Carbohydrates Flashcards
(23 cards)
List three different roles of sugars in biology
- Carbohydrates are an important source of energy/stored fuels
- Glycogen granules (dark patches) in a liver cell - Carbohydrates provide structure to cells and organisms
- Cellulose in plants and chitin in arthropods - Cell Biology
- Major component of the cell surface
- Important in influencing the function of proteins
- Important in specific recognition interactions
Provide examples of different roles of carbohydrates in biology
> Information molecules (The Sugar Code)
- Carbohydrates have enormous structural diversity
Cell surface sugars are important in:
- Cell-cell adhesion
- Bacteria adhesion
- Virus attachment to host cells
- Binding of toxins to cell surface
- Genes encode proteins – not sugars
- How are glycoconjugates synthesised
Explain the difference between aldose and ketose sugars.
Monosaccharides
- basic unit of carbohydrates
- are aldehydes or ketones that have two or more hydroxyl groups
> Aldose because it contains an aldehyde group
Glyceraldehyde has a single assymetric (chiral) carbon atom
- Two sterioisomers D- and L-glyceraldehyde are enantiomers or mirror images of each other
- most naturally occuring sugars belong to D-series
Explain the structural basis of the interconversion between the α and β anomers of cyclised sugars
Draw the Fischer and Haworth projections of D-glucose.
- Provide a clear and simple view of the stereochemistry at each carbon centre
- Horizontal lines project out of the plane
- Vertical lines project behind the plane
Draw and recognise other sugars, based on their similarity to D-glucose.
D-glucose
- All OH on right hand side apart from C3
Be able to draw and/or identify glycosidic linkages in linear and branched oligosaccharides.
Linear Oligosaccharide
A linear oligosaccharide is a short chain of monosaccharide units connected in a straight, unbranched sequence by glycosidic bonds.
All sugars are connected end-to-end.
Each monosaccharide (except the ends) is linked to two others.
Example: Maltotriose (glucose–glucose–glucose via α(1→4) linkages).
Branched Oligosaccharide
A branched oligosaccharide contains monosaccharide chains that diverge from a central core, forming branch points.
At least one sugar is attached to more than two others.
Branched points usually occur at hydroxyl groups like C6 of a glucose unit.
Example: N-linked glycan chains on glycoproteins, or the branching in glycogen.
Identify reducing and non-reducing sugars in an oligosaccharide.
The redox activity of a sugar — its ability to act as a reducing agent — is specifically associated with the hemiacetal (or hemiketal) group at the anomeric carbon, not the acetal (or ketal) formed during glycosidic bond formation.
Explain how the linkages between glucose units in cellulose and glycogen give rise to their different properties.
Cellulose
- beta (1,4) bonds, homopolymer of glucose, has intrachain and interchain H bonds so forms stable and strong supramolecular fibres leading to insoluble molecule great for structure and rigiditiy of plant cell walls
Glycogen
- alpha (1,4) bonds, homopolymer of glucose, hollow open helix, branched, storage molecule
Cellulose’s β(1→4) linkages → straight chains → strong intermolecular H-bonding → tough, insoluble fibers.
Glycogen’s α(1→4) and α(1→6) linkages → flexible, branched chains → compact and easily mobilized for energy.
Draw the structural difference between cellulose and chitin.
List the amino acids that can act as glycosylation sites on proteins.
Carbohydrate chains can be attached to proteins to form glycoproteins
- Carbohydrate groups are covalently attached to many proteins
- Called glycoproteins
- Short oligosaccharide chains
- Many glycoproteins and glycolipids are components of cell membranes
- Oligosaccharides play important functions in recognition (e.g. cell adhesion)
Recognize N-linked and O-linked protein glycosylation.
Carbohydrates link to proteins in two ways:
1. Via Asparagine residues (N-glycans)
- sugars are linked to the amide side chain of asparagine
- therefore called N-linked oligosaccharides or N-glycans
- Via Serine or Threonine residues (O-glycans)
- Sugars are linked to the oxygen atoms in the side chain of serine or threonine residues (O-linkage)
- Therefore, called O-linked oligosaccharides or O-glycans
Sugar –> amino acid link –> type of glycosylation –> bond –> location
N-acetylglucosamine –> Asn –> N-glycan –> N glycosidic bond to amide nitrogen –> ER
O-acetylgalactosamine –> Thr/Ser –> O glycosidic bond to hydroxyl oxygen - Golgi
Describe the general reaction of a glycosyl transferase.
Specific enzymes are responsible or oligosaccharide assembly
- Nucleotide-monosaccharide (UDP-Glc) + Acceptor (carb) –> (glyosyl (sugar) transferase) –> monosaccharide-acceptor + nucleotide
Note: monosaccharide is transferred from the nucleotide sugar to the non-reducing end of the carbohydrate acceptor
Describe the structural difference between A, B and O group blood antigens
- Human ABO blood groups are carbohydrate antigens
- Carbohydrates are attached to glycoproteins (and glycolipids) on surfaces of red blood cells
- ABO blood group antigens are glycosphingolipids
- A and B blood group antigens differ from O antigen by one extra monosaccharide
All blood groups antigens have:
- Ceramide group consisting of sphingosine and fatty acid
- A chain of monosaccharides
Identify/draw A, B and O antigens.
Predict the blood group type of an individual, based on the functional properties of the ABO glycosyltransferase alleles they have inherited.
Glycosyltransferases and blood groups
- There is a single gene controlling the synthesis of ABO blood groups
- The ABO blood group gene encodes a glycosyltransferase (enzyme)
- Three forms (or alleles) of this exist in the human population
- An individual has one allele inherited from their mother and one allele inherited from their father
- The single gene controlling ABO blood groups encodes a glycosyltransferase
o A allele: encodes an active glycosyltransferase
GalNAc transferase
o B allele: encodes an active glycosyltransferase
Gal transferase
o O allele: encodes a dead enzyme
No active transferase
Explain ABO blood group incompatibility.
- The only difference between blood group A and B antigens Is the substitution of C2 of the terminal galactose
- Antibodies are highly specific and very small chemical differences in oligosaccharide structure binding change affinity
- Antibodies to A and B group antigens occur naturally in individuals
Predict the outcome of blood transfusions.
- antibodies will cross link the “foreign” RBCs (agglutination)
- incompatibility leads to hemolysis and can be deadly
Describe, with an example, the interaction between a C-type lectin and a carbohydrate.
- Antibodies are not the only proteins which can recognise oligosaccharides
- Lectins
o Broad family of proteins that recognise carbohydrates
o Animals, plants and micro-organisms
o One large class of lectins are known as C-type lectins - C-type lectins- How do they recognise carbohydrate?
o C type for calcium binding
o A common 120 amino acid domain responsible for carbohydrate binding
o Calcium acts a bridge between the protein and the sugar through direct interaction with sugar hydroxyl groups
o Different C-type lectins have different carbohydrate-binding specificities
o Changes in residues that interact with the sugar alter the carbohydrate-binding specificity of the lectin
- C-type lectins and cell-cell adhesion
o Selectins are C-type lectins
o Are involved in cell-cell adhesion
o Bind white blood cells to sites of injury and allow movement of cells from bloodstream to site of infection
Explain how the antiviral drug Relenza interferes with the influenza life cycle.
Many animal viruses attach to host cells via carbohydrate
- To infect a host cell, viruses need to bind to receptors on the cell surface
- In some cases these receptors are carbohydrates
- For example, influenza virus recognise sialic acid residues present on cell surface glycoproteins
Influenza virus binds to sugar residues on the host cell surface
o The sugar-binding site of sialic acid binding proteins has been the target of “carbohydrate-based drug design”
o Influenza antivirals
o The anti-flu drug “Relenza” is a sialic acid analogue
- Relenza inhibits the enzyme neuriminidase from flu virus
Use the GLUT1 glucose transporter as an example of passive transport by an integral membrane protein.
Describe the model for conformational change allows GLUT 1 to able to translocate glucose across membranes.
Glucose uptake occurs by GLUT transporters
- Two conformational states – T1 and T2
- As the concentration of glucose goes up outside the cell the rate of transport gets faster and then plateaus
Be able to rationalize the kinetic properties of GLUT family transporters with their tissue distribution and functional role.
