Carbohydrates for energy, structure, and signals Flashcards
What are carbohydrates essential for?
- energy storage
- structure
- recognition
Define carbohydrates
molecules with molecular formula (CH2O)n (n≥3)
List some monosaccharides
- glucose
- ribose
- fructose
- mannose
List some disaccharides
- sucrose
- maltose
- lactose
- mannose
- trehalose
List some polysaccharides
- starch
- glycogen
- cellulose
- amylopectin
- amylose
- dextran
- cellulose
- chitin
- GAGs
Describe aldose
carbohydrate with aldehyde
Describe ketose
carbohydrate with ketone
Describe enantiomers
- mirror images
- stereoisomers)
- L = left
- D = right (dextral)
Stereo/chiral center
atom with four different attachments
L stereoisomer carbohydrates are
very rare
Organisms prefer
- D-carbohydrates
- L-amino acids
List some aldose pentoses
- D-Ribose
- D-Arabinose
- D-Xylose
List some ketose pentoses
D-Ribulose
List some aldose hexoses
- D-Glucose
- D-Mannose
List some ketose hexoses
D-Fructose
Enantiomers are
rare
Diastereomers
stereoisomers that are not mirror images
Epimers
diastereomers that differ only at one stereo center
Compare and contrast D-Glucose and D-Fructose
- D-Glucose: aldose
- D-Fructose: ketose
- both hexoses
- differ only in position of carbonyl
Describe Pyranoses
- six-membered sugar rings
- more stable
Describe Furanoses
- five-membered sugar rings
- less stable
Four-membered
sugar rings are unstable
Describe intramolecular cyclization
- (preferably secondary) hydroxyl can react with carbonyl
- pentoses and hexoses form rings
What does aldehyde cyclisation result in?
hemiacetal with anomeric carbon
What does ketone cyclisation result in?
hemiketal with anomeric carbon
Describe cyclisation of aldoses
- α and β are anomers
- α: OH on C-1 is opposite side of C-6
- β: OH on C-1 is on same side as C-6
- equilibrium in solution: 36% α, 64% β
- <1% linear
anomers
type of stereoisomer
Describe Pyranose rings
- chair or boat
- chair is energetically more favorable
- less steric hindrance between equatorial groups
Describe cyclisation of fructoses
- α and β are anomers
- α: OH on C-1 is opposite side of C-6
- β: OH on C-1 is on same side as C-6
- Furanose rings have envelope conformations
- fructose mostly as pyranose via primary OH at C6
Compare epimers and anomers
epimers are fixed, anomers are dynamic
Describe a reduction reaction
- β-D-Deoxyribose from ribose in DNA to D-Mannitol
- not cyclic
- from D-mannose
- fungal energy
Describe an oxidation reaction
- β-D-Glucuronic acid
- detoxification in liver
Describe a phosphorylation reaction
- β-D-Glucose-6- Phosphate (G6P) to D-Ribulose-1,5-Biphosphate (RuBP)
- in glycolysis
Describe amino sugars
- α-D-Glucosamine to α-N-acetyl-glucosamine (GlcNac, in chitin)
- chitin degradation
Describe glycosidic bonds
- in acidic conditions, anomeric carbon reacts with alcohols to form O-glycosidic bonds
- common linkage in di/polysaccharides
Disaccharides are often
soluble
Describe lactose
- Galactose-β(1→4)-Glucose
- milk sugar: 2-8% in milk
- hydrolysis by β-galactosidase in bacteria; lactase in humans
- reducing sugar: carbonyl is still available
- can act as reducing compound
Describe maltose
- Glucose-α(1→4)-Glucose
- malt sugar: starch degradation product
- malting barley during beer brewing
- hydrolysis by maltase in small intestine
- reducing sugar: carbonyl is still available
- can act as reducing compound
Describe sucrose
- Glucose-α(1→2)β-Fructose - table sugar from cane or beet
- hydrolysis by sucrase/invertase
- O-Glycosidic bond between anomeric carbons locks carbonyl
- ‘nonreducing’
Describe trehalose
- Glucose-α(1→1)α-Glucose
- stable: heat/acid resistant
- antifreeze in plants, bacteria
- flying insects hydrolysis by trehalase
- O-Glycosidic bond between anomeric carbons locks carbonyl
- ‘nonreducing’
What happens in non-reducing disaccharides
second residue is inverted 180 degrees
polysaccharides are … than disaccharides
less water soluble
Describe glycogen, starch and dextran
- branching α(1→6)
- glucose polymers
- accessible, quickly mobilisable energy store
Describe glycogen
- animals
- branching every ~10 units
Describe starch
mixture of amylopectin and amylose
Describe amylopectin
branching every ~30 units
Describe amylose
no branching
Describe dextran
bacteria; yeast
- only α(1→6) linkages
Describe cellulose
- unbranched glucose polymer with β(1→4) glycosidic bonds
- every second unit turned 180 degrees (extra H-bond)
- long straight chains assemble into fibers (structural role)
- plant cell wall: most abundant organic compound in biosphere!
Describe chitin
- decorated cellulose
- 2nd most abundant polymer in biosphere
- structural role in fungi, anthropods, squid, algae
- GlcNac instead of glucose
- carries acetylated amino group at C-2
GlcNac
N-acetylglucosamine
Describe GAG
- major component of ECM in animals
- Hyaluronan in cartilage of joints
- inear disaccharide polymer
- water soluble
- linked to proteins
GAG
Glycoaminoglycans (GAG)
Describe some possible glycogen conjugates
- to nitrogen bases
- to proteins
- to lipids
How does glycogen bond to nitrogen bases?
N-glycosidic bond
How does glycogen bond to proteins?
N- and O-glycosidic bond
How does glycogen bond to lipids
diverse linkages
Describe glycogen conjugates with nitrogen bases
- in all (deoxy)nucleotides
- in many cofactors
- e.g. adenosine
- β-(1-N)-Glycosidic bond
Describe protein glycosylation
- common for secreted proteins
- N-glycosylation
- O-glycosylation
Describe N-glycosylation
- on Asn(N) residues in N-x-S/T motifs
- similar core
- diverse antenna
- quality control and trafficking
Describe O-glycosylation
- on Ser/Thr residues
- highly diverse
and complex - roles in recognition
- e.g. animal proteoglycan (with GAG)
Describe lipid glycosyation
common and diverse
List some glycosylated lipids
Ganglioside
GPI anchor
LPS
Describe ganglioside
- diverse glycosphingolipids
- signaling in nervous system
Describe GPI anchor
- glycosylphosphatitylinositol
- anchors membrane proteins
Describe LPS
- lipopolysaccharide
- Gram-negative bacteria
- antigenic
- can cause sceptic shock
List the 3 main roles of carbohydrates
- energy storage
- structure
- recognition
Describe the carbohydrates used in energy storage
- mannitol
- sucrose
- lactose
- trehalose
- mannose
- glycogen
- starch
- dextran
Describe the carbohydrates used in structure
- cellulose
- chitin
- GAGs
Describe the carbohydrates involved in recognition
protein/lipid glycosylation
