Carbohydrates Flashcards
Carbohydrates
Functions
- structural, e.g.
- -glycosaminoglycans in cartilage
- -chitin in exoskeletons
- -cellulose in cell walls
- coenzymes in biological processes e.g. immune system, fertilisation, prevention of blood clotting
- energy storage and metabolism: glycogen/glucose
Carbohydrates
Structure
-biomolecules made of carbon, hydrogen and oxygen
-usually with a hydrogen to oxygen atom ratio of 2:1:
Cm(H2O)n
-carbohydrates have a cyclic form and an acyclic form, there is constant turn over between these forms in aqueous solution
How do monosaccarides form complex polysaccacrides?
- variation in spatial orientation of OHs and side groups
- chemical modification of side groups can add complexity
- side groups can sit almost anywhere on the ring
- type of linkage/branching
- combination of monosaccarides
Enantiomers
Definition
- swapping the orientation of all OH or other side groups creates enantiomers (mirror image molecules) e.g. D-glucose and L-glucose
- only D-glucose is created naturally in biological systems, but L-glucose is possible to chemically construct
Epimers
Definition
- swapping position of OH or side groups on only one carbon
- if it is the first carbon group that swaps, the two molecules are called anomers
- if it is any other carbon group that swaps, the two molecules are distinct monosaccharides
Ring Puckering
- the 6-membred ring is not flat, it can adopt different conformations
- conformers can interconvert with each other but this may be a slow process
Glycosidic Linkage
- the linkage type effects interactions between monosaccharides and their rigidity
- α(1-4) bonds are more flexible than β(1-4)
Do polysaccharides have a secondary structure?
many polysaccharides have no defined secondary structure e.g. hyaluronan
- but some have defined secondary structures and higher order structure e.g. amylose and cellulose
- amylose has α(1-4) bonds and a helical secondary structure stabilised by hydrogen bonds
- cellulose has β(1-4) bonds and a sheet secondary structure stabilised by hydrogen bonds
Hydrogels
- heterogeneous polysaccharides can for hydrogels
- e.g. pectin can form a hydrogel structure stabilised by ion bridges of carboxyl groups with Ca2+ ions
Cumulus Matrix
- an example of an ultrasoft hydrogels, what the oocyte moves through
Stretching Polysaccharide Chains
Small Forces
- at small forces, polysaccharides are entropically elastic chains
- follow the worm-like chain model
- pyranose ring and glycosidic linkage lead to rigidity in bending
Stretching Polysaccharide Chains
Large Forces
- at higher forces, conformational transitions of pyranose ring add enthalpic elasticity
- mechanical forces could modulate protein binding to polysaccharides
Carbohydrates in Cell Recognition
- cells present specific patterns of carbohydrates on surface (attached to proteins and lipids)
- these are important for cell-cell recognition and hijacked pathogens
- these surface carbohydrates can form catch bonds with cognate proteins:
- -holding firm at high shear flow to avoid washing away
- -holding weak at low shear flow to enable surface migration
Polysaccharides vs. Proteins
Force-Extension Curve
- polysaccharides are stiffer than polypeptides
- when considering the WLC model, Lp is greater for polysaccharides than for polypeptides so the force is smaller for a given extension
- polysaccharides also feature enthalpic elasticity in addition to entropic elasticity
α(1-4) bonds vs. β(1-4) bonds
Force-Extension Curve
- polysaccharides with β(1-4) bonds (e.g. cellulose, hyaluronin) are stiffer and have a sharp narrow peak
- polysaccharidse with α(1-4) bonds (e.g. amylose, heparin) are more flexible, have a wider peak with a hump, wavy lines
