably, the sweetness of the berries encourages their consumption by animals that then disperse the seeds so that new plants are established.)
What is the asymetric center of monosaccarides?
all monosaccharides (except dihydroxyacetone) contain 1+ chiral carbon atom
occur in optically active isomeric forms
enantiomers = two different optical isomers that are mirror images
in general, a molecule with n chiral centers can have 2n stereoisomers
What are fischer projections?
used to represent three-dimensional sugar structures on paper
bonds drawn horizontally indicate bonds that project out of the plane of the paper
bonds drawn vertically project behind the plane of the paper
Whats the difference between D-isomers and L-isomers?
reference carbon = chiral center most distant from the carbonyl carbon
two groups of stereoisomers:
-D isomers = configuration at reference carbon is the same as D-glyceraldehyde
on the right (dextro) in a projection formula
most hexoses of living organisms
-L isomers = configuration at reference carbon is the same as L-glyceraldehyde
on the left (levo) in a projection formula
How are carbons on sugar numbered?
carbons are numbered beginning at the end of the chain near the carbonyl group
What are epimers?
epimers = two sugars that differ only in the configuration around one carbon atom
How are monosaccharides have cyclic structure?
in aqueous solution, aldotetroses and all monosaccharides with 5+ backbone carbon atoms occur as cyclic structures
covalent bond between the carbonyl group and the oxygen of a hydroxyl group
What are hemiacetals or hemiketals?
hemiacetals or hemiketals = derivatives formed by a general reaction between alcohols and aldehydes or ketones
product of the first alcohol molecule addition
a five- or six-membered ring forms if the —OH and carbonyl groups are on the same molecule
What is an acetal or ketal?
acetal or ketal = product of the second alcohol molecule addition
forms a glycosidic bond
How are hemiacetals and hemiketals?
What are alpha and beta stereoisomeric configurations?
reaction with the first alcohol molecule creates an additional chiral center (the carbonyl carbon).
produces either of two stereoisomeric configurations: α and β
What are anomers?
anomers = isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom
anomeric carbon = the carbonyl carbon atom
reaction between the aldehyde group at C-1 and the hydroxyl group at C-5 forms a hemiacetal linkage
mutarotation = the interconversion of α and β anomers
What is pyranoses?
pyranoses = six-membered ring compounds
form when the aldehyde group at C-1 reacts with hydroxyl group at C-5
What are furanoses?
furanoses = five-membered ring compounds
form when the hydroxyl group at C-5 reacts with the keto group at C-2
What is a Haworth Perspective formula?
Haworth perspective formulas = more accurate representation of cyclic sugar structure than Fischer projections
six-membered ring is tilted to make its plane almost perpendicular to that of the paper
bonds closest to the reader are drawn thicker than those farther away
How do you convert D-hexose fischer to Haworth?
step 1: draw the six-membered ring (five carbons, and one oxygen at the upper right)
step 2: number the carbons in a clockwise direction beginning with the anomeric carbon
step 3: place the hydroxyl groups
hydroxyl groups on the right in a Fischer projection are placed pointing down and those on the left are placed pointing up
step 4: place the terminal —CH2OH group
projects upward for the D enantiomer, downward for the L enantiomer
step 5: place the anomeric hydroxyl group
for a β structure, the hydroxyl group is placed on the same side of the ring as C-6
for an α structure, it is placed on the opposite side
What should we know about chair conformations?
Conformational Formulas of Pyranoses
pyranose rings tend to assume either of two “chair” conformations
interconvertible without breaking covalent bonds
requires energy input
What are aldonic and Uronic acids?
aldonic acids = form following oxidation of the carbonyl carbon of aldoses, ex. Gluconic acid
uronic acids = form following oxidation at C-6, ex. Glucuronic acid
both form stable intramolecular esters called lactones
What is a phosphorylated derivative?
some sugar intermediates are phosphate esters
example: glucose 6-phosphate
stable at neutral pH and bear a negative charge
functions to trap sugar inside the cell because most cells do not have membrane transporters for phosphorylated sugars
What is a reducing sugar?
Sugars That are, or can Form, Aldehydes are Reducing Sugars
Reducing sugars = undergo a characteristic redox reaction where free aldehyde groups react with Cu2+ under alkaline condition
reduction of Cu2+ to Cu+ forms a brick-red precipitate: Benedict’s test
ketoses that can tautomerize to form aldehydes are also reducing sugars
Lactose, Maltose (Reducing Sugar)
Sucrose, Threhalose (Non-reducing sugar)
What are O-Glycosidic Bonds?
O-glycosidic bond = covalent linkage joining two monosaccharides
formed when a hydroxyl group of one sugar molecule reacts with the anomeric carbon of the other (acetal formation from hemiacetal)
readily hydrolyzed by acid but resist cleavage by base
What is the reducing end?
formation of a glycosidic bond renders a sugar nonreducing
reducing end = the end of a disaccharide or polysaccharide chain with a free anomeric carbon a
How does name reducing oligosaccharides work?
step 1: with the nonreducing end on the left, give the configuration (α or β) at the anomeric carbon joining the first unit to the second
step 2: name the nonreducing residue using “furano” or “pyrano”
step 3: indicate in parentheses the two carbon atoms joined by the glycosidic bond, with an arrow connecting the two numbers
step 4: name the second residue and repeat for additional residues
What are the three common disaccharides?
lactose is a reducing disaccharide
sucrose and trehalose are nonreducing sugars
What are some symbols and abbreviations for monosaccharides and derivatives?
What are homopolysaccharides?
homopolysaccharides = contain only a single monomeric sugar species
serve as storage forms and structural elements
Starch, Glycogen, Cellulose
What are heteropolysaccharides?
heteropolysaccharides = contain 2+ kinds of monomers
provide extracellular support
Glycosaminoglycans (Dermatan sulfate, Chondroitin sulfate)
Why do polysaccharides not have defined lengths of molecular weights?
this distinction between proteins and polysaccharides is a consequence of the mechanisms of assembly
there is no template for polysaccharide synthesis
the program for polysaccharide synthesis is intrinsic to the enzymes that catalyze the polymerization of monomer units
What is the structure of starch and glycogen?
What is chitin?
chitin = linear homopolysaccharide composed of N-acetylglucosamine residues in (β1→4) linkage
acetylated amino group makes chitin more hydrophobic and water-resistant than cellulose
What are NS (N-sulfo) Domains?
NS domains are highly sulfated domains that alternate with domains having unmodified GlcNAc and GlcA residues (NA domain)
the same core protein can display different heparan sulfate structures when synthesized in different cell types
What are the Four Types of Protein Interactions with NS Domains of Heparan Sulfate?
What is the Role of Lectin-Ligand Interactions in Leukocyte Movement?
Two other selectins participate in this “lymphocyte homing”: E-selectin on the endothelial cell and l-selectin on the leukocyte bind their cognate oligosaccharides on the leukocyte and endothelial cell, respectively.
What is the binding site for Neuraminidase?
The lectin of the influenza virus, known as the HA (hemagglutinin) protein, is essential for viral entry and infection. After the virus has entered a host cell and has been replicated, the newly synthesized viral particles bud out of the cell, wrapped in a portion of its plasma membrane. A viral sialidase (neuraminidase) trims the terminal sialic acid residue from the host cell’s oligosaccharides, releasing the viral particles from their interaction with the cell and preventing their aggregation with one another. Another round of infection can now begin. The antiviral drugs oseltamivir (Tamiflu) and zanamivir (Relenza) are used clinically in the treatment of influenza. These drugs are sugar analogs; they inhibit the viral sialidase by competing with the host cell’s oligosaccharides for binding (Fig. 7-30). This prevents the release of viruses from the infected cell by sialidase, and also causes viral particles to aggregate, both of which block another cycle of infection.
What are some Methods of Carbohydrate Analysis?
