Carbohydrates Flashcards
the general formula of carbohydrates
Cn(H2O)n
monosaccharides
single monomer
disaccharides
2 covalently linked monosaccharides
polysaccharides
many-covalently linked monosaccharides by O- glycosidic bonds
what do monosaccharides contain
a carbon chain, an aldehyde/keton, and hydroxyls on each carbon atom except the carbonyl.
aldopentose
5 carbon saccharide with an aldehyde group
keto hexane
a 6-carbon saccharide with a ketone group
how do we establish a chiral center?
we resort to Fischer projections to establish the configuration of the chiral center by comparison with D- or L- glyceraldehyde
how do we determine if a carbohydrate is D or L?
based on the configuration of the chiral C furthest away from the aldehyde/ketone
d-glucose
- known as dextrose
- building block of the disaccharides sucrose and lactose and of polysaccharides such as cellulose and glycogen
d-galactose
- aldohexose with the formula C6H12O6
- obtained from lactose found in milk
galactosemia
a condition where the enzyme needed to convert D-galactose to D-glucose is missing. galactose accumulates in the blood and tissue which can lead to cataracts, mental retardation, and liver disease
d-fructose
ketohexose with the formula C6H12O6
obtained as one of the hydrolysis products of sucrose
d-ribose
- an aldopentose with the formula C5H10O5
- component of RNA
- has 3 chiral centres
- reduced form is 2 deoxy-D-ribose which is part of DNA
mutarotation
takes place in the formation of cyclic forms of monosaccharides. nucleophilic attack from the -OH causes the carbon atom to undergo a change in hybridization from sp2 to sp3.
anomers
isomers with different chirality at the carbon atom that in the open-chain form is a carbonyl
alpha anomer
the OH group is pointing down
beta anomer
the OH group is pointing up
mutarotation in fructose
makes the glycosylation in glucose and galactose possible
what structure do saturated 6-membered rings assume?
a half-chair conformation due to sp3 hybridisation
which form of monosaccharides can undergo oxidation?
open-chain
oxidation of monosaccharides with aldehydes
if there is an aldehyde group with an adjacent hydroxyl group it can be oxidised to a carboxylic acid by an oxidizing agent such as Benedicts solution
oxidation of monosaccharides with ketones
ketoses should not undergo oxidation but intermolecular arrangements make monosaccharides oxidizable.
reduction of monosaccharides
the carbonyl group can be reduced to the corresponding agents
reducing agents used for the reduction of monosaccharides
Pd + H2 or NaBH4 or enzymatically with the NADH cofactor as a hydride donor
describe the condensation of 2 monosaccharides to form a disaccharide
- one monosaccharide provides the hemiacetal in the form of anomeric carbon
- one provides a hydroxyl
- reaction leads to a stable acetal releasing a water molecule
the bond between 2 monosaccharides
glycosidic bond which can have a configuration of alpha or beta
maltose
has an alpha (1,4) glycosidic bond
lactose
glucose + galactose
found in breastmilk
hydrolyzed by the enzyme lactase
sucrose
-consists of a-D-glucose and b-D-glucose
- has an alpha-beta (1->2) glycosidic bond between C1 of glucose and C2 of fructose
cannot undergo mutarotation and cannot be oxidised
taste receptors
proteins that bind to substances that depolarise the cells and transmit a taste signal through facial and cranial nerves.
sweet receptors
TAS1R2 and TAS1R3 are found in the palate and the back of the tongue and any molecule that can bind to TAS1 will deliver a sensation of sweetness
what cannot the monosaccharide providing the anomeric carbon in the condensation reaction undergo
mutarotation
what is the glycosidic bond?
is an acetal
hemiacetal + alcohol
nucleophilic substitution. the cyclic hemiacetal can react with other nucleophiles such as NH3 to yield other products
N-glycosidic
bond between the nitrogenous base and the ribose
homopolysaccharides
single repeating unit
how does heterogeneity arise in homopolysaccharides?
from differences in monosaccharides linked together and in the pattern of glycosidic bonds: branching
amylose
poly alpha 1,4 glucose
linear chains
found in plants
broken down by enzymes called alpha-amylases
amylopectin
poly alpha 1,4 glucose and alpha 1,6 branches
branching occurs every 30 glucose units
amylopectin associated with amylose via non-covalent hydrogen bonding interactions to form starch
cellulose
-poly beta 1,4 glucose
- strands of cellulose can establish interchain hydrogen bonds that reduce the solubility and promote the formation of a fibrillar structure
- nondigestible: animals lack the enzyme required for the hydrolysis of the beta 1,4 glycosidic bond
glycogen
poly alpha 1,4 glucose with a 1,6 branching
more branching than amylopectin
branching every 8-12 residues
why is branching important in glycogen?
- when glucose is needed, monosaccharides are detached at the non-reducing end of the chain
- branching offers many ends to the enzymes that catalyze the removal
- in a time unit, much glucose is released, and glucose increases quickly
hemiacetal+ alcohol
acetal + water
heteropolysaccharides
when more than one type of monosaccharide is used
what can heteropolysaccharides function as
- structural
- increasing solubility of proteins
- shielding proteins from the immune system
- stabilising extracellular proteins
- anticoagulants
- blood groups
glycans
polysaccharides associated with other molecules
glycosylated proteins
glycans associated with proteins to yield glycosylated proteins
n-glycosylation
glycans are attached to Asn residues that are found in consensus sequences -Asn-non Pro- Ser/Thr
o-glycosylation
glycans are attached to Ser or Thr residues
N-acetyl glucosamine
modified sugar
which proteins undergo glycosylation?
extracellular proteins produced in the cell and then secreted or translocated to the cell membrane
glycosylation
when a carbohydrate is covalently attached to a macromolecule such as a protein or a lipid
Glycosaminoglycans (GAGs)
linear polymers of repeating disaccharides
what is the composition and mass of GAGs determined by?
by the relative activities of processing enzymes
when do mucopolysaccharides arise
when the activity of breakdown enyzmes is reduced
what does a disaccharide of glycosaminoglycan consist of?
uronic acid (sugar with a COOH function) and an amino sugar (sugar with an amino group replacing the OH)
how can sugars be modified?
by formation of sulfate esters
where are GAGs synthesised?
in the golgi apparatus
3 classes of GAGs
heparin/ heparan sulfate
chondroitin sulfate/dermatan sulfate
keratan sulfate
how are GAGs linked to proteins
via O-glycosidic bond or N-glycosidic bond
proteoglycans
heavily-glycosylated proteins with one or more GAGs attached to the core polypeptide
cartilage composition
composed of collagen, GAGs and proteoglycans
heparin GAG
GlcUA/ IdoUA
keratan sulfate GAG
Gal neutral or sulfate
Chondroitin sulfate GAG
GlcUA
Heparan sulfate GAG
GlcUA or IdoUA
Dermatan sulfate GAG
GlcUA or IdoUA
Hyaluronic acid GAG
GlcUA
